BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

BBC Microcomputer service manual 

SECTION 1 BBC Microcomputer Models A and B (ANA01 - ANB04) 

SECTION 2 BBC Microcomputer Model B+ (ANB51 - ANB54) 

SECTION 3 Additional Upgrades 

1770 Disc interface daughter board upgrade 

64K Sideways RAM upgrade 

Part No. 0433,001 

Issue 1 

October 1985

Within this publication the term 'BBC' is used as an 

abbreviation for 'British Broadcasting Corporation'. 

Copyright ACORN Computers Limited 1985 

Neither the whole or any part of the information contained in, 

or the product described in, this manual may be adapted or 

reproduced in any material form except with the prior written 

approval of ACORN Computers Limited (ACORN Computers). 

The product described in this manual and products for use with 

it, are subject to continuous development and improvement. All 

information of a technical nature and particulars of the product 

and its use (including the information and particulars in this 

manual) are given by ACORN Computers in good faith. However, it 

is acknowledged that there may be errors or omissions in this 

manual. A list of details of any amendments or revisions to this 

manual can be obtained upon request from ACORN Computers 

Technical. Enquiries. ACORN Computers welcome comments and 

suggestions relating to the product and this manual. 

All correspondence should be addressed to:- 

Technical Enquiries 

ACORN Computers 

Limited Newmarket 

Road 

Cambridge 

CB5 8PD 

All maintenance and service on the product must be carried out 

by ACORN Computers' authorised dealers. ACORN Computers can 

accept no liability whatsoever for any loss or damage caused by 

service or maintenance by unauthorised personnel. This manual is 

intended only to assist the reader in the use of this product, 

and therefore ACORN Computers shall not be liable for any loss 

or damage whatsoever arising from the use of any information or 

particulars in, or any error or omission in, this manual, or any 

incorrect use of the product. 

This manual is for the sole use of ACORN Computers' authorised 

dealers and must only be used by them in connection with the 

product described within. 

First published 1985 

Published by ACORN Computers Limited

SECTION 1 BBC Microcomputer Models A & B

BBC Microcomputer Service Manual 

Contents 

1 - Introduction 

1.1 Nature and purpose of the manual 

1.2 Technical specification 

1.2.1 Model A specification 

1.2.2 Model B specification 1.2.3 Expansion 

1.2.4 Software 

1.2.5 Machine operating system 1.2.6 BASIC 

1.3 Packaging 

1.4 Mechanical assembly of the case etc 

2 - General Circuit Description 

2.1 Introduction 

2.2 Hardware description 

3 - Detailed Circuit Descriptions 

3.1 Processor + clock circuitry + reset circuitry 

3.2 Memory + address decoding 

3.3 DRAMs + CRT controller + video processor + Teletext hardware 

3.4 RGB + PAL encoder + UHF output 

3.5 Cassette + RS423 + serial processor 

3.6 Internal VIA 

3.7 Keyboard 

3.8 Sound + speech + serial ROM interfaces 

3.9 A to D converters 

3.10 Disc interface 

3.11 Printer + user port interfaces 

3.12 Econet 

3.13 1 MHz bus 3.14 Power supply 

4 - Upgrading the PCB 

4.1 Convert from EPROM MOS to ROM MOS 

4.2 Convert model A to model B 

4.3 Add speech option 

4.4 Add 5 1/4 inch disc interface to model B 

4.5 Add Econet interface to model A 

4.6 Add 8 inch disc interface to model B 

4.7 Partial upgrading 

5 - Selection links & Circuit changes 

5.1 Selection link survey 

5.2 Table of link options 

5.3 Circuit modifications from Issue 1 to Issue 7 

5.3.1 Changes from issue 2 to 3 

5.3.2 Changes from issue 3 to 4 

5.3.3 Changes from issue 4 to 7

6 - Servicing + Fault-finding 


6.2 Test equipment 

6.3 Fault isolation 

6.4 Most common faults 

6.5 Test programs + sample waveforms 

6.5.1 Test program 

6.5.2 Test ROM 

7 - Interfacing Survey 

7.1 Purpose of each interface 

7.2 Interfacing to various printers 

7.3 Hardware hints and tips 

8 - Component location tables 

8.1 Integrated circuits 

8.2 Transistors 

8.3 Diodes 

8.4 Capacitors 

8.5 Resistors 

8.6 Links 

9 - Appendices 

9.1 Circuit block diagram 

9.2 Assembly Drawing 

9.3 Case Lower Assembly Drawing 

9.4 Main PCB Layout 

9.5 Main PCB Circuit Diagram 

Issue 9 (issue 7 board) as full foldout 

9.6 Keyboard Circuit Diagram 

9.7 Power Supply Circuit Diagram 

9.8 Parts list for B + disc + Econet 

9.9 Glossary of abreviations

WARNING: THE COMPUTER MUST BE EARTHED 

IMPORTANT: The wires in the mains lead for the computer are coloured 

in accordance with the following code: 

GREEN & YELLOW - EARTH 

BLUE - NEUTRAL 

BROWN - LIVE 

As the colours of the wires may not correspond with the coloured 

markings identifying the terminals in your plug, proceed as follows: 

The wire which is coloured green and yellow must be connected to the 

terminal in the plug which is marked by the letter E, or by the safety 

earth symbol =|= , or coloured either green or green and yellow. 

The wire which is coloured blue must be connected to the terminal 

which is marked by the letter N, or coloured black. 

The wire which is coloured brown must be connected to the terminal 

which is marked by the letter L, or coloured red. 

If the socket outlet available is not suitable for the plug supplied, 

the plug should be cut off and the appropriate plug fitted and wired as 

previously noted. The moulded plug which was cut off must be disposed 

of as it would be a potential shock hazard if it were to be plugged in 

with the cut off end of mains cord exposed. 

The moulded plug must be used with the fuse and the fuse carrier firmly 

in place. The fuse carrier is of the same basic colour* as the coloured 

insert in the base of the plug. Different manufacturers' plugs and fuse 

carriers are not interchangeable. In the event of loss of the fuse 

carrier the moulded plug MUST NOT be used. Either replace the moulded 

plug with another conventional plug wired as previously described, or 

obtain a replacement fuse carrier from an authorised BBC Microcomputer 

dealer. In the event of the fuse blowing, it should be replaced, after 

clearing any faults, with a 3 amp fuse that is ASTA approved to BS 

1362. 

This computer was designed and manufactured to comply with BS 415. In 

order to ensure the continued safety of Acorn products, power supplies 

should be returned to Acorn for repair. 

Do not use the Microcomputer in conditions of extreme heat, cold, 

humidity or dust or in places subject to vibration. Do not block 

ventilation under or behind the computer. Ensure that ,no foreign 

objects are inserted through any openings in the Microcomputer. 

*Not necessarily the same shade of that colour.

1 Introduction 

1.1 Nature and Purpose of the Manual 

The purpose of this manual is to provide technical and diagnostic 

information about the BBC Microcomputer. 

After giving general information about the technical specification and 

the mechanical assembly of the BBC Microcomputer, it gives a detailed 

description of the operation of the whole of the circuit. Information 

is also given about how to upgrade the various models of microcomputer 

and the purpose of the various links on the circuit board. Some details 

are also given of the ways in which the circuit has changed in its 

evolution from issue 1 up to issue 7. There is some guidance about 

servicing and fault-finding, further information about interfacing and 

a few suggestions about possible applications. Finally there is a 

section of hardware hints and tips which is a compilation of ideas from 

various sources. 

1.2 Technical Specification 

The BBC Microcomputer is supplied with two levels of hardware 

provision, designated, model A and model B, the former being fully 

upgradable to the latter. 

1.2.1 Model A Specification 

A fast, powerful self-contained computer system generating high 

resolution colour graphics and capable of synthesising 3 part music + 1 

channel of noise. The computer is contained in a rigid injection 

moulded thermoplastic case. The following are contained within the 

computer thus ensuring the minimum of connecting wires. 

* 73 key full travel QWERTY keyboard including 10 user definable 

function keys. The keyboard has two key rollover and auto repeat. 

* Internal power supply is fully encased and manufactured to BS 415 

Class 1. 

* The internal loudspeaker is driven from a 4-channel sound synthesis 

circuit with full ADSR envelope control. 

* A colour television signal, for connection to a normal domestic 

television aerial socket, is available through a phono connector. This 

signal is 625 line, 50Hz, interlaced, fully encoded PAL and is 

modulated on UHF channel 36. 

* A BNC connector supplies a composite video output to drive a black 

and white or PAL colour monitor. 

* A standard audio cassette recorder can be used to record computer 

programs and data at 300 or 1200 baud using the Computer Users Tape 

Standard tones. The cassette recorder is under full automatic motor 

control and is connected to the computer via a 7 pin DIN connector. 

1

* An interrupt driven elapsed time clock enables real-time control 

and timing of user responses. 

* The unit uses a 2 MHz 6502A and includes 16K of Random Access Memory. 

* A 16K Read Only Memory (ROM) integrated circuit contains a Machine 

Operating System designed to interface easily to high level languages. 

* A further 16K Language ROM contains a fast BASIC interpreter. The 

interpreter includes a 6502 assembler which enables BASIC statements to 

be freely mixed with 6502 assembly language. 

* Up to four 16K "sideways" ROMs may be plugged into the machine at 

any time. These four ROMs are "paged" and may include Pascal, word 

processing, computer aided design software, disc and Econet filing 

systems or Teletext acquisition software. 

* The full-colour Teletext display of 40 characters by 25 lines, known 

as mode 7, has character rounding, with double height, flashing, 

coloured background and text plus pixel graphics - all to the Teletext 

standard. 

* The non-Teletext display modes (modes 0 to 6) provide user definable 

characters in addition to the standard upper and lower case alphanumeric 

font. In these modes, graphics may be freely mixed with text. 

Text characters can be positioned not only on, for example, a 40 x 32 

grid, but at any intermediate position. 

* Separate or overlapping text and graphic windows can be easily userdefined 

over any area of the display. Each of these windows may be 

filled separately and the text window scrolls independently of the rest 

of the screen. 

* The Model A is able to support the following modes:- 

Mode 4: 320 x 256, 2 colour graphics and 40 x 32 text (10K) 


Mode 6: 40 x 25, 2 colour text only (8K) 

Mode 7: 40 x 25, Teletext display (1K) 

* All graphics access is "transparent" (see section 2.2), resulting in 

a fast, snow-free display. 

* Extensive support is provided in the Machine Operating System for 

the graphics facilities, and this is reflected in the BASIC 

interpreter. These facilities include the ability to draw lines very 

rapidly and to fill large areas of colour. In addition, very rapid 

changes of areas of colour can be effected by the use of a colour " 

palette". 

* The Model A BBC Microcomputer can be expanded at any time to the 

Model B. In addition, or as an alternative, other facilities such as 

the Econet may be fitted within the computer system. 

2

1.2.2 Model B Specification 

The Model B BBC Microcomputer is an enhanced version of the Model A 

Microcomputer with the following differences:- 

* 32K Random Access Memory (RAM). This enables the following extra 

graphics modes to be used:- 




Mode 3: 80 x 25, 2 colour text only (16K) 

* The installed RAM is divided between the high resolution graphics 

display, the user's program and Machine Operating System variables. If 

higher resolutions are required with large programs, then the second 

processor option may be fitted. 

* 6 pin DIN connector provides separate RGB and sync outputs at TTL 

levels. RGB are all high true, and sync is link selectable as high or 

low true, pulse duration 4.7 microseconds. 

* Serial interface to RS423 standard. The new standard has been 

designed to be inter-operable with RS232C equipment but offers a 

considerably enhanced specification - for example in maximum length of 

cable and maximum data transfer rates. Baud rates are software 

selectable between 75 baud and 9600 baud. The interface provides not 

only two-way data transfer, but also two-way hand-shaking using RTS and 

CTS lines. The software for implementing this interface is only 

provided with operating systems 1.2 onwards. 

* An 8 bit input/output port with 2 control bits is also provided. 

* Four analogue input channels are provided. Each channel has an input 

voltage range of 0 - 1.8V. The conversion time for each channel is 10 

milliseconds. These analogue inputs can be used not only as inputs for 

games-paddles or joysticks but also in laboratory control situations. 

The resolution of the ADC chip is 12 bits, but its conversion is such 

that only 9 or 10 bits are significant. However with suitable 

averaging, this can be extended to the full 12 bits accuracy. 

* A 1 MHz buffered extension bus is provided for connection to a 

variety of external hardware such as a Teletext acquisition unit, IEEE 

488 interface, Winchester disc drive etc. 

3

1.2.3 Expansion 

The following expansion options are available, some of which may be 

fitted internally at purchase, but all of which could be fitted by 

Dealers at a later date:- 

* Floppy disc interface (fitted as an option at purchase) 

* Econet network interface (fitted as an option at purchase) 

* Voice synthesis circuit with cartridge ROM pack interface 

* Various alternative high-level languages in ROM 

External options which plug directly into the machine include:- 

* Games paddles 

* Cassette Recorder 

* Black and White and colour monitors and televisions 

* 5 1/4" disc drives, ranging from single-sided single density (100K) 

to dual double sided double track density (800K). 

* Dot-matrix or daisy wheel printers, serial or parallel interface 

* Teletext acquisition unit enabling Tele-software to be downloaded 

into the BBC Computer as well as providing access to the normal 

Teletext services. Pages may be "grabbed" and stored for later use. 

* 3 MHz 6502 second processor with 64K of RAM. 

* Z80 second processor with 64K of RAM and a fully CP/M-compatible 

operating system. 

* IEEE interface 

* Winchester 10 megabyte disc drive 

* Prestel adaptor unit 

1.2.4 Software 

Considerable attention has been paid to the overall design of both 

systems and applications software. A modular approach has been adopted 

specifically to ease the interfacing of various high-level languages ( 

such as BASIC and Pascal) to the operating system. 

1.2.5 Machine Operating System 

A 16K ROM is used for the MOS. This software controls all input/output 

devices using a well defined interface. The MOS supports the following 

interrupts (the full implementation only being available from MOS 1.2 

onwards):- 

* Event Timer (10ms) (used as an elapsed time clock) 

* 4 channel analogue to digital converter 

* Vertical sync 

* Keyboard and keyboard buffer 

* Music tone generation and buffer 

* Serial interface, input and output buffers 

* Parallel input/output port 

and 'hooks' are provided to support other devices such as:- 

* Teletext acquisition 

* Prestel acquisition 

* Econet file system 

* Disk file system 

* Byte transfer to second processor 

The majority of the operating system calls are vectored to enable the 

user to change them if required. 

4

1.2.6 BASIC 

The BASIC interpreter is an extremely fast implementation, with 

numerous powerful extensions:- 

* Long variable names 

* Integer, floating point and string variables 

* Multi-dimension integer, floating point and string arrays 

* Extensive support for string handling 

* IF ... THEN ... ELSE 

* REPEAT ... UNTIL 

* Multi-line integer, floating point and string functions 

* Procedures 

* Local variables 

* Full recursion on all functions and procedures 

* Effective error trapping and handling 

* Cassette loading and saving of programs and data 

* Full support for the extensive colour graphics facilities 

* Easy control of the built-in music generation circuits 

* Built-in 6502 mnemonic assembler enabling BASIC and assembler to be 

mixed, or pure assembly language programs to be produced. 

1.3 Packaging 

The BBC Microcomputer is supplied in a two part moulded polystyrene 

packing which is further packaged within a cardboard sleeve. With the 

Microcomputer, a User's Manual, a Welcome Cassette package and a UHF TV 

lead are also supplied. The packaging should be kept intact in case it 

becomes necessary to transport the unit at a later date. 

1.4 Mechanical assembly of case etc 

The lid of the Microcomputer case may be removed after undoing four 

fixing screws, two on the rear panel and two underneath. When 

reassembling, press the lid down at the rear whilst tightening the two 

rear fixing screws. Take care not to lose the two spire clips pushed 

onto the case lid, into which the rear fixing screws locate. NB Do not 

remove the lid with the mains power connected. 

Inside the Microcomputer are three main sub-assemblies: 

power supply unit, keyboard and the main printed circuit board. 

To remove the keyboard, undo the two or, in some cases, three screws 

and nuts holding it to the case bottom, take care to note the positions 

of the associated washers. Unplug the 17 way keyboard connector and the 

2-way loudspeaker connector from the main printed circuit board, and 

the 10 way serial-ROM connector, if fitted. 

The power supply unit is connected to the main circuit board by seven 

push-on connectors which may be unplugged. Three screws on the 

underside of the case are undone allowing the unit to be removed. On 

reassembly, ensure that the same type of screw is used. 

The main printed circuit board can be removed after the two wires to 

SK2 (composite video BNC socket) have been disconnected. Undo the four 

fixing screws (five or seven screws on later issue boards) and remove 

the circuit board from the case by sliding it forwards and then lifting 

it from the rear. 

5

2 General Description of Hardware 


This next section gives a general description of the hardware of the 

computer, and reference is made to the functional block diagram ( 

section 9.1) which is laid out approximately as the components are 

situated on the printed circuit board. General areas and component 

orientations are referred to by using compass points, as shown on the 

block diagram. When any reference needs to be made to the specific 

position of a component, then X-Y co-ordinates will be used, giving the 

distances in millimetres from the SW corner. This is also shown on the 

block diagram. A list is given in section 8 of this manual of all the 

integrated circuits, transistors, diodes, capacitors, resistors and 

selection links by number, including their X-Y co-ordinates on the PCB 

and on the main circuit diagram. 

As each section of the hardware is described, reference is made to 

sections of the following chapter in which more detailed descriptions 

are given. The heart of the hardware is the 6502 microprocessor, and in 

this general description we shall move around 'he 6502 in an anticlockwise 

direction starting from the SE corner c the PCB. 

2.2 Hardware description 

The 6502 accesses an area of just less than 32 Kbytes of ROM. (3/4K of 

this memory allocation is actually used for memory-mapped input/output.) 

The ROM is arranged in such a way that one group of 16K bytes forms a 

fixed part of the memory map (15 1/4K ROM for the operating system + 

3/4K of I/O), whilst the other 16K has been organised to give as much 

flexibility as possible. There is a ROM select facility for accessing 

up to 16 different memory devices, although only four sideways ROM 

sockets are available on the PCB. It is expected that the normal way in 

which these four sockets will be used is to provide 2 MHz access to 

each of 4 chips which could be either 16K or 8K, ROMs or EPROMs. [See 

section 3.2] 

The RAM is also divided into two sections of 16 Kbytes, each of which 

contains eight 16K by 1 bit DRAM chips. In the model A microcomputer, 

only one bank of 16K is present whilst both are present in the model B. 

This RAM has to be accessed by both the processor itself and also the 

CRT controller. This is done by using a form of "transparent access" in 

which both the processor and the CRT controller can access the RAM at 

the full clock speed by interleaving the accesses on alternate phases 

of the system clock. [See section 3.3] 

The display is extremely versatile, and uses two entirely different 

methods depending on screen mode. Mode 7 uses Teletext hardware which 

produces RGB signals by having its own character generator and accepting 

data from the RAM as ASCII characters. This means that it uses very 

little RAM (only 1 Kbyte), and apart from providing the addressing for 

the RAM, the only thing which the CRT controller has to do is to add 

the cursor information and sync signals. 

6

In the other screen modes, the information is stored in RAM as actual 

bit patterns for every character that is written to the screen. This is 

expensive in terms of memory usage, (between 8K and 20K in the 

different modes) but it makes it extremely versatile, especially when 

mixing graphics with text. The addressing of the RAM for the different 

modes is performed by the 6845 CRT controller, whilst the data is taken 

from the RAM and serialised by a custom designed circuit, known as the 

video processor. This data is not used directly to produce RGB 

information, but can be thought of as a set of logical colour numbers 

which are passed to an area of high speed RAM within the video 

processor referred to as the colour palette. This determines, for each 

logical colour number, which combination of red, green and blue is 

produced, and whether or not the colour is flashing. The video 

processor is also responsible for selecting either the RGB signals 

coming from the Teletext chip or the signals coming from the palette 

and sending them out to the RGB buffers and the PAL encoder. 

This RGB information is presented, after buffering, on the RGB 

connector. To provide a UHF output, the RGB signals are combined with 

the sync signals and fed into a UHF modulator. A video output is also 

provided which consists of a summing of the RGB signals in such a way 

as to give an appropriate grey scale. On issue 4 boards onwards, the 

option is given of adding colour to the video signal in order to 

provide a PAL encoded video output. [See section 3.4] 

Moving on round in an anti-clockwise direction we come to the two 

serial interfaces, the cassette interface and the RS423. These 

facilities are both provided by a standard ACIA (Asynchronous 

Communications Interface Adaptor) - the 6850, and a custom designed 

circuit referred to as, the serial processor. This processor contains 

the programmable baud rate generators for transmit and receive which 

provide the clocks for the ACIA. The ACIA itself is responsible for 

serialising the data, providing the control lines for the RS423 and 

generating interrupts, whilst the serial processor switches these data 

and control lines between the cassette and RS423 interfaces. The serial 

processor also provides data separator and sinewave synthesis circuits 

for the cassette recorder as well as a means of detecting the presence 

of the incoming tone from the recorder. [See section 3.5] 

The next section is the analogue input port which is a four channel 12 

bit converter which is discussed in more detail in section 3.9 and the 

interfacing survey (see chapter 7). 

In the NW corner is the Econet section which centres around a 68B54 

Advanced Data-Link Controller (ADLC). This is a sophisticated serial 

communications device allowing the sending and receiving of data at a 

variety of speeds between as many as 254 computers. The data transfer 

is synchronised by a clock signal fed to all the computers as a 

differential signal on one pair of cables, whilst the data itself uses 

another pair of cables. Data is both transmitted and received on the 

same pair of cables, but obviously only one computer at a time is able 

to "broadcast" onto the data highway. [See section 3.12] 

7

There are two 6522 versatile interface adaptors (VIA) on the PCB (one 

on the model A), the first being used mainly for internal control and 

the second for external interfacing. VIA-A is used both for control of 

internal hardware and also for generating interrupts from various 

devices such as the ADC and the keyboard. Of its two internal timers, 

the first is used for generating regular interrupts at one centisecond 

intervals and the second is used occasionally by the operating system. 

[See section 3.6] 

Of the two ports on this VIA, PA is used to provide a slow (1 MHz) data 

bus for the sound and speech chips and also for the keyboard, whilst PB 

is used to provide control lines for various functions throughout the 

circuit board. The sound is produced by a four-channel sound generator 

chip (SN76489) whilst the speech is produced by a TMS 5220 which can 

get its data either from RAM through VIA-A or from a serial ROM, the 

TMS 6100. This facility for accessing serial ROMs is also used to 

provide an external serial ROM facility on the keyboard. [See sections 

3.7 and 3.8] 

Moving down to the SW corner we have the disc controller interface 

based on an 8271 floppy-disc controller. This is responsible for 

sending out the command signals for a floppy disk drive, and for 

reading and writing the data from and to the disk drive. [See section 

3.10] 

The next device is VIA-B, referred to as the external VIA, which is 

used to provide interfaces for a printer and user applications. It also 

has two timers which are available to the user for his own applications 

programs. [See section 3.11] 

The last two sections of the circuit board are the 1 MHz extension bus 

and the TUBE. These provide two different ways of accessing various 

external devices. The 1 MHz bus is available for more general use but 

works at the slower speed of 1 MHz, whilst the TUBE works at the full 2 

MHz but is only intended for use with second processors. [See section 

3.13] 

8

3. Detailed Circuit Description 

3.1 Processor + clock circuitry + reset circuitry 

The microprocessor is a 6502A and runs at either 1 or 2 MHz. Most 

processing is done at 2 MHz, including accesses to the RAM and ROM, but 

the processor slows down to 1 MHz when addressing slow devices, viz. 

the 1 MHz extension bus, the ADC, the two VIA's, the 6845 CRT 

controller, the ACIA, and the serial processor. Clock signals for the 

microprocessor are produced by a 16 MHz crystal oscillator (IC43) in 

conjunction with divider circuitry in part of the video processor (IC6) 

which produces 8, 4, 2 and 1 MHz signals. The 1 MHz signal coming 

directly from the video processor is only used for the Teletext 

generator chip, whilst a D-type flip-flop (half of IC34) divides the 2 

MHz clock signal in order to produce the system 1 MHz clock (1 MHzE). A 

2 MHz signal of suitable phase is produced at the output of another Dtype 

(half of IC31) which remembers when a 1 MHz cycle has been 

requested. At the appropriate time, as governed by the 2 MHz clock, one 

of the 2 MHz clock cycles is masked off by the D-type (half of IC34) 

and when this happens the D-type that remembered that a request had 

been made, is cleared. Depending on the phase relationship between the 

1 and 2 MHz clocks at the time of the request, the delay on the 2 MHzE 

clock is different as illustrated by the diagrams below. The following 

simple program will produce these conditions alternately, so that they 

may be viewed with an oscilloscope. 

10 P%=&3000 

20 [ SEI 

30 .start 

40 STA &FC00 

50 STA &FC00 

60 JMP start 

70 ] 

80 CALL &3000 

9

A 555 timer circuit (IC16) provides a reset signal both at power up and 

also when the BREAK key is pressed. There is also a separate reset 

circuit using a CR combination from the +5 volt power supply (C10 and 

R20 and D1), to provide a signal called Reset A which is fed to IC3, 

the internal VIA. The idea is that although the 555 timer produces a 

general reset at power up or when the BREAK key is pressed, Reset A 

goes low only on power up. By interrogating the interrupt register on 

IC3 on the occurrence of a general reset, the microprocessor can 

discover Whether it was a "cold start", ie power up, or a "warm start", 

ie the BREAK key has been pressed when the system has already been in 

use for some time. 

3.2 Memory and address decoding 

31 1/4 Kbytes of ROM are catered for in the address map. 15 1/4 Kbytes 

of this ae contained in the operating system (IC51). This is in fact a 

16K device but 3/4K of it is left unused and it is in this area that 

the I-0 device memory map is situated. Four other ROMs (ICs 52, 88, 

100 and 101) are on the main circuit board. They may all be 16 Kbyte 

devices, in which case any one of them may be switched into the 16 

Kbyte space in the memory map by writing to the ROM select latch (IC76) 

. Alternatively, four 4 Kbyte ROMs may be in these four sockets in 

order to fill the 16 Kbyte space assigned. In this case, a two line to 

four line decoder (half of IC20) is used to select which of the four 

devices is being addressed by the address lines Al2 and A13. Mixtures 

of these two cases are allowed for, for instance two pairs of 8 Kbyte 

ROMs, one pair or the other being selected by the ROM select latch and 

then the ROM to be used in each pair being selected by the 2-4 line 

address decoder. Address decoding for the ROMs is by IC21 which decodes 

memory addresses &8000 to &C000 and &C000 to &FFFF. Locations from 0 - 

&7FFF are assigned to the dynamic RAM, and this is decoded by feeding 

A15 into pin 4 of IC21. All the rest of the hardware is mapped within 

locations &FC00 to &FEFF. This is decoded by IC22, whilst ICs 20 and 25 

are used to mask off the ROM over this range of addresses. ICs 24 and 

26 decode the individual devices within this range, some of which are 

read or write only. IC23 detects when a slow 1 MHz device is being 

addressed and it calls for the 6502 to execute a slow clock cycle. 

Note that in early versions of the BBC Microcomputer, the operating 

system was contained within 4 EPROMs in IC positions 52, 88, 100 and 

101 while the BASIC interpreter was located in IC51. This arrangement 

is abnormal and has been phased out. Refer to the link selection survey 

(5.1) for more detailed information on this. 

10

3.3 CRT controller + video processor + Teletext hardware 

Random Access Memory on the Microcomputer is provided by either 8 or 16 

dynamic memory devices (ICs 53-68). These devices store 16K bits each 

and therefore in the Model B, the data inputs and outputs of one pair 

of devices are paralleled for each of the 8 data bits, DO to D7. To 

address 16K bits requires 14 address lines, and this is achieved on the 

4816 by having 7 inputs and latching in the addresses in two halves by 

using a row address strobe (RAS) and a column address strobe (CAS). Two 

octal buffers (81LS95) have to be used to multiplex the appropriate 

processor address lines onto the RAM address lines. (ICs 12 and 13). 

However, the 6845 CRT controller (IC2) also needs to access the RAM, 

and what is more, it accesses it differently depending on whether it is 

working in the Teletext mode or in one of the other graphics modes. 

Therefore two more pairs of octal buffers are used, ICs 10 and 11 for 

the Teletext mode, and ICs 8 and 9 for the other modes, the main 

difference being that in these modes the three least significant 

address bits are produced by the character row address lines from the 

CRTC in order to give the bit-mapping of the characters in the RAM 

memory rather than having the ROM character generator as in the 

Teletext mode. 

12

The 6502 microprocessor runs from a constant clock and so its 

requirements for memory access are predictable. Every 250 nanoseconds, 

control of the RAM address lines is switched between the microprocessor 

and the CRTC. Thus, in each one microsecond period, the microprocessor 

has two RAM accesses and the CRTC has two RAM accesses. Because the 

CRTC generates a sequence of addresses in order to refresh the VDU 

display, all the row address lines of the RAMs are constantly cycled. 

The addressing methods have been designed so that in each screen mode 

the dynamic RAMs are automatically refreshed by virtue of the 

sequential CRTC accesses. 

13

The row address strobe signal is produced by a D-type flip-flop 

connected to the 8 and 4 MHz clock signals (half of IC44). This RAS 

signal then drives all of the dynamic RAMs via R106. The two banks of 

RAM are enabled by virtue of having their column address strobes 

individually available. in model A computers, with only one bank of 

RAM, CAS 1 is used. In the model B, CAS 0 controls the lower 16K and 

CAS 1 the upper 16K. The second bank of RAMs is selected by a 74LS51 

circuit (half of IC28) which controls the ,74S139 (half of IC45) 

producing the CAS signals. When A14 is high the B input is low thus 

selecting CAS 1. The other half of IC45 is used to select between the 

processor and CRT address lines. 

Using this technique, two bytes of information are available per 

microsecond for refreshing the raster scanned video display. With each 

horizontal line having a period of 64 microseconds, a 40 microseconds 

active display area is usual. Thus, 640 bits (2 bytes x 8 bits x 40 

microseconds) of information per horizontal line are produced from the 

memory-mapped display. At the end of each 250 nanosecond CRTC access 

period, the video processor (IC6) latches the byte from the RAM and, 

according to the display mode in operation, serialises the byte into a 

single bit stream of 8 bits, or two bit streams of 4 bits or four bit 

streams of 2 bits. In this way, display modes varying from 640 pixels 

in 2 logical colours to 160 pixels in 16 logical colours can be 

produced. 

14

The video processor contains a piece of high speed (16MHz) static RAM 

called a palette. This memory can be programmed to define the 

relationship between the logical colour number produced by the RAM and 

the physical colour which will appear on the display. Note that the 

information in the main RAM is unchanged by changing the palette; it is 

its interpretation into physical colours which changes. Modes 0 to 6 in 

the Microcomputer use software generated characters, that is to say, 

the character font to be produced on the screen is held in the memorymapped 

display area of the RAM so that graphics and/or characters may 

be held. The definition of these characters is stored in the operating 

system ROM from 0000 to C2FF. 

15

The speed of printing on the screen is much increased by the use of 

hardware scrolling. There is a register in the CRTC which is used to 

define the start of screen address in the screen memory. Thus in order 

to scroll the screen, it is only necessary to increment this register 

by the number of characters per line and then write to the memory 

address where the last screen data was. The number of address lines 

from the CRTC, used to address the screen memory, has to be sufficient 

to cater for the biggest screen (20 Kbytes). Thus 14 address lines have 

to be used which means that when using the hardware scrolling 

technique, the picture scrolls around in 32 Kbytes. Consider a scroll 

of 8 Kbytes in a 20 Kbyte screen. The original start of screen for the 

20 Kbyte mode was &3000. After an 8 Kbyte scroll, the current start of 

screen address is &5000 with the end of the screen as seen by the CRTC 

at &5000 plus 20 Kbytes, which comes to &9FFF, as illustated below. 

16

Since there is only 32 Kbytes of RAM this would mean that instead of 

accessing addresses &8000 to &9FFF you would be accessing locations 

&0000 to &1FFF. Therefore when the address produced by the CRTC is 

greater than &7FFF (ie MAl2 = 1) you have to add to the address from 

the CRTC, a number which will bring the actual address back up to the 

area of RAM which is currently being used for the screen ie above 

HIMEM. Thus for numbers greater than &7FFF you simply add the number 

&3000 which brings the addresses back to the range &3000 to &4FFF, as 

illustrated in the diagram above. In the 20K modes you add &3000 (=12K) 

, in the 16K mode you add &4000 (=16K), in the 10K mode you add &5800 ( 

=22K) and in the 8K mode you add &6000 (=24K). This number to be added 

is defined by the control lines CO and Cl from the 74LS259 (IC32), and 

computed by some AND gates with the result being added to the higher 

CRTC refresh address lines by a 74LS283 adder (IC39). The CRTC address 

line MAl2 is used as a "carry" to determine whether zero or the number 

computed by the AND gates is added to the address lines. (Confusion may 

arise when looking at 1C 9 on the circuit diagram since it looks as if 

AA0 to AA2 are being buffered to AO to A2. But if you look at the pin 

numbers and compare them with the other 81LS95's you will see that they 

are in fact buffered to the top three bits, A4 to A6. MA4 to MA7 are 

buffered to AO to A3.) 

Display mode 7 is a Teletext mode and to implement this an SAA 5050 ( 

IC5) Teletext character generator Read Only Memory is used. IC15 

latches the information coming from the RAM prior to the SAA 5050. When 

using this mode, only 1K of RAM is devoted to the display memory and 

the characters are held within it as ASCII bytes. The SAA 5050 then 

translates these bytes into a standard Teletext/Prestel format display. 

A 6 MHz clock signal is required for the Teletext character generator ( 

IC5). This signal is produced by knocking a reset flip-flop (two 

quarters of IC40) backwards and forwards from the 8 MHz and 4 MHz clock 

signals. The output of this flip-flop is then itself inverted according 

to the state of the 2 MHz clock signal by an exclusive OR gate (1/4 of 

IC38). Glitches on this output are removed by R119 and C48 to produce 

the 6 MHz clock signal at Pin 8 of IC37. 

17

The CRTC is still used to generate the RAM addresses even in the 

Teletext mode, but using only 1K means that only 10 address lines are 

needed hence the top four address lines on the 81LS95 (IC11) are tied 

to logic 1. The Teletext mode is selected by setting the value of video 

address start (registers 12 and 13 in the CRTC) so high that an extra " 

carry" is generated on MA13. This is used to enable ICs 10 and 11, 

disable ICs 8 and 9 and also enable the data latch (IC15). 

18

3.4 RGB + PAL encoder + UHF output _ 

The red, green and blue logic signals -produced by the -_video 

processor are buffered by transistors Q4, Q5 and Q6 and fed out 

together with a composite sync signal to the RGB connector (SK 3). The 

red, green and blue lines are summed together by binary weighted 

resistors to feed Q7 which produces a 1V composite video signal 

suitable for feeding to monochrome monitors, on which the different- 

colours will appear as different shades of grey. Also available, from 

the main printed circuit board, is a UHF TV signal on channel 36, 

suitable for feeding to the aerial input of a domestic television. This 

output is modulated using a UM1233 for PAL. Colour is provided for 

domestic televisions by a PAL (phase alternating line) encoder circuit 

which modulates the colour information on to the colour subcarrier - 

frequency. Q10 is a 17.73 MHz oscillator circuit which is divided by a 

ring counter (IC46) giving 2 outputs at the colour subcarrier frequency 

of 4.433618 MHz. One of these two outputs is switched by the horizontal 

line frequency in order to produce the alternate phase on each TV line. 

Thus on IC46 pin 9, we have the 'U' signal and on IC48 pin 11, the '+/- 

V' signal. A row of exclusive OR gates is used to select different 

phases of the 'U' and 'V' signals according to whether a red, green, 

blue, cyan, magenta or yellow colour is to be produced. These signals 

then drive resistors via a row of NAND gates in order to produce the 

colour subcarrier signal which is added to the luminance output from Q8 

by the buffer Q9. In order for the receiving television to interpret 

the colour information, a reference colour burst has to be provided at 

the beginning of each line. A burst gate pulse of approximately 5uS 

immediately after the horizontal sync pulse for each line is produced 

at pin 4 of IC41, and it is timed by C45 and R109. This burst gate 

allows through a standard colour subcarrier signal which the television 

uses as its reference for the rest of- that line. The PAL signal may be 

added to the 1V video connector, with the addition of a 470 pF 

capacitor between the emitter of Q9 and the base of Q7. This is 

provided as a link selectable option on later issues of the PCB (issue 

4 on). In modulated PAL, diodes D20, 21 and 22 increase the luminance 

of the darker colours, eg blue, in order to make coloured text displays 

more readable. 

3.5 Cassette + RS423 + serial processor 

For both the cassette and RS423 interfaces, a 6850 asynchronous 

communications interface adaptor (ACIA) (IC4) is used to buffer and 

serialise or deserialise the data. The serial processor (IC7), 

specifically designed for the BBC Microcomputer, contains two 

programmable baud rate generators, a cassette data/clock separator, 

switching to select either RS423 or cassette operations and also a 

circuit to synthesise a sinewave to be fed out to the cassette 

recorder. IC42 divides the 16 MHz clock signal by 13 (1.23 MHz) and 

this signal is divided further (by 1024) within the serial processor to 

produce the 1200 Hz cassette signal. Automatic motor control of an 

audio cassette recorder is achieved by using a small relay driven by a 

transistor (Q3) from the serial processor. The signal coming from the 

cassette recorder is buffered, filtered and shaped by a three stage 

amplifier (IC35). The RS423 data in and data out signals and the 

request to send output (RTS) and clear to send input (CTS) signals are 

interfaced by ICs 74 and 75 which translate between TTL and standard 

RS423/232 signal levels (+5V and -5V). The control register, which is 

memory-mapped at &FE10, specifies the frequencies for the transmit 

clock (bits 0-2) and the receive clock (bits 3-5) used by the 6850 ( 

IC4). The switching between the cassette and RS423 inputs and outputs 

19

is also determined by the control register (bit 6), and so is the motor 

control (bit7). R75 and C28 provide the necessary timing elements for 

delay between receiving the high tone run-in signal and asserting the 

data carrier detect signal to the ACIA. The value of resistor needed is 

affected by the output impedance of that pin on the serial. processor 

which has been subject to a certain amount of variation. Thus the value 

of R75 has changed through the evolution of the circuit. 

20

3.6 Internal VIA 

One 6522 VIA device (IC9) is devoted to internal system operation. Port 

B drives an addressable latch (IC32) which is used to provide read and 

write strobe signals for the speech interface, the keyboard and the 

sound generator chip. Also coming from this latch are control lines CO 

and Cl which provide the memory address addition for the CRT controller 

depending on the amount of RAM devoted to the display memory. Pins 6 

and 7 of the addressable latch drive the capitals lock and shift lock 

LEDs on the keyboard. The rest of Port B on the internal system VIA is 

used to input the two "fire button" signals from the analogue to 

digital convertor interface and two response lines from the speech 

interface. Each time the system VIA is written to, any changes on Port 

B which should affect the addressable latch are strobed into it by a 

flip-flop (IC31) which is triggered from the 1 MHz clock signal. Port A 

of this VIA is a slow data bus which connects to the keyboard, the 

speech system chip and the sound generator. 

3.7 Keyboard'` 

The keyboard circuit (Section 9.5) connects via PL 13. A 1 MHz clock 

signal is fed to a 74LS163 binary counter, the outputs of which are 

decoded by a 7445 decoder driver circuit. These outputs drive the rows 

of the keyboard matrix, each row being driven in turn. If any key is 

depressed, the 74LS30 gate will produce an output when that row is 

strobed and this will interrupt the computer through line CA 2 of IC3. 

On this interrupt, the computer will enter the key reading software. In 

order to discover which key was pressed, the microprocessor loads 

directly into the 74LS163 the address of each key matrix row allowing 

it to interrogate each row in turn. Also, the microprocessor loads into 

a 74LS251 data selector, the address of each specific key on that row. 

ie column addresses. In this way, the microprocessor can interrogate 

each individual key in turn until it discovers which one was depressed 

and causing the interrupt. Once read, the keyboard assumes its free 

running mode. 

3.8 Sound + speech + serial ROM interfaces 

The speech system device used is a TMS 5220 (IC99) which, on 

instructions from the Microprocessor, will either produce at its audio 

output speech from its associated memory (IC98) or from speech data fed 

to it directly from the Microcomputer's memory. On later issue boards a 

variable resistor is provided (VR 2) to adjust the clock frequency to 

give the best effect of the speech. IC18 is a four channel sound 

generator chip which may be programmed to give varying frequency and 

varying attenuation on each channel. The audio output of the speech 

system device is filtered by an operational amplifier circuit with a 

cut-off frequency of 7 kHz. This signal is then added to an amplified 

and level shifted signal from the sound generator by a virtual earth 

amplifier to which is also added an extra analogue input from the 1 MHz 

extension bus. This summated audio signal is then finally filtered by 

an 8 kHz low pass filter. All of these operations are done by a quad 

operational amplifier (IC17). IC19 provides audio power amplification 

to drive a speaker from PL15. A low level audio output is provided from 

PL16 for feeding the auxiliary input of an external power amplifier. 

21

3.9 A to D convertors 

A four channel analogue to digital convertor facility is provided by 

IC73. This device connects straight to the Microcomputer's data bus and 

is a dual slope -convertor with its voltage reference being provided by 

the three diodes, D6, D7 and D8. Each time a conversion is completed, 

the microprocessor is interrupted through CB1 of the internal VIA which 

responds by reading the value and storing it in a memory location. 

3.10 Disc interface 

IC78 is a floppy disc controller circuit which is used to interface to 

one or two, single or double sided 5 1/4 inch floppy disc drives. Logic 

signals from the controller to the disc drive are buffered by two open 

collector driver packages IC79 and 80. The incoming signal from the 

disc drive is first conditioned by monostable IC87 producing a pulse 

train with each pulse of fixed width. These pulses are then fed to the 

data separation circuits ICs 81 and 82. These form a digital 

monostable. 1C86 divides the 8 MHz clock signal down to 31.25 kHz. ICs 

83, 84 and 85 are then used to detect index pulses coming in from the 

drive which show that the drive is ready for a read or write operation. 

3.11 Printer + user port interfaces 

1069 is a versatile interface adaptor. Port A is used to provide a 

centronics standard parallel printer interface, with an octal buffer, 

IC70, to improve on the current driving capabilities of the data lines. 

Control line CA2 is used as the strobe line having been buffered by 

part of IC27 and Q11. It is asserted low for approximately 5uS to 

signal that the data is ready. This circuit has been changed on the 

various issues of the PCB as explained in section 4.4. 

Port B is left uncommitted and is free for user applications as either 

input or output. For full details of what can be done with the user 

port you should refer to the 6522 data sheet, but basically, apart from 

being used as a straightforward input/output port, PB7 can be used as a 

programmable pulse output using one of the timers, PB6 can be used as 

an input to the other timer for pulse counting, and CB1 and CB2 can be 

used for automatic hand-shaking and in conjunction with the VIA's own 

shift register. 

22

3.12 Econet 

ICs 89 to 96 are concerned with the Econet interface. IC89 is an 

Advanced Data Link Controller Circuit, type 6854 which handles the 

Econet protocol. Data to be transmitted on to the network is fed from 

the ADLC to the line drive circuit (IC93) via an inverting Schmitt 

trigger circuit (part of IC91). Transmit data then goes through the 

line driver circuit which produces a differential signal drive to the 

Econet cables. Received data is detected and converted to a logic 

signal by one half of IC94 which is a dual comparator circuit, type 

LM319. The received data is then fed back to the data link controller 

circuit. 

An Econet installation has a single master clock station which provides 

the clock for the whole of the network. This clock signal is 

transmitted around the network as a second differential line signal and 

it is used to clock the data in and out of the data link controller 

circuits. The network clock is detected using the other half of IC94, 

and the detected clock signal is then fed to both receive clock and 

transmit clock inputs on the 6854. In the presence of a network clock, 

the monostable circuit (IC87) is permanently triggered and thus 

providing a data carrier detect signal for the data link controller 

chip. Once the network clock is removed, the monostable immediately 

drops out and the data carrier is no longer detected. 

The Econet is a broadcast system on which a number of stations may 

attempt to transmit their data over the network at any given time. In 

this case, a situation called a collision can occur and then the 

transmitting stations should detect the collision and back off before 

trying again to transmit over the network. Collision arbitration 

software is included in the Econet system and is based on the station 

ID number. Collisions on the network data lines result in the 

differential signal on the two data wires being reduced and this 

condition is detected by IC95 which is another dual comparator circuit. 

When there is a good differential data signal on the network one output 

of IC95 or the other will be low, in which case the output of IC91 pin 

6 will be high, indicating no collision. When there are no collisions 

on the network, and the network clock is detected by the clock 

monostable, the data link controller is clear to send data over the 

network. When there is a collision on the network both outputs of IC95 

will go high and the clear to send condition will cease. Note that when 

the computer is not connected to the network a collision-like situation 

results, in which case again the data link controller will not get a 

clear to send condition. 

Up to 254 stations may be connected to each Econet with each station 

being indentified by a unique station identification number. This 

station ID is programmed on the links S11 and the ID can then be read 

by the octal buffer IC96. The data link controller circuit produces 

interrupts which are fed to the processor's NMI line. These interrupts 

can be enabled and disabled under software control by using the 

address-decoded signals, INTOFF which is achieved by reading the 

station ID at &FE18, and INTON which is generated by reading &FE20. ( 

Writing &FE20 loads the Video processor register.) 

23

3.13 1 MHz bus 

The address and data lines, AO - A7 and DO - D7, together with two page 

select lines are available as the 1 MHz extension bus to which various 

peripheral devices may be connected, eg Teletext interface. All 

accesses to this bus will be at a 1 MHz processor speed, although links 

are, provided to increase this to 2 MHz if desired (see the selection 

link survey). The octal buffer (IC71) and the octal transceiver (IC72) 

are used to interface these signals to the internal data and address 

buses, IC72 being enabled only when either "FRED" or "JIM" is accessed 

(pages &FC00 and &FD00). 

3.14 Power supply 

The power supply unit produces 5 volts at 3.75 amps and -5 volts at 100 

milliamps for use on the main circuit board. Some auxiliary power for 

accessories is also available on an external connector and this 

includes +12 volts at 1.25 amps, but the amount of power avalable 

depends on what hardware is connected internally - Econet, disc 

interface, sideways ROMs etc. 

The power supply connects to the main circuit board by seven push-on 

connectors with the +5 volts being fed to three different points across 

the main circuit board. These points are all connected together 

electrically. However, by distributing the power in this way the need 

for very large copper tracks to distribute power around the board is 

avoided. Most computers in production will have a switched-mode power 

supply, the circuit diagram for which is given. However it is not 

recommended that attempts should be made to repair this power supply, 

instead it should be treated as a module to be exchanged. This is 

because of the stringent safety regulations relating to such units. A 

small number of early computers have a linear power supply unit with a 

conventional mains transformer and regulator circuit. These also should 

be treated as modules to be exchanged rather than serviced, though it 

should be noted that the three outputs are from separate regulators, 

thus it is possible for power to appear say on two out of the three 

pairs of connectors. 

24

4 Upgrading the PCB 

In these instructions about how to add extra hardware to the PCB for 

disc, Econet, speech etc, some differences may occur depending on which 

hardware is already fitted. This is made clear within each set of 

instructions. In order to locate the positions of various of the 

selection links, reference should be made to section 5.2 which gives 

the X-Y coordinates of each link. Dealers and service centres 

performing these upgrades must also conform to upgrade procedures and 

requirements as notified by their supplier, and should refer to any 

available information updates for latest details. 

4.1 Modification A 

Convert from EPROM MOS to ROM MOS 

i) Remove the four MOS EPROMs from their sockets IC52, IC88, IC100 and 

IC101. 

ii) Remove the BASIC ROM from the IC51 socket and replace it in the 

IC52 socket. 

iii) Insert the MOS ROM into the IC51 socket. 

iv) Set the following link positions using MOLEX jumpers (if fitted or, 

tinned copper wire):- 

S18 - North 

S19 - East 

S20 - North 

S21 - 2 x East/West 

S22 - North 

S32 - West 

S33 - West 

v) Test using a FIT and, if available, a PET (see section 6.2). 

4.2 Modification B 

Convert Model A to Model B 

i) The following parts are required:- 

8 off 4816AP-3 IC61 to 68 

1 off 6522 IC69 

2 off 74LS244 IC70, 71 

1 off 74LS245 IC72 

1 off uPD7002 IC73 

1 off 88LS120 IC74 

1 off DS3691 IC75 

1 off 74LS163 IC76 

1 off 74LS00 IC77 

1 off 6-pin DIN socket MAB6H SK3 

1 off 5-pin DIN socket MAB5WH SK4 

1 off 15-way D-type socket 164801-1 SK6 

2 off 34-way header 3431-1302 PL8, PL11 

1 off 26-way header 3429-1302 PL9 



25

ii) Insert the above ICs into the sockets provided on the main 

circuit board. Solder the connectors on to the printed circuit board. 

iii) Cut the wire links at link positions S12 and S13. Move the MOLEX 

link at position S25 from South to North. 

iv) On issue 1, 2 or 3 circuit boards only, add a 2k2 ohm resistor 

between PL9 pin 1 and +5v on the solder side of the circuit board using 

a resistor with sleeved leads. +5v is available at IC85, pin 16 (33 mm 

due North of pin 1). 

v) On issue 1, 2 or 3 circuit boards only, cut the track connected to 

PL9 pin 23 (this may have previously been cut), then link IC69 pin 40 

to PL9 pin 19. This modification may have been made, and, if so, a 

check should be made to ensure that it has been correctly performed. 

vi) On issue 1 and 2 circuit boards only, PL9 pin 26 should be cut out 

of the header. Care should be taken to ensure that the pin is cut right 

back so that no connection can be made to it. 

vii) On issue 1 and 2 circuit boards only, a BC239 transistor should be 

added in place of link S1 as follows:- Cut the track between the centre 

and South pins of S1 on the solder side of the circuit board. Cut the 

two tracks connected to the North pin of S1 on the solder side of the 

circuit board, then reconnect the ends of these tracks leaving the 

North pin isolated. Insert a BC239 transistor into the S1 position with 

the base in the South pin, the emitter in the North pin, and the 

collector in the centre pin. Finally, link the North pin of S1 to IC27 

pin 7 with a short length of insulated wire. 

viii) On issue 1 or 2 circuit boards only, add a 4k7 ohm resistor ( 

R162) between the existing two holes located approximately 5 mm East of 

IC70 pins 11 and 13, as shown below. 

26

ix) On issue 1 circuit boards only, disconnect the LPSTB signal between 

IC69 pin 18 and PL10 pin 2 by cutting the track on the solder side of 

the circuit board which is connected to IC69 pin 18. 

x) Test using a FIT and, if available, a PET. 

4.3 Modification C 

Add Speech Option 

i) The following components are required:- 

1 off Integrated Circuit TMS6100 IC98 

1 off Integrated Circuit TMS5220 IC99 

ii) On issues 2 and 3, the following modifications are needed. 

On the component side of the main PCB: 

- Cut the track between IC3 pin 16 and the through-hole 8mm to the 

west. 

- Cut the track between IC3 pin 17 and the through-hole 10mm to the 

west. 

On the solder side of the PCB: 

- Link the through-hole 10mm to the west of IC3 pin 17 to IC3 pin 16 

- Link the through-hole 8mm to the west of IC3 pin 16 to IC3 pin 17. 

(These operations switch the signal lines to IC3 pins 16 and 17.) 

Then, also on the solder side of the PCB: 

- Cut the track between IC98 pins 13 and 14. 

- Link IC98 pin 13 to PL14 pin 3 (0 volts). 

iii) Issue 1 keyboard PCBs also need modifying as follows:- 

On the solder side of the PCB, cut the track between pins 14 and 15 of 

the edgecard connector. The pins are those furthest from the speaker. ( 

ie further east). 

On the solder side of the PCB, link pin 14 of the edgecard connector to 

0 volts. This can be found on either of the capacitor legs nearer the 

centre of the PCB. 

iv) When the modifications are complete, procede as follows:- 

Reconnect the keyboard to the main PCB. Add the new connector for PL14, 

then, with the computer turned off, test for continuity between the 

following points: 

27

Edge connector pin number |6|7|8|9|10|11|12|13|14|15| 

1C98 pin number |1|3|4|5| 6| 7|10|11|13|14| 

Note: On the edge connector, pin 1 is nearest the speaker, thus the 

polarising key is pin 3 and pins 4 and 5 are "empty". 

Also check that there are no short-circuits between any of the edge 

connector pins. Repeat the tests for the other edge connector. 

v) Insert ICs 98 and 99, turn the machine on and type: 

REPEAT SOUND-1,GET,0,0:UNTIL0 

Now press any alphanumeric key and you should hear the voice synthesis 

operating. If the pitch is wrong, follow the instructions in vi) below. 

If there is no speech, double check the modifications and try again. 

vi) The pitch of the speech must be set. From Issue 4 PCBs onwards this 

is a simple matter of adjusting VR2 which is situated just west of 

IC98. On Issue 1, 2 and 3 PCBs, the resistor R32 (between ICs 98 and 

99) may need to be changed to achieve the best result. The method for 

setting the pitch is to connect a frequency meter to pin 3 of IC99 and 

to adjust VR2 until the meter reads 160 kHz (+ or - 100 Hz), or as 

close as is obtainable by changing R32. 

vii) Reassemble the machine. Before fitting the ROM socket cover into 

the 

the case lid, remove the rforated section of the black label above the 

ROM sockets. It may be necessary to trim the label to match the case 

cutout. For early version cases (without a rib on the underside behind 

the keyboard cutout), remove the two small lugs on the ROM socket cover 

before fitting. 

viii) Test using a FIT and, if available, a PET. 

4.4 Modification D 

Add 5 1/4 inch Disc Interface to Basic Model B 


1 off 8271 IC78 

2 off 7438 ICs 79,80 

1 off 74LS10 IC82 

2 off 74LS393 ICs 81,86 

2 off CD4013B ICs 83,84 

1 off CD4020B IC85 

1 off 74LS123 IC87 (Not required if Econet already fitted) 

1 off 2764 EPROM (DFS) IC88 (or IC100 if Econet fitted; but not 

required if DNFS already fitted) 

ii) Insert the ICs listed above into the sockets provided on the main 

circuit board. 

iii) On issue 1 or 2 circuit boards only, connect the two pads of link 

position S8 with a wire link. 

28

iv) If the MOS ROM version 0.1 is fitted in position IC51 then it 

must be replaced by a 1.2 MOS, see modification A. 

v) If the existing power supply does not incorporate an auxiliary power 

output socket it must be exchanged for a suitable unit (eg ASTEC type). 

vi) On issue 1, 2 or 3 circuit boards only, cut the leg of IC27 pin 9 

as close to the PCB as possible and the track connected to it on the 

component side of the circuit board between IC27 and IC89, then 

reconnect the cut IC leg to the East pad of link S9 with a short Length 

of insulated wire. 

vii) On issue 4 boards onwards, cut the TCW link at position S9. 

viii) Set the following link positions using MOLEX jumpers:- 

S18-North 

S19-East 

S20-North 

S21-2 x East/West 

S22-North 

S32-West 

S33-West 

ix) Test using a FIT and, if available, a PET. 

4.5 Modification E 

Add Econet Interface to Model A 


5 off 14-pin DIL IC sockets 



1 off 74LS163 IC76 (already fitted on model B) 

1 off 74LS123 IC87 (Not required if disc already fitted) 

1 off 68B54 IC89 

1 off 74LS132 IC91 

1 off 75159 IC93 

2 off LM319 ICs 94,95 

1 off 74LS244 IC96 

1 off 74LS74 IC97 

1 off 10uF Tantalum Capacitor C18 

1 off 10uF Ceramic Capacitor C23 

1 off 5-pin 180 degree DIN socket SK7 

1 off 8X22K SIL resistor pack RP2 

1 off 2764 EPROM with NFS IC88 (not required if DNFS already fitted) 

2 off Rows of 8 MOLEX pins S11 

20 off 2% tolerance 1/4W resistors as follows:- 

R34-10k R40-100k R48-1k R62-56k 

R35-10k R41-100k R51-10k R63-56k 

R64-1M5 R44-1M5 R52-1k0 

R36-1M5 R45-10k R59-56k 

R38-100k R46-1k0 R60-56k 

29

R39-100k R47-1k5 R61-1k0 

ii) Solder all of the above passive components onto the main PCB. 

iii) Insert all of the above integrated circuits into their sockets. 

iv) Cut the wire links at link positions S2, S12 and S13. (S12 and S13 

should already have been cut on Model B's) 

v) Set the following link positions using MOLEX jumpers:- 

S18-North 

S19-East 

S20-North 

S21-2 x East/West 

S22-North 

S32-West 

S33-West 

vi) On issue 1, 2 or 3 boards only, the following modifications are 

requi red: 

Remove the capacitor C17 and replace it with a 2. 2nF capacitor. Cut 

the PCB track from IC26 pin 6 to IC96 pins 1 and 19 leaving the track 

from IC26 pin 6 to IC97 pin 2 intact. Cut the track from IC89 pin 26 to 

IC97 pin 4 and link IC26 pin 9 to IC96 pins 1 and 19 and also to IC97 

pin 4. 

vii) Test using a FIT and, if available, a PET. 

4.6 Modification F 

Add 8 inch disc interface to Model B 

(As Modification D - Add 5 1/4 inch disc interface, but add....) 

x) Set the following link positions by cutting the indicated PCB track 

and inserting a wire link. 

LINK CUT TRACK WIRE LINK. 

S4 East (Solder side) West 

S10 West (Component side) East 

S27 West (Solder side) East 

4.7 Partial upgrading 

If you want to upgrade a Model. A to enable it to run software intended 

for use with a model B, but do not want all the various interfacing 

facilities, then it is only really necessary to add the RAM and the 

6522 VIA and change link 525. The VIA is needed as some professional 

software uses its hardware timers. 

If you want to use sideways ROMs then you will need to add the 74LS163 

(IC76) and be sure that links S12 and S13 are cut. 

30

5 Selection links and circuit changes 

5.1 Selection Link Survey 

Here is a survey of the options which may be selected on the 

Microcomputer by selection links S1 to S39. These links may take the 

form of tracks on the circuit board which can be cut, soldered wire : 

inks, or shorting jumpers, plugging on to the rows of pins. This is 

followed by a tabular survey of the options selected in production on a 

standard model B Microcomputer. 

Option Select Links are as follows:- 

1. Used only on issue 4 and succeeding boards to select printer strobe 

or direct output from CA2. 

2. OPEN enables ECONET NMI 

CLOSED disables ECONET NMI 

- Do not fit this link with IC91 in place. 

3. Clock base frequency selection for ECONET 

Not used after issue 3. 

4. EAST selects 5 1/4" disc 

WEST selects 8" disc. 

- This changes the pin connection of the "side select" line on the 

disc interface. 

5. NORTH enables ECONET clock 

SOUTH disables ECONET clock. 

- Not used after issue 3. 

6. NORTH divides ECONET clock by 2 

SOUTH divides ECONET clock, by 4. 

- Not used after issue 3. 

7. WEST applies 4-5v to pin 30 of disc controller (IC78). 

EAST applies 0v to pin 30 of disc controller. 

- Readable by software, bit 0 of the result register of the 8271. Not 

used. 

8. CLOSED links disc head load signal to PL8. 

OPEN isolates disc head load signal from PL8. 

9. CLOSED disables DISC NMI. 

OPEN enables DISC NMI. 

Do not fit 1C78 with this link closed. Due to PCB faults, various. 

different modifications are necessary with different issue boards in 

order to use the disc interface. (See section 4, modification D.) 

31

10. WEST selects 5 1/4" disc. 

EAST selects 8" disc. 

- Changes the pin connection of the "index" line on the disc 

interface. 

11. Selects Econet station ID. (NORTH is LSB) 

- See Econet upgrade instructions - section 4, modification E. 

12. CLOSED ties ROM select line A to 0V. 

OPEN ROM select line A driven by IC76. 

- On model A's, IC76 is not fitted because sideways ROM'S are not 

used. ROM 0 (IC52) is permanently selected. Do not fit IC76 with this 

link closed. 

13. CLOSED ties ROM select line B to 0V at IC20. 

OPEN ROM select line B driven by IC76. 

- Do not fit IC76 with this link closed. See comments on link 12. 

14. CLOSED disables ROM output from page FD, enables JIM. 

OPEN enables ROM output from page FD, disables JIM. 

- If link 14 is open then link 15 must be closed and R72 must be 

fitted. The purpose of this link was to provide access to an extra page 

of the OS ROM for development purposes. It is unlikely to be used in 

production machines as it disables the 1MHz bus. 

15. CLOSED disables fast access to page FD via IC23. 

OPEN enables fast access to page FD via IC23. 

- Link 15 must be closed if link 14 is open and R72 must be fitted. 

See comments on link 14. 

16. CLOSED disables fast access to page FC via IC23. 

OPEN enables fast access to page FC via IC23. 

- Link 16 must be closed if link 17 is open and R73 must be fitted. 

See comments on link 14. 

17. CLOSED disables ROM output from page FC, enables FRED. 

OPEN enables ROM output from page FC, disables FRED. 

- If link 17 is open then link 16 must be closed and R73 must be 

fitted. See comments on link 14. 

18. SOUTH forces slow access to IC100 ROM. 

NORTH allows fast access to IC100 ROM. 

- To allow the use of 1MHz EPROMs. 

32

19. WEST forces slow access to ROMs IC52, IC88 and IC101. 

EAST allows fast access to ROMs IC52, IC88 and IC101. 

- Diodes D10, D11 and D12 may be selectively added to slow down ROMs 

IC101, IC88 and IC52 respectively when link 19 is in WEST position, but 

for any ICs to have slow access, R55 must be added. 

D10, 11 & 12 and R55 are not fitted from issue 7 onwards. 

20. SOUTH connects high ROM select bit to IC20 decoder from A 13. 

NORTH connects high ROM select bit to IC20 decoder from ROMSEL 1. 

21. 2 x NORTH/SOUTH selects blocks 8 to B in IC51 and blocks C to F in 

ICs 52, 88, 100, and 101. (4 EPROMs for OS) 

2 x EAST/WEST selects blocks C to F in IC51 and blocks 8 to B in ICs 

52, 88, 100 and 101. (OS in IC51) 

22. SOUTH connects low ROM select bit to IC20 decoder from A 12 . 

NORTH connects low ROM select bit to IC20 decoder from ROMSEL 0. 

23. OPEN RS 423 receiver not terminated (DATA). 

CLOSED RS 423 receiver terminated (DATA). 

24. OPEN RS 423 receiver not terminated (CTS). 

CLOSED RS 423 receiver terminated (CTS). 

25. SOUTH selects CAS 1 only, for 16K RAM configuration. 

NORTH selects CAS 0 and 1 for 32K RAM configuration. 

- If removed altogether, this selects CAS 0 only, but this should only 

be used for testing purposes on a Model B. 

26. WEST selects normal video output. 

EAST selects inverted video output. 

27. WEST selects 8 MHz clock for 5 1/4" disc. 

EAST selects 16 MHz clock for 8" disc. 

28. WEST selects base baud rate. (1200 baud) 

EAST selects 1300 baud cassette rate. 

- If link 28 EAST position RS 423 baud rate is also changed by the 

same factor:- 13/12. 

29. EAST selects base baud rate. (1200 baud) 

WEST selects 1300 baud cassette rate. 

- If link 28 is in the WEST position, RS 423 baud rates are also 

affected. 

30o increase the flexibility used for the addition of extra sideways 

ROM sockets. This would be in connecton with other links (S20,21,22) 

to enable a total of 16 sideways ROMS to be selected. 

31. WEST selects +ve CSYNC to RGB video output. 

EAST selects -ve CSYNC to RGB video output. 

33

32. WEST selects A 13 input to pin 26 of ROMs IC52 and IC88. 

EAST selects +5v input to pin 26 of ROMs IC52 and IC88. 

- This enables 24 pin ROMs to be used in the 28 pin socket. 

33. WEST selects A 13 input to pin 26 of ROMs IC100 and IC101. 

EAST selects +5v input to pin 26 of ROMs IC100 and IC101. 

- This enables 24 pin ROMs to be used in the 28 pin socket. 

34-38. These are used to provide contact with the ROM decoder (IC20) 

and the chip select lines of ROMs 52, 88, 100 and 101, in order to 

allow the use of extra ROM sockets on an external PCB. (Implemented 

from issue 4.) 

39. CLOSED adds colour burst signal to the black and white video 

signal to produce PAL encoded video on the BNC socket. 

OPEN Black and white video on BNC socket. (Implemented from issue 4.) 

34

5.2 Table of link options. 

The following table gives a list of selection links showing their 

positions on the circuit board (mm E,N from SW corner) and on the 

circuit diagram (grid reference, see main PCB circuit diagram). The 

links made in production on a standard model B without disc or Econet 

interfaces are also given. 

P = plugable link, T = track, W = wire link, C = closed, 0 = open. 

N S E and W refer to orientation of tracks or plugs. 

Some links have been omitted on later issue boards, whilst others have 

been added. 

LINK PCB Circuit Options 

position diagram (Model B) 

1. 2,1082,5 T N (Not used on issues 2 and 3) 

2. 2,16112,7 W C 

3. 2,17314,14- - (Not fitted after issue 4) 

4. 12, 12 1,9 T E 

5. 26,195 14,9 - - (Not fitted after issue 4) 

6. 26,205 15,9 - - (Not fitted after issue 4) 

7. 30, 65 4,9 T E 

8. 32, 15 2,9 T C 

9. 35,128 3,10 W C 

10. 45, 15 1,8 T W 

11. 75,210 13,10 P - (When Econet fitted) 

12. 97, 70 9,8 W 0 (Wire link in Model A) 

13. 100, 67 9,8 W 0 (Wire link in Model A) 

14. 101, 53 7,10 T C 

15. 107, 97 7,9 T C 

16. 108, 90 7,9 T C 

17. 108, 52 7,10 T C 

18. 110, 52 7,8 P N 

19. 102,102 7,8 P E 

20. 123, 55 9,8 P N 

21. 122, 65 9,8 P 2 x EW 

22. 127, 70 9,8 P N 

23. 177,215 13,3 W 0 

24. 181,195 13,3 W 0 

25. 215,185 7,5 P N 

26. 221, 68 10,1 P W 

27. 226, 95 1,7 T W 

28. 237,144 12,6 T W 

29. 237,146 12,6 T E 

30. 284, 20 8,8 - - (For external connections) 

31. 270,170 14,3 P W 

32. 295, 65 9,9 P W 

33. 295, 67 10,9 P W 

34. 200, 65 9,8 T C (From issue 4 onwards) 





39. 255,215 9,8 T 0 (From issue 4 onwards) 

35

5.3 Circuit Modifications from issue 1 to issue 7. 

In this next section are listed the more important changes which have 

taken place in the circuit design as it has evolved from issue 1 to 

issue 7. Since there are so few issue 1 boards in circulation at the 

moment, we will ignore the changes from 1 to 2 and suggest that if you 

come across an issue 1 board and cannot solve any fault which occurs on 

it, that you should consult the Technical Services Department of Acorn 

Computers Ltd. 

5.3.1 Changes from issue 2 to 3. 

1. The ACK line on PL9, the printer port, was moved from pin 23 to pin 

19. 

2. A 4k7 resistor (R162) was added to the ACK line to pu11 it up to +5v. 

3. Link S1 was removed in order to put in a transistor inverter, Q11. 

4. Pin 26 of PL9 was left unconnected to avoid the problem that this 

pin is used on some printers as the reset line. 

(Changes 1-4 were made retrospectively on most of the issue 1 and 2 

circuit boards). 

5. Various modifications were made in the region of S9 and IC27 at 

various stages, and so for the correct implementation, see section 4 on 

the modification for adding the disc interface. 

6. A 2k2 pull up resistor (R170) was added to the strobe line of the 

printer port (pin 1 of PL9). 

7. R109 became select on test (SOT) with a value between 1k8 and 2k7 

in order to set the correct colour burst length. 

8. C51 also became SOT at a value between 15pF and 22pF in order to set 

the colour burst frequency to the correct value of 4.4336 MHz + or - 

100 Hz. 

9. In order to improve the waveform of the 16MHz signal, C42 was at one 

stage a fixed capacitor with a trimming capacitor in parallel. 

Therefore various issues of boards will have various different values 

for C42. Also the gate used (IC40) was changed from a 74LS00 to a 

74S00. 

5.3.2 Changes from issue 3 to issue 4. 

1. The Econet circuitry was modified in various ways. The clock 

generator and terminator components were removed, certain component 

values were changed in order to improve circuit performance and the 

layout was altered in order to improve the shielding and to reduce 

cross-talk. 

2. A 22k resistor (R174) was introduced from pin 20 of IC7, the serial 

processor, to 0 volts to ensure that if IC74 was absent that the CTSI 

line was held low. 

3. Having added the 2k2 pu11 up resistor (R170) to printer port strobe 

line and put Q11 in its own position, S1 was reinstated. 

4. The circuitry associated with S14 to S17, which change the 1MHz bus 

to 2MHz, on the issue 3 PCB was incorrect. This should be checked 

against the current circuit diagram if it is to be used. 

5. The position of D13 was changed to put it in parallel with the relay 

coil rather than across the collector and emitter of the transistor ( 

Q3). 

6. The connections from the speech circuit (IC99) to the VIA were 

changed. VSPRDY and VSPINT were changed over to connect to PB7 and PB6 

respectively. 

36

7. A resistor (R171) was connected in series with the EOC line of the 

ADC (1C73) in order to prevent momentary output contention which may 

occur during power-up. 

8. A 4k7 resistor (R173) was connected between pin 7 of IC89 and +5v, 

as the output is open collector. 

9. Resistors R104, R125, R142, R149, and R153 which were in series with 

the ROM chip select lines were replaced by copper links formed on the 

component side of the PCB. (S34 to 38) 

10. A 10k resistor (R172) was introduced between the analogue input on 

the 1MHz bus and 0 volts in order to reduce the input impedance and 

hence improve the signal to noise ratio. (See section 6.4) 

11. Link S39 was added in order to connect the 470pF capacitor, C58 

from the base of Q7 to the emitter of Q9. 

12. A 220nF capacitor (C59) was added in series with R90 in order to 

AC couple the log amplifier on the cassette interface. 

13. A number of changes were made to the Econet control lines in order 

to speed up software control. For details of how to bring earlier issue 

boards up to the current issue, see the section on upgrading the Econet 

system (section 4, modification E). 

14. Provision was made for mounting a right-angled phono socket as an 

alternative to the free-wired BNC socket normally used for video 

output. 

15. A 200k potentiometer (VR2) was added in parallel with R32 in order 

to adjust the operating frequency of IC99 for the appropriate pitch of 

the speech output. 

16. At some stage between issues 3 and 4, C34, the cassette output 

coupling capacitor was increased from 47nF to 220nF. 

5.3.3 Changes from issue 4 to issue 7. 

(Issues 5 and 6 never went into production). 

1. R114 changed to its present value of 18 ohms 1W, and C42 changed to 

33pF. 

2. R75 went to its final value of 82k. (The reason for the change in 

value of R75 was to control the data carrier detect delay time to avoid 

loosing the first bit of the first byte of the first block when 

recording data.) 

3. The diodes and resistors on the ROM select circuitry which can be 

used to produce 1MHz operation were omitted. 

4. Links S18 and S19 are made with tinned copper wire. 

5. When the video processor ULA was replaced by the first set of 

custom-designed ICs, a modification was necessary. S26 was left 

unconnected and a wire link was made from the TTX-VDU line (pin 17 of 

IC2) to the invert input of the videoprocessor (pin 27 of IC6). Later 

versions of the custom IC made this modification unnecessary. 

A11 other changes from issue 4 to issue 7 were cosmetic changes 

including some thickening up of the tracks to improve the power supply 

distribution. 

37

6 Servicing and Fault-finding 


Before starting, it should be realised that attempt at repair by any 

person other than a registered dealer or service agent will void the 

warranty. 

6.2 Test Equipment 

The very minimum test equipment required in order to trace even the 

simplest fault is a digital multi-meter and an oscilloscope (or 

possibly a logic probe). It is difficult in a book such as this to do 

more than give a few general guide-lines as to the sort of problems to 

look for, and a few techniques which might be used. 

Acorn Computers Ltd supply two pieces of test equipment which are 

specifically designed for the BBC Microcomputer which are known as the 

PET (Progessive Establishment Tester) and the FIT (Final Inspection 

Tester). Whilst the service agent or dealer might be expected to have 

these pieces of equipment, the average user is unlikely to feel that it 

is worth purchasing them for the limited amount of fault-finding he or 

she would be likely to do. The purpose of the PET, which is the more 

expensive of the two items, is to take an apparently lifeless computer 

and attempt to find out where the fault lies. The FIT on the other hand 

is somewhat simpler and its aim is not to isolate a known fault but to 

check whether an apparently working computer is in fact working in all 

respects. Both the PET and the FIT are the subject of entirely separate 

documents produced by Acorn Computers PLC. 

Two other very useful pieces of "test equipment" are a can of freezer 

spray and a hair-dryer! It is fairly common for faults in some of the 

ICs to be associated with temperature conditions. Therefore if you have 

reason to suspect a particular component, it is sometimes helpful to " 

exercise" the device by the use of these two items. This can sometimes 

show up a fault quite clearly. However, this can be slightly misleading 

in cases where the fault is caused by a timing problem on some device. 

This is because changing the temperature conditions of one device which 

may not itself be at fault, may, by changing the relative timing, bring 

the timing back into a working condition. Therefore having discovered a 

device which apparently has a temperature fault, before de-soldering 

it, it is well worth temperature cycling the associated components. 

38

6.3 Fault Isolation 

Having checked that the apparent fault is not a problem with the 

program, the first thing, to do is to isolate the problem to a 

particular area of the computer. For example, if the problem is in 

loading and saving programs with a cassette recorder then attention 

should be focussed on the cassette interface itself. However, this is 

not as easy as it sounds in some cases because of the links between 

various sections of the circuit. It would probably be worthwhile 

reading through most of the circuit description given in this book, in 

order to try to gain an understanding of the operation of the computer 

as a whole before trying to deal with one apparently isolated section 

of the computer's hardware. 

The simplest fault to check for is malfunctioning of the power supply. 

Voltages can be measured at the terminals on the PCB where the power 

leads are attached, but it is worth checking, particularly with the 

older linear type power supplies, that the +5 volts is available on 

each of the three pairs of connectors. It is also worth checking that 

the -5 volts is present because although the processor, memory and VDU 

will all function normally if the -5 volts is not present, it is 

essential for cassette, sound, speech and RS423 interfaces. 

If the power supply is NOT working then you should NOT attempt to 

repair it. The reason is that in order to maintain the safety 

specification to which the computer was designed, any repair to the 

power supply, including replacement of power supply cable, must be 

checked for earth continuity at a current of not less than 10 A, and 

must undergo a Dielectric Withstand Test between both live and neutral 

to ground of 1500 V AC. This requires specialist equipment and training 

and should not even be attempted by dealers, unless they have the 

necessary equipment and expertise. 

The worst kind of fault with a microprocessor system is that the 

processor is unable to fetch instructions from the ROM, process them 

and then produce some sort of result which the operator can see or 

hear. In the case of such faults, the whole system appears completely 

dead and it is very difficult to locate the specific fault. This kind 

of problem is made worse on this particular computer because of the 

technique used to refresh the dynamic RAM. Not only must the processor 

fetch instructions from ROM and process them, but also it must 

successfu11y program the CRT controller which, in turn, must begin to 

produce refresh addresses for the dynamic RAM before the system memory 

can operate. 

Assuming then that the machine appears totally dead even though the 

power supply unit is apparently working, and that you do not have 

access to a PET, then here are a number of things you could check:- 

i) Check that the reset line on the 6502A (pin 40) is high, and only 

goes low when BREAK is pressed. 

ii) Check that the IRQ line is not permanently in either a low or high 

state. (Pin 4 of the 6502A) 

39

iii) Check for the presence of the various clock signals, for example, 

the clock input and output on the 6502A (pins 37 and 3), and the 1, 2, 

4 and 8 MHz signals on pins 4, 5, 6 and 7 of the video processor (IC6). 

iv) A very useful pin to check is pin 7 on the 6502A. This is the sync 

pin and, although it is not actually used in the circuit, it gives an 

indication of whether or not the 6502A is fetching any instructions. If 

this is permanently high or low then the 6502A is totally stalled. 

v) Check that the read-write line (pin 34) of the 6502A is working 

normally and also check that the same signal, having been inverted and 

re-inverted, is available at pin 10 of IC33. 

vi) Check for the horizontal and vertical sync signals coming from the 

CRTC (pins 39 and 40 of IC2) which will reveal whether or not the CRTC 

has been successfu11y programmed at system reset. 

If you do detect something abnormal in one of these tests then the next 

stage would be to remove from the board any devices in IC sockets which 

are unnecessary to the basic operation of the computer. For example, 

the 6850 ACIA (IC4), the serial processor (IC7), the ADC Converter ( 

IC73) and the external 6522 (IC69). Having removed these devices, if 

the fault disappears, then it may simply be a case of replacing them 

one by one until the fault reappears. If the fault remains, then if you 

have any spare ICs, or another machine with which you could exchange 

ICs, it would be worth replacing the internal VIA (IC3), the 6502A ( 

IC1), the 6845 (IC2) and the video processor (IC5). 

At this stage the next thing to try is to examine each of the 

individual address and data lines to see if one or more of these lines 

is permanently high or low. If so, look for short circuits, solder 

bridges etc on that line. It is worth checking these lines both on the 

6502A itself and also IC51, the operating system ROM. 

When looking around the board at various points with an oscilloscope, 

try to find any waveforms which either have "slack" edges, ie sloping 

rather than square, or which have voltage levels which are not within 

the normal TTL range. (Logic 1 must be greater than 2.8V and logic 0 

less than 0.8V, though normally one would not expect to see voltages of 

less than about 3.4V or more than 0.4V.) 

Another very useful test with a model B, is to move link S25 to the 

south position to see if the computer will operate in the 16K mode, in 

which case, it suggests a problem with the CAS 0 area of RAM. Then if 

you remove S25 altogether, it puts the machine again into the 16K mode 

but this time with the CAS 0 area enabled and the CAS 1 area 

inoperative. 

40

6.4 Most Common Faults 

In the following section, we shall try to give some ideas which have 

been colllected from various people who have been doing a good deal of 

servicing and repair work on BBC Microcomputers. There will be no 

particular order to the comments but reading through all of them should 

give some useful ideas about faults which are likely to occur. 

* A common reason for getting sound-on-vision effects is that the 

power leads have become intermittent. To check whether they are giving 

a problem, a quick flick with one finger is what the experts recommend. 

If this causes the display to flicker then switch off the unit, remove 

the power leads, pull back the insulating sleeves, solder along the 

area where the wire is crimped by the terminal, and replace them, being 

careful not to exchange the 0 and +5 volt connectors. 

* It is possible for the ROM sockets to develop bad contacts. This is 

sometimes caused by heavy-handed use of the "butterfly" carrier boards 

which were used at various stages to put two 8K eproms into one single 

16K socket. The only solution for this is to replace the ROM socket 

entirely, and you would be well advised to use the best quality socket 

available. This is not an easy task unless you are experienced in the 

use of desoldering equipment. 

* The most common reason for the cassette system becoming inoperative 

is a damaged LM324. Another problem is with the clock input to the 

serial processor and it is worth checking that this is the correct 

frequency (ie 1.23 MHz = 812 ns periodic time). Another problem which 

sometimes occurs is that the value of R75 needs to be changed. The 

optimum value is different for different issues of the serial processor 

ULA because of the variation in impedence of pin 15 to earth. This 

affects the timing between receiving a high tone lead-in and asserting 

the data-carrier detect on the ACIA. For ICs numbered 2C199E and 

2C199E-3, R75 should be 100x or 56k as required for consistent loading 

of data. For 2C199E-7, R75 should be 82k. 

* One simple problem, but unfortunately fairly common, is that the 

pins of ICs tend to get bent as they are pushed into the sockets. If 

you have isolated the fault to a particular area, then this is something 

to look out for. It is also not unknown for the IC socket itself to 

have a pin bent underneath. This may not have been noticed by quality 

control if the IC socket was empty at the time of production eg speech 

IC socket. 

* If you wish to get two BBC Microcomputers to send programs to each 

other on the cassette system, then it is possible to do so by a direct 

connection, provided a 1.5k resistor is connected between the signal 

line and ground. With the later issue boards, that have the 220nF 

output capacitor on the cassette system, a smaller value may be 

necessary. The resistor is necessary to adjust the relative phase of 

the two tones of the cassette signal. 

41

* On a number of occasions, the tracks on the right hand side of the 

keyboard PCB have become broken in transit. This occurred more 

frequently on earlier versions and less so since the newly modified 

case has been used, but if any of the keys on the right hand side are 

inoperative then this is a likely cause. Excessively hard use of the 

keyboard may also cause solder pads to lift. A group of nonfunctional 

keys would indicate that this has happened. This could be 

checked quite simply by the use of a meter to test continuity. 

* A number of people complain of interference on the sound signal. 

This is caused by the pick up of digital noise as the track goes from 

the 1MHz extension bus input to the audio stages. The solution is to 

connect a 10k resistor across from this line to ground. This should 

only be necessary on issues 1 to 3 of the PCB. It can be done by 

connecting the resistor between pin 8 of IC20, which is ground, to the 

plated-through hole just to the south of that pin. It is necessary to 

scratch away the solder resist very carefully from around this hole, 

before you can successfully solder into it. 

* One simple problem that sometimes occurs is that of getting 

twinkling characters in some of the higher modes of graphics, or 

smeering of the cursor. If this was not originally a problem but has 

developed after some months of use, then it may well be that the heat 

sink on the video processor has become dislodged. This can be put right 

by applying firm pressure to the heat sink and also possibly by 

applying more heatsink compound between it and the top of the 

integrated circuit. 

42

* Certain other faults on the VDU display associated' with the UHF 

output can be cured by adding extra decoupling to the supply to the 

modulator to improve its stability. A 10 ohm series resistor with a 4.7 

uF capacitor to earth is usually sufficient. 

* It has been noticed that problems can occur with some of the 74LS74 

ICs, especially from certain manufacturers. If there is any problem 

therefore with the cleaness of the clock pulses applied to the 6502A, 

or problems with the PAL encoder circuit, or more likely with the RAS 

signal, it would be worth checking the output of these ICs to see if 

they are driving to the full TTL levels. 

* Unfortunately, a number of problems also arise when people have 

tried to do their own upgrades and have made mistakes or used bad 

soldering techniques. In particular, a number of people seem to get the 

printer upgrade on the issue 2 board wrong and therefore this should be 

checked very carefully (see section 4). Also it is worth checking the 

soldering very carefully, particularly around the area of the IDC 

connectors. This is because it is easy to get solder bridges over the 

tracks which are fed in between the pins on this connector. The worst 

place seems to be in the area of the tube connector. 

* If there are problems associated with the PAL or cassette circuitry, 

it is worth checking very carefully whether the correct resistor values 

have been used. Since there are so many resistors so closely packed 

together, it is very easy to get resistors in the wrong places. To 

check this, it is best to remove the circuit board from the case 

entirely and use a strong light source in order to view the resistor's 

colour codes carefully. This is well worth doing, as it can save a lot 

of time looking for faults which are basically simple but which would 

be difficult to diagnose. 

* Our service centres tell us that there is a series of rather obscure 

faults which they have detected which is associated with timing 

problems with the RAM. One symptom is twinkling characters in mode 7 

but not in the other modes of graphics, and another is that when 

playing Acornsoft's "Defender" (not the later version of "Planetoids") 

some very strange effects occur as the game continues. Also there is a 

program of 3D Noughts and Crosses from Beebug which produces a strange 

fault, stopping inexplicably at one particular line and giving a No 

Room error. These faults, being related to relative timing, can 

sometimes be cured by changing the 6502 processor, or the 74LS245 ( 

IC14) and are more often noticed where the RAM in CAS 1 is a different 

type from the RAM in CAS 0, when an A to B upgrade has been done. In 

particular it seems to be that the Fujitsu RAMs do not mix well with 

the Mostek or Hitachi RAMs. 

43

6.5 Test programs and sample waveforms 

6.5.1 Test program 

The following program allows you to test the chip select lines of any 

of the devices on the computer. It sets up a machine code loop which 

accesses the address which you specify as hexadecimal number. Since it 

is a closed loop, the only way to escape is to use the break key which 

is programmed to re-enter the BASIC program. To escape, enter a zero 

address. If you are accessing a slow device, its chip select line 

should go low for a full microsecond, but with a fast device, it will 

only be low for 500 nanoseconds. In either case, the waveform should be 

high for 3 microseconds. 

10 *KEY10 OLD|M RUN|M 

20 CLS 

30 DIM' CODE 20 

40 P%=CODE 

50 INPUT "ADDRESS",M$ 

60 M%=EVAL("&" + M$) 

70 IF M%=0 THEN END 

80 [SEI 

90 SEC 

100 .again 

110 LDA M% 

120 BCS again 

130 ] 

140 CALL CODE 

The following is a reproduction of a photograph showing the waveform on 

pin 23 of the ADC chip, IC73, when the above test program is running 

with address &FECO selected. Scope parameters are 1V/cm, 1us/cm. 

44

The following shows pin 24 of the disc controller chip, 1C78, 

selected by using address &FE80. Scope parameters are 1V/cm, 1us/cm. 

The following shows two traces while the test program is running: top 

trace is 2MHz clock pin 37 IC1, and bottom trace is CAS0 pin 7 1C45. 

Scope parameters are 2V/cm, 100ns/cm. 

45

The 'following shows two more traces while the test program is running: 

top trace is 2MHz pin 37 IC1, and bottom trace is CAS1 pin 5 IC45. 


The following shows two more traces while the test program is running: 

top trace is 2MHz pin 37 IC1, and bottom trace is RAS pin 12 IC43. 


46

6.5.2 Test ROM 

The listing shown below is the object code for a ROM which could prove 

extremely useful for fault-finding an apparently dead machine, 

especially if you do not have a PET. There are three routines given, 

but you could extend the idea for up to 8 different routines if you 

wanted to do so. The idea is that a 2764 ROM is put in place of the 

operating system ROM and the routine which the system starts on powerup 

or break is determined by taking the address lines that would 

normally be connected to A10, All and Al2 (pins 21, 23 and 2 of the 

2764 respectively) and have some means of attaching them to +5 volts or 

0 volts. 

This can be done crudely by bending up the three pins so that they don' 

t engage in the IC socket, and soldering on to them three leads 

terminating in crocodile clips. These can then be used to select the 

address by clipping on to the +5V and OV rails, being careful not to 

let them short out. For a system that is to be used regularly for 

fault-finding, it is wise to use either a DIL switch or better still a 

thumbwheel switch, properly mounted. 

The three routines given are:- 

Routine "0": Provides a chip select pulse for each memory-mapped device 

around the board in turn. The pin numbers at which each pulse should 

appear are given in the program. 

Routine "1": This sets up the teletext mode of graphics by programming 

the 6845 and the video processor appropriately. Then codes 0 to 255 are 

stored in the first four pages of video RAM. If there is a RAM fault 

then the display will not be the succession of ASCII character which 

you would expect, and by careful thought about which characters are in 

error, you should be able to diagnose where the problem lies. The 

pattern is periodically re-written so that intermittent faults will 

show up and you can try temperature exercising any suspect chips. 

Routine "2": This is similar to the previous routine but by 

incrementing a location on the screen it checks the combination of 

reading and writing; ie if the RAM can be written to but not read then 

the character at location &7C01 will not cycle through 0 to 255 since 

the read instruction will be in error. All these routines are working 

in machine code at high speed and therefore it is easy to use an 

oscilloscope to probe around the circuit to see what has gone wrong. 

10 FOR N% = 0 TO 2 : REM ie 3 tests available 

20 PROCtest(N%) 

30 NEXT 

40 *SAVE ROMIMAG 3000 + 2000 

50 END 

60 

70 DEFPROCtest(N%) 

80 offset% = &400*N% 

90 0% = &3000 + offset% 

100 P% = &F800 

110 table% = P% + &200 

120 !(&33FC + offset%) = &F800 : REM RESET vector 

130 opt% = 5 

140 IF N% = 0 PROC strobe select lines 

150 IF N% = 1 PROC DRAM test 

160 IF N% = 2 PROC RNW exercise 

47

170 !(&3200 + offset%) = &4433283F 

180 !(&3204 + offset%) = &1B19021E 

190 !(&3208 + offset%) = &13721293 

200 !(&320C + offset%) = &002C002C 

210 ENDPROC 

220 

230 DEFPROC strobe select lines 

240 [OPT opt% 

250 

260 .loop 

270 LDA &FE00 \ IC2 pin 25 (CRTC) 

280 LDA &FE08 \ IC4 pin 9 (ACIA) 

290 LDA &FE10 \ IC7 pin 9 (SER PROC) 

300 LDA &FE18 \ IC96 pin 1 (STATID) or IC97 pin 4 (INTOFF) 

310 LDA &FE20 \ IC97 pin 2 (INTON) 

320 STA &FE20 \ IC6 pin 3 (VID PROC) 

330 STA &FE30 \ IC76 pin 9 -(ROMSEL) 

340 LDA &FE40 \ IC3 pin 23 (VIA A) 

350 LDA &FE60 \ IC69 pin 23 (VIA B) 

360 LDA &FE80 \ IC78 pin 24 (FDC) 

370 LDA &FEAO \ IC89 pin 9 (ADLC) 

380 LDA &FECO \ IC73 pin 23 (ADC) 

390 LDA &FEE0 \ PL 12 pin 8 (TUBE) 

400 LDA &FC00 \ PL 11 pin 10 (FRED) 

410 LDA &FD00 \ PL 11 pin 12 (JIM) 

420 JMP loop 

430 ] 

440 ENDPROC 

450 

460 DEFPROC DRAM test 

470 [OPTopt% 

480 LDX £&OF 

490 STX &FE20 \ Write to Vidproc 

500 

510 .setup_6845 

520 LDA table%,X \ table of 6845 data 

530 STX &FE00 \ Register number 

540 STA &FE01 \ Contents of register 

550 DEX 

560 BPL setup_6845 

570 LDA £3 

580 STA &FEFE \ send down TUBE 

590 

600 .restart 

610 LDY £0 

620 

630 .loop2 

640 NOP 

650 TYA 

660 STA &7C00,Y 

670 STA &7D00,Y 

680 STA &7E00,Y 

690 STA &7F00,Y 

700 INY 

710 BNE loop2 

720 

730 LDA £&42 

740 STA &FE20 \ Write to Vidproc 

750 JMP restart 

760 ] 

48

770 ENDPROC 

780 

790 DEFPROC_RNW_exercise 

800 

810 [OPT opt% 

820 LDX £&0F 

830 STX &FE20 \ Write to Vidproc 

840 

850 .setup 6845 

860 LDA table%,X \ table of 6845 data 

870 STX &FE00 \ Register number 

880 STA &FE01 \ Contents of register 

890 DEX 

900 BPL setup_6845 

910 

920 LDA £3 

930 STA &FEFE \ send down TUBE 

940 

950 LDA £&41 \ Character "A" 

960 LDY £0 

970 

980 .write 

990 INC &7C01 \ Change character on screen 

1000 STA &7D00,Y 

1010 STA &7E00,Y 

1020 STA &7F00,Y 

1030 INY 

1040 BNE write 

1050 

1060 LDA £&42 \ Character "B" 

1070 STA &FE20 \ Write to Vidproc 

1080 JMP write 

1090 ] 

1100 ENDPROC 

The following is a reproduction of a photograph showing the waveform on 

pin 34 of the 6502 (R/W of IC1). Scope parameters are 1V/cm, 2us/cm. 

49

7 Interfacing Survey 

7.1 Purpose of each interface 

Since there are so many different interface connections on the BBC 

microcomputer, it may be a help to look at each in turn and talk about 

possible applications for each. Working from left to right on the back 

of the computer, we start with the UHF output, which provides a PAL 

colour TV signal for use with a normal colour television. Next is a 

video output on a BNC connector which is intended to be used with a 

black and white video monitor. However it is possible to introduce the 

colour burst information onto this signal in order to produce a PAL 

composite video signal. On circuit boards issue 4 onwards, it is 

possible to introduce this signal by adding a simple link, S39. On 

previous issue boards it is necessary to introduce a 470pF capacitor 

from the emitter of Q9 to the base of Q7. This capacitor would have to 

be soldered directly on to the circuit board. 

The third connector which is provided for video output is a 6-way, 240 

degree DIN plug. This provides the red, green, blue and sync signals 

needed for an RGB monitor. The sync signal is a 5 volts, negative going 

pulse of 4.7uS duration, but it can be changed to positive going by 

changing link S31. Also provided on this connector are a 0 volt and a 

+5 volt supply, but these should not be used for providing more than a 

few milliamps to external circuits. 

The next connector is a serial port of the RS423 standard. This is a 

standard which has superior drive capabilities to the RS232 interface 

and is run in this case between +5 volt and -5 volt levels. The speed 

is software selectable at 75, 150, 300, 1200, 2400, 4800 or 9600 baud. 

There is a higher speed of 19200 baud, but this is not guaranteed to be 

error-free. It is also possible to get the interface to work at 110 

baud, but this requires a modification which would also change the 

speed of the cassette interface. (See section 7.3 on hardware hints and 

tips for more information.) The control signals provided are the normal 

CTS and RTS lines, the RTS output also working on +5 volt and -5 volt. 

NB When making up a connector for the RS423, note that connections as 

shown on the circuit diagram refer to the socket. For the plug 

connections, refer to page 504 of the User Guide which gives the 

connections as seen from outside the case. 

Apart from using the RS423 interface to run a serial printer it is also 

possible to use it to communicate with other computers. For example, it 

is possible to communicate with a mainframe computer either directly 

within a building on a wired link or, by using the telephone network, 

to a computer in another building or even another country. This would 

of course require the use of an acoustic coupler. 

There are various levels at which this link could be used. Firstly the 

computer could be used as a "dumb terminal" which would simply be 

capable of sending characters typed on the keyboard to the mainframe 

computer and receiving characters from the mainframe and printing them 

on the screen. The following program will allow you to do so. 

50

10 REM Dumb Terminal Program 

20 REM Only works on OS 1.0 & following 

30 REM Works even if Tube fitted.' 

40 REM 

50 OSASCI = &FFE3 

60 OSBYTE = &FFF4 

70 OSWRCH = &FFEE 

80 CLS 

110 DIM CODE 50 

120 FOR J=0 TO 2 STEP2 

130 P%=CODE 

140 [OPT J 

150 .RS423 

160 LDA £&91 

170 LDX £1 

180 JSR OSBYTE\character in RS423 buffer? 

190 BCS keyboard 

200 TYA 

220 JSR OSWRCH\or OSASCI for CRLF 

230 .keyboard 

240 LDA £&91 

250 LDX £0 

260 JSR OSBYTE\character in keyboard buffer? 

270 BCS RS423 

280 TYA 

290 JSR OSWRCH\or OSASCI for CRLF 

300 LDA £&8A 

310 LDX £2 

320 JSR OSBYTE\Put character in RS423 output buffer 

330 JMP RS423 

340 ] 

350 NEXT 

360 *FX 7,7 

370 *FX 8,7 

380 *FX 2,2 

390 CLS 

400 CALL CODE 

The next level would be its use as a semi-intelligent terminal which 

would enable you to use some of the processing of the BBC Microcomputer 

to deal with file handling, so. that the text could be prepared off 

line, stored on disc and then spooled down to the mainframe when the 

link is made. The third level then would be to use the graphics 

facilities of the BBC Microcomputer in addition to its ability to print 

text. This produces the possibility of using the computer as a colourgraphics 

terminal to a mainframe computer at a fraction of the cost. 

All that is needed is for someone to write the appropriate terminal 

emulation software and put it in sideways ROM. There are now a number 

of such packages commercially available. 

51

The other way in which the RS423 can be used is to link two BBC 

computers together. One reason for doing this would be to enable 

software to be downloaded from a disc system to another computer which 

does not have a disc interface. A 15K program can be downloaded using 

an RS423 link in approximately 20 seconds which is clearly faster than 

using a cassette to cassette link. The only software involved in doing 

this is to type in, on the receiving computer: 

NEW 

*FX2,1 

(This sets the RS423 as input instead of the keyboard.) 

and then on the sending computer you would type in: 

*FX3,7 

LIST 

*FX3,0 

(This enables the RS423 as the output and sends a listing of the 

program so that, to the receiving computer,, it is as if it were being 

typed in from the keyboard.) 

If you have a number of transfers to do, then these commands could be 

programmed onto a single key on each machine. When the program has been 

sent down, you simply have to press BREAK on the receiving computer, 

type OLD and then the program is ready for use. If you are 

doing a BBC to BBC link over a short distance and want to use the full 

speed of the interface then you will have to connect the hand-shake 

lines as well as the data lines. The "data out" from one computer 

should be connected to the "data in" of the other computer and viceversa, 

and in a similar way for the control lines, the RTS on one 

should be connected to the CTS on the other and vice-versa. If you are 

working over a longer distance and want to use only three cables, data 

in, data out, and ground and are prepared to work at a slower speed 

without any handshaking, then you have to loop back the RTS to the CTS 

on each of the computers so that each is permanently enabled for 

sending. If you do not do so, the RS423 output buffer fills up and 

printing stops after a number of characters have been sent to the 

screen. 

If you want to use the RS423 interface over a long distance at high 

speed using the hand shake lines, it might be necessary to terminate 

the receivers by making links S23 and S24. This terminates the line 

with its characteristic impedance of 180 ohms. 

The cassette interface is a standard CUTS cassette interface. It has 

two speeds, 300 and 1200 baud, controllable by software. There is also 

motor control provided on pins 6 and 7 of the 7-way DIN plug. The 

rating of the relay contacts is 24V at 1A DC, and they should not, 

under any circumstances, be used to switch mains voltages, no matter 

how small the current. If you wish to get two BBC Microcomputers to 

send programs to each other on the cassette system, then it is possible 

to do so by a direct connection, provided a 1.5k resistor is connected 

between the signal line and ground. 

52

The analogue input is on a 15-way D-type connector and provides four 

A to D converter channels and two digital input lines which work on the 

internal 6522 VIA. The pin connections are arranged so that the signals 

are divided into two sets intended for use with two games paddles, each 

of which will have two A to D inputs and a voltage reference source as 

well as the ground and one of the digital input lines. 

The conversion time for each channel is 10 milliseconds, but you can 

choose to have as few or as many of the channels working as you wish by 

using the *FX16 command. Therefore if all four converters are required, 

you need to allow 40 milliseconds to be sure of a successful conversion 

on any one channel. However ADVAL(0) in BASIC, or OSBYTE call 128 in 

machine code, can be used to see which channel has just converted, and 

OSBYTE call 17 allows you to force a particular ADC channel to convert, 

out of turn. The resolution is software selectable between 8 and 12 

bits resolution using OSBYTE 190. However in the 12 bit resolution 

mode, the true resolution is somewhat less than 12 bits. It is probably 

more realistic to think in terms of a 9 or 10 bit accuracy. 

Also provided on this connector is an input to the light pen strobe on 

the 6845 CRT controller. The hardware involved in setting up a light 

pen system is quite simple. The light pen circuit has to produce a 

positive-going 5 volt pulse with a duration of greater than 100 ns. 

However the software involved is quite complicated if you want to do 

more than identify character blocks. This is because of the way in 

which the 6845 has been extended beyond its normal memory range in 

order to provide bit-mapped graphics. 

The final connector which is provided on the back of the computer is 

only available when the unit has been upgraded with an Econet 

interface. This is a standard 5 pin 180 degree DIN plug to provide the 

necessary data and clock signals for the Econet interface. 

Underneath the computer is a set of standard IDC connectors. First of 

all there is a 34 way connector for the disc drive(s). This connector 

carries the standard connections for a 5.25" disc drive. The next two 

are a 26-way and a 20-way IDC connector which are used to provide 

connections to the two ports of the external 6522 VIA. The 26-way 

connector links to port A, and is arranged in a standard format for use 

with a Centronics-type parallel printer. The 8 port lines are buffered 

to provide better drive capabilities, but it does mean that they can 

only be used for output. If you want to use these lines to drive some 

other device, you should work on the basis of each line being able to 

sink 10 mA at logic 0 or to source 400 uA at logic 1. Of the two 

control lines, CA1 is available as an input with a single 4k7 pull up 

resistor on it, whilst the CA2 line is available as an output, the line 

being buffered by a single transistor (Q11) to improve the current 

sinking. On boards from issue 4 onwards, there is a selection link (S1) 

which will enable this line to provide direct connection to CA2 so that 

it can be used as either input or output. 

53

The 20-way User Port is much simpler in that all connections go 

directly to the lines on the VIA (PBO to PB7 and also control lines CB1 

and CB2). If you are using these lines for output you should consult 

the 6522 data sheet to establish the amount of drive current available. 

The next connector, another 34-way IDC, provides an extremely versatile 

interface known as the 1MHz extension bus. This interface is the 

subject of a separate Application Note and at this stage it is 

sufficient to say that it provides two "pages" (2 x 256 bytes) of 

memory locations mapped between &FC00 to &FDFF. By using one of these 

locations (&FCFF) as a paging register it is possible to extend the 

memory addressing capability to a full 64K bytes. 

The final connector is a 40-way IDC which provides an interface known 

as the "Tube", intended for use with a second processor. Although the 

hardware on the BBC microcomputer side is very simple, the hardware on 

the second processor is extremely complex and it really requires a ULA 

to incorporate all the hardware necessary to handle the protocol. It is 

therefore suggested that any interfacing to the Tube should be done 

only using products from Acorn Computers Ltd. 

7.2 Interfacing to various printers. 

It is possible to interface to a wide variety of printers using the two 

interfaces provided. First of all, for serial printers, the RS423 

connector can be used and secondly, for parallel printers the 

Centronics interface can be used. It is also possible to link up to 

various other non-standard printers such as the IEEE 488 printers used 

for the Commodore Pet computers. This is more difficult and requires 

connection not only to the Printer Port, but also the User Port to 

provide extra control lines and also requires extra software within the 

machine. 

It is possible to write your own printer driver routine which works 

through the operating system. 

In order to link up to teletype printers, which run at 110 baud, you 

have to change the position of link S28. This is explained in detail in 

section 7.3. 

One important point to note is that on issue 1 printed circuit boards, 

the pin connections on the printer port are not quite the same as later 

issues. Pin 19 is the ACK line and pin 26 is not open circuit as it 

should be for certain printers. If you wish to use this interface, for 

example with the Seikosha GP80A, you will encounter problems since pin 

26 is used as a reset line. Another problem which may occur on the 

early issue boards is that there is no pull-up resistor on the CA2 line 

thus leaving the collector of transistor Q11 open circuit. 

54

7.3 Hardware Hints and Tips 

Here are a number of miscellaneous hints and tips which have come from 

various sources. 

* RS423 at 110 baud 

In order to get the RS423 interface to work at 110 baud, all that has 

to be done is to use the *FX8,1 command in order to set 75 baud and 

then change the position of link S28. This link is made by a track on 

the PCB, between the centre pin and the west pin. This link has to be 

broken and a solder link made from the centre pin to the east pin. This 

has the effect of speeding up all of the baud rates, both send and 

receive by 44%, and also makes the cassette port run fast by the same 

amount, so a single pole double throw switch could be wired to S28 in 

order to select the normal speed or the fast speed for the 110 baud. 

The actual speed produced is 108.333 but this is near enough for most 

teletype printers. 

* Disabling Break 

For certain applications, particularly when the computer is being used 

by very young children, it is most frustrating when the user presses 

the break key. It is easy enough to disable the escape key within a 

program, but the break key is mare of a problem. You could use *KEY10 

OLD|M RUN|M but even this is not very satisfactory. The alternative is 

to disable the break key electrically, which can be done by removing a 

link from the keyboard PCB. You could then wire it up to a miniature 

ON-OFF toggle switch which can be mounted on the back of the case by 

drilling a suitable hole, or leave it open circuit and use the contacts 

at the back of the main PCB marked "RST SW", to provide an alternative 

break key. The disadvantage of doing this though is that if you are 

using a disc system, you may want to be able to do a SHIFT break in 

order to boot the disc. The position of the link to be removed is shown 

below. 

55

8 Component location tables 

The following lists of components should enable you to locate any 

component on the main circuit diagram by its X and Y grid reference ( 

see grid numbers on main PCB circuit diagram). For ICs and selection 

links, their positions on the PCB itself are also given. These are 

defined by X and Y coordinates in millimetres, measuring from the SW 

corner of the PCB. 

8.1 Integrated circuits 

NB Some ICs which contain more than one circuit will appear at more 

than one place on the circuit diagram. 

IC Type PCB Circuit Comments 

Position Diagram 

1 6502A 160,85 10,6 

2 6845 160,140 5,3 CRT controller 

3 6522 90,75 5,9 Internal VIA 

4 6850 128,141 11,5 ACIA 

5 SAA5050 187,102 10,2 Teletext ROM 

6 5C094 214,71 7,6 10,1 Video ULA 

7 2C199 128,182 12,4 Serial ULA 

8 81LS95 241,62 5,1 

9 81LS95 241,93 5,2 

10 81LS95 252,63 5,3 

11 81LS95 252,93 5,4 

12 81LS95 275,62 5,4 

13 81LS95 264,62 5,5 

14 74LS245 184,71 9,4 

15 74LS273 196,71 9,2 

16 LM555 7,210 6,7 

17 LM324 6,28 4,6 

18 76489 .22,44 5,6 Sound generator 

19 LM386 52,23 5,5 

20 74LS139 120,23 7,10 9,8 

21 74LS00 120,55 10,8 

22 74LS30 137,55 6,10 

23 74LS30 120,80 8,9 

24 74LS138 135,78 6,10 

25 74LS20 120,108 7,10 10,6 

26 74LS139 137,105 6,8 

27 7438 3,124 2,4 4,2 4,10 12,7 

28 74LS51 50,143 7,7 9,6 

29 74LS32 65,143 5,6 8,7 8,8 

30 74LS74 78,143 8,6 

31 74LS34 90,143 5,9 8,7 

32 74LS259 105,143 5,10 

33 74LS04 58,164 4,9 10,9 10,3 

34 74LS74 71,164 8,7 

35 LM324 151,205 13,5 

36 74LS10 215,122 4,2 10,3 

37 74LS04 228,122 8,6 10,3 10,6 

38 74LS86 241,122 5,3 

39 74LS283 197,145 5,2 

40 74S00 206.149 4,2 6,5 8,6 (Previously 74LS00) 

41 74LS02 228,149 4,4 7,4 

42 74LS163 241,158 12,6 

43 74S04 187,174 7,6 8,4 

56

44 74LS74 206,174 12,1 

45 74S139 228,172 7,5 

46 74S74 252,183 12,2 

47 74LS86 264,183 13,2 

48 74LS86 275,183 13,1 

49 74LS00 286,183 13,2 

50 74LS00 298,183 12,6 13,1 

51 (27128) 214,24 8,9 Operating system ROM 

52 (27128) 233,24 9,9 BASIC ROM 

53 4816 287,69 8,2 

54 4816 287,93 8,2 

55 4816 275,93 8,3 

56 4816 298,146 8,4 

57 4816 287,146 8,2 

58 4816 275,146 8,2 

59 4816 264,146 8,3 

60 4816 252,146 8,4 

61 4816 298,68 6,2 

62 4816 298,93 6,2 

63 4816 264,93 6,3 

64 4816 298,122 6,4 

65 4816 287,122 7,2 

66 4816 275,122 7,2 

67 4816 264,122 7,3 

68 4816 252,122 7,4 

69 6522 160,29 2,6 External VIA 

70 74LS244 137,24 2,6 

71 74LS244 184,29 2,4 

72 74LS245 199,29 2,3 

73 uPD7002 100,173 14,6 ADC convertor 

74 88LS120 183,201 13,4 

75 3691 207,199 14,4 

76 74LS163 70,44 10,8 

77 74S00 38,143 4,8 4,9 

78 8271 60,75 3,9 Disc controller 

79 7438 42,46 2,9 

80 7438 57,46 2,9 

81 74LS393 82,46 2,8 

82 74LS10 97,46 3,8 

83 4013 67,24 4,7 

84 4013 82,24 3,7 

85 4020 97,21 2,7 

86 74LS393 108,23 2,7 

87 74LS123 43,164 2,10 13,8 

88 2764 252,24 9,9 

89 68B54 15,125 12,8 ADLC - Econet 

90 40178 16,162 13,10 (Not fitted) 

91 74LS132 27,166 12,7 14,9 

92 74LS74 16,188 14,10 (Not fitted) 

93 75159 13,166 14,9 

94 LM319 45,93 13,8 

95 LM319 20,193 13,7 

96 74LS244 82,185 12,10 

97 74LS74 4,171 13,9 

98 TMS6100 15,75 4,8 Speech ROM 

99 TMS5220 35,75 5,7 Speech generator 

100 (27128) 272,24 10,9 Sideways ROM 

101 (27128) 290,24 11,9 Sideways ROM 

57

8,2 Transistors 

Q Type Circuit 

Diagram 

1 BC239 13,5 

2 BC239 13,5 

3 BC239 13,5 

4 BC239 11,3 

5 BC239 12,3 

6 BC239 12,3 

7 BC309 14,2 

8 BC309 13,2 

9 BC239 14,1 

10 2N3906 11,2 

11 BC239 2,4 

8.3 Diodes 

D Type Circuit 

Diagram 

1 1N4148 6,8 

2 1N4148 4,6 

3 1N4148 14,7 

4 1N4148 10,8 Not fitted 


6 1N4148 14,6 

7 1N4148 14,6 

8 1N4148 14,6 


10 1N4148 8,8 Not fitted, issue 7 onwards 



13 1N4148 13,5 

14 1N4148 3,3 

15 1N4148 3,3 

16 1N4002 11,1 

17 1N4002 11,1 

18 1N4002 11,1 

19 1N4148 13,1 

20 1N4148 13,3 

21 1N4148 13,3 

22 1N4148 13,3 

8.4 Capacitors 

C Type Circuit 

Diagram 

1 2n2F Plate ceramic 4,6 

2 4u7F 16V Elec 4,5 


4 Not used 

5 10uF 16V Elec 4,5 

6 100nF Disc ceramic 6,7 



58

9 10uF 16V Elec 4,4 

10 lOnF Plate ceramic 6,7 


12 10pF Plate ceramic 5,7 

13 1nF Plate ceramic 2,10 

14 47uF 10V Elec 3,1 


16 47uF 10V Elec 5,5 


18 10uF 10V Tant 14,8 

19 Not used 



22 Not used 

23 l0nF Plate ceramic 13,8 


25 33 n F Polyester 14,6 

26 47uF 10V Elec 4,1 

27 luF 35V Tant 14,6 

28 4u7F 10V Tant 12,5 


30 10uF 10V Tant 12,5 




34 200/220nF 14,5 


36 47uF 10V Tant 4,1 








44 Not used 



47 10uF 10V Tant 11,1 




51 15/22pF Plate ceramic 11,2 (SOT) 



54 47uF 10V Tant 4,1 



57 10uF 10V Tant 3,1 


59 220nF 13,5 

60 4u7F 10V Tant 3,1 

59

8,5 Resistors 

R Value Circuit 

Diagram 

1 10k 4,6 

2 10k 5,6 

3 10k 5,6 

4 22k 4,6 

5 100k 3,6 

6 4k7 1,5 

7 100k 3,6 

8 10k 4,5 

9 39k 4,6 

10 3k3 4,2 

11 100k 3,6 

12 220k 3,6 

13 1M 6,7 

14 10R 4,5 

15 39k 3,5 

16 22k 4,5 

17 10k 4,5 

18 10R 4,4 

19 Not used 

20 1M 6,8 

21 1M 6,7 

22 150R 3,9 

23 150R 3,9 

24 39k 4,4 

25 Not used 

26 Not used 

27 10k 3,5 

28 4k7 3,5 

29 1k 3,5 

30 10k 5,6 

31 4k7 5,6 

32 150k 5,7 

33 1k 2,10 

34 10k (2%) 14,8 

35 10k (2%) 14,8 

36 1M5 13,8 

37 k 2,9 

38 100K (2%) 14,8 

39 100K (2% 14,8 

40 100K (2%) 14,8 

41 100K (2%) 14,8 

42 Not used 

43 Not used 

44 1M5 13,9 

45 10k (2%) 13,8 

46 k 13,8 

47 k5 (2%) 14,8 

48 k (2%) 14,8 

49 150R 2,10 

50 39k 12,9 

51 10k (2%) 14,8 

52 k 13,9 

53 1k 4,7 

54 Not used 

55 3k3 8,8 Not fitted, issue 7 onwards 

60

56 Not used 

57 10R 5,5 

58 150R 1,7 

59 56k (2%) 13,7 

60 56k (2%) 13,7 

61 1k 13,8 

62 56k (2%) 13,7 

63 56k (2%) 13,7 

64 1M5 13,7 

65 3k3 3,10 

66 10k 5,10 

67 10k 5,10 

68 3k3 2,7 

69 3k3 10,8 

70 3k3 2,6 

71 2k7 14,6 



74 2k2 12,5 

75 82k 12,5 

76 10k 13,5 

77 100k 13,5 

78 150k 14,4 

79 820k 15,5 

80 39k 14,5 

81 3k3 10,6 

82 150k 14,4 

83 4k7 5,4 

84 10k 13,5 

85 3k3 10,6 

86 220k 14,5 

87 8k2 14,5 

88 8k2 14,5 

89 4k7 13,4 

90 4k7 13,5 

91 820R 7,6 

92 820R 7,6 

93 3k3 14,4 

94 ,100R 8,6 

95 2k2 14,4 

96 3k3 14,4 

97 2k2 14,4 

98 1k2 7,5 

99 1k2 10,2 

100 1k2 11,2 

101 1k0 11,2 

102 100R 6,5 

103 1k0 10,2 

104 100R 9,8 Not fitted, issue 4 onwards 

105 100R 8,6 

106 56R 8,4 

107 1k0 7,5 

108 3k3 3,3 

109 1k8/2k713,1 SOT 

110 68R 14,2 

111 68R 14,3 

112 68R 14,3 

113 68R 14,3 

114 18R 1W 11,1 

115 1k0 14,2 

61

116 3k9 14,2 

117 2k2 14,2 

118 1k0 14,2 

119 100R 8,5 

120 82R 12,3 

121 82R 11,3 

122 82R 11,3 

123 470R 14,2 

124 56R 7,4 


126 3k9 14,1 

127 1k5 14,2 

128 470R 13,2 

129 68R 14,2 

130 68R 13,2 

131 56R 7,4 

132 120k 14,1 

133 120k 14,1 

134 1k0 14,2 

135 1k0 14,1 

136 2k2 13,3 

137 3k9 13,2 

138 1k5 13,2 

139 1k0 13,2 

140 1k0 14,2 

141 2k7 14,2 


143 12k 11,2 

144 15k 11,2 

145 1k0 11,2 

146 1k5 11,1 

147 3k9 11,1 

148 820R 14,2 


150 680R 14,2 

151 470R 14,1 

152 2k2 14,2 


154 1k2 14,1 

155 680R 14,1 

156 3k3 14,1 

157 680R 14,1 

158 470R 14,1 

159 270k 13,5 

160 3k3 12,4 

161 5k6 13,4 

162 4k7 1,6 

163 Not used 

164 Not used 

165 Not used 

166 Not used 

167 Not used 

168 Not used 

169 Not used 

170 2k2 2,5 

171 100R 13,6 

172 10k 4,6 

173 4k7 12,8 

174 22k 12,3 

62

8.6 Links 

Some links have been omitted on later issue boards, whilst others have 

been added. 

LINK PCB Circuit position diagram 

1. 2,108 2,5 (Not used on issues 2 and 3) 

2. 2,161 12,7 

3. 2,173 14,14 (Only fitted on issues 1,2,3) 

4. 12, 12 1,9 



7. 30, 65 4,9 

8. 32, 15 2,9 

9. 35,128 3,10 

10. 45, 15 1,8 

11. 75,210 13,10 

12. 97, 70 9,8 

13. 100, 67 9,8 

14. 101, 53 7,10 

15. 107, 97 7,9 

16. 108, 90 7,9 

17. 108, 52 7,10 

18. 110, 52 7,8 

19. 102,102 7,8 

20. 123, 55 9,8 

21. 122, 65 9,8 

22. 127, 70 9,8 

23. 177,215 13,3 

24. 181,195 13,3 

25. 215,185 7,5 

26. 221, 68 10,1 

27. 226, 95 1,7 

28. 237,144 12,6 

29. 237,146 12,6 

30. 284, 20 8,8 

31. 270,170 14,3 

32. 295, 65 9,9 

33. 295, 67 10,9 

34. 200, 65 9,8 (From issue 4 onwards) 





39. 255,215 9,8 (From issue 4 onwards) 

63

6 4

9 Appendices 

65

9.8 Parts list 

CASE LOWER ASSEMBLY MODEL B + ECONET + DISC 

ITEM PART No DESCRIPTION QTY REMARKS 

1 A2/210,232 CASE LOWER 1 

2 A2/103,500 MAIN PCB ASSEMBLY MODEL B+ECONET+DISC 1 

3 A1/103,004/A CASE LOWER ASSY REF 'STANDARD' DRG 

4 A1/103,001 KEYBOARD ASSEMBLY (INC SPKR) 1 

5 A2/21,111 CUT RIGID PVC LABEL BOTTOM ACCESS 1 

6 A2/201,098 CUT RIGID PVC LABEL REAR ACCESS 1 

7 

8 890,000 STICK ON FEET 4 

9 

10 800,600 B.N.C. CONNECTOR 75R 1 PANEL MOUNT SK2 

11 870,109 WIRE 7/0,2 WHITE PVC 2 2" LENGTH 

12 

13 882,644 No 8 x 9.5 FLANGE HEAD POSIDRIV 5 BLACK SELF TAP 

14 882,712 No 4 x 7/16" PAN HD SUPERDRIVE 2 PLASTITE 

15 882,986 NYLON WASHER I/D 5mm 5 

16 

17 

18 

19 882,022 M3 x 8mm CHEESE HEAD POST DRIVE 3 USE WITH ITEM 24 

20 

21 

22 

23 

24 103.003 POWER SUPPLY UNIT 1 ASTEC SMPS 

25 

26 

27 882,988 4 BA INTERNAL TOOTH SHAKEPROOF WASHER 2 

28 882,914 4BA NUT FULL 2 

29 882,343 4BA x 5/8" PAN HD POSIDRIV 2 

30 

81

MAIN CIRCUIT BOARD MODEL B + DISC + ECONET 

ITEM PART No DESCRIPTION QTY REMARKS 

1 203000 PRINTED CIRCUIT BOARD 1 

2 103,500/A ASSEMBLY DRAWING MODEL B +DISC+ECONET 1 PER WORKS ORDER 

3 

4 520,180 RESISTOR 18R 1W 10% CARBON FILM 1 R114 

5 

6 500,100 RESISTOR 10R 1/4W 10% CARBON FILM 3 R14 18 57 

7 500,560 RESISTOR 56R 10% CARBON FILM 3 8106,124,131 

8 500,680 RESISTOR 68R 1/4W 10% CARBON FILM 6 R110,113,129,130 

9 500,820 RESISTOR 82R 1/4W 10% CARBON FILM 3 R120,121,122 

10 500,101 RESISTOR 100R 1/4W 10% CARBON FILM 5 R94,102,105,119, 

171 

11 500,151 RESISTOR 150R 14W 10% CARBON FILM 4 R22,23,49, 

12 500,471 RESISTOR 470R 1/4W 10% CARBON FILM 4 R123,128,15518,158 



15 

16 500,102 RESISTOR 1K 1/4W 10% CARBON FILM 20 R29,33,37,46,52, 

53,61,101,103, 

107,115,118,134, 

135,139,140,145, 

98,99,100 

17 

18 

19 500,122 RESISTOR 1K2 1/4W 10% CARBON FILM 2 R154,127 


21 500,222 RESISTOR 2K2 1/4W 10% CARBON FILM 7 R74,95,97,117, 

136,152,170 

22 500,272 RESISTOR 2K7 1/4W 10% CARBON FILM 1 R71 

23 500,332 RESISTOR 3K3 1/4W 10% CARBON FILM 12 R10,65,68,69,70, 81,85,93,96, 

108, 156,160 

24 


26 500,392 RESISTOR 3K9 10% CARBON FILM 4 R116 126,137 147 

27 500,472 RESISTOR 4K7 1/4W 10% CARBON FILM 8 R6,28,31,83,89, 

90,162,173 



30 

31 500,103 RESISTOR 10K 1/4W 10% CARBON FILM 12 R1-3,8,17,27,30, 

66,67,76,84,172 

32 

33 500,123 RESISTOR 12K 1/4W 10% CARBON FILM 1 R143 


35 500,223 RESISTOR 22K 1/4W 10% CARBON FILM 3 R4,16,174 

36 500,393 RESISTOR 39K 1/4W 10% CARBON FILM 5 R9,15,24,50,80 


38 500,104 RESISTOR 100K 1/4W 10% CARBON FILM 4 R5,7,11,77 

39 500,124 RESISTOR 120K 1/4W 10% CARBON FILM 2 R132,133 

40 500,154 RESISTOR 150K 1/4W 10% CARBON FILM 2 78,82 

41 

42 500,224 RESISTOR 220K 1/4W 10% CARBON FILM 2 R12,86 



45 

46 500,105 RESISTOR 1M 1/4W 10% CARBON 3 R13,20,21 

47 500,155 RESISTOR 1M5 1/4W 10% CARBON 3 R36,44,64 

48 590,682 RESISTOR PACK S.I.P. 6K8 x 8 1 RP1 

49 590,223 RESISTOR PACK S.I.P. 22K x 8 1 RP2 

50 580,103 POTENTIOMETER 10K VERT. MIN. PRESET 1 VR1 

51 580,204 POTENTIOMETER 200K VERT. MIN. PRESET 1 VR2 

52 

53 500,182/272 RESISTOR 1K8/2K7 1/4W 10% CARBON FILM 1 R109,S.O.T. ON 

BATCH BASIS 

54 

55 505,102 RESISTOR 1K 1/4W 2% CARBON 1 R48 

56 505,152 RESISTOR 1K5 1/4W 2% CARBON 1 R47 

57 505,103 RESISTOR 10K 1/4W 2% CARBON 4 834,35,45,51 



60 

61 631,010 CAPACITOR 10pF PLATE CERAMIC 1 C12 

62 631M015/022 CAPACITOR 15722pF PLATE CERAMIC 1 C51,S.O.I. ON 

BATCH BASIS 

63 631,033 CAPACITOR 33pF PLATE CERAMIC 2 C37,42 


82




68 







75 629,001 ^ CAPACITOR 1nF PLATE CERAMIC 1 C13 

76 629,002 CAPACITOR 2n2F PLATE CERAMIC 6 C1,3,7,11,17,29 

77 634,004 CAPACITOR 4u7F PLATE CERAMIC 2 C32,33 

78 629,010 CAPACITOR l0nF PLATE CERAMIC 4 C10,23,40,45 

79 650,333 CAPACITOR 33nF POLYESTER 1 C25 

80 680,001 CAPACITOR, DECOUPLER 33/47 nF 76 A 

81 628,470 CAPACITOR 47nF DISC CERAMIC 1 C20 

82 628,101 CAPACITOR 100nF DISC CERAMIC 5 C6,8,15,21,24 

83 634,002 CAPACITOR 2n2F PLATE CERAMIC 2 C38,39 

84 651,224 CAPACITOR 220nF 1 C59 

85 613,100 CAPACITOR luF 35V TANTALUM 1 C27 

86 610,005 CAPACITOR 4u7F 10V TANTALUM 2 C28,60 

87 635,047 CAPACITOR 4u7F 16V RADIAL ELEC 1 C2 

88 610,010 CAPACITOR 10uF 10V TANTALUM 4 C18,30,47,57 

89 635,100 CAPACITOR 10uF 16V RADIAL ELEC 2 C5,9 

90 610,047 CAPACITOR 47uF 10V TANTALUM 2 C36,54 

91 621,470 CAPACITOR 47uF 10V AXIAL ELEC 3 C14,16,26 

92 651,204/224 CAPACITOR 200/220nF 1 C34 

93 

94 699,001 TRIMMING CAPACITOR 2-22pF 1 VC1 

95 

96 

97 860,005 CHOKE 33 uH 1 L1 

98 

99 

100 820,160 CRYSTAL 16 MHz 1 X1 

101 820,177 CRYSTAL 17,7345 MHz 1 X2 

102 

103 

104 794,002 DIODE IN 4002 3 D16,17,18 

105 794,148 DIODE IN 4148 12 D1,2,6,7,8, 

13-15,19-22 

106 

107 

108 810,001 RELAY 5V 1 RL1 

109 

110 

111 780,239 TRANSISTOR BC 239 8 Q1-6,9,11 

112 780,309 TRANSISTOR BC309 2 Q7,8 

113 783,906 TRANSISTOR 2N3906 1 Q10 

114 

115 

116 800,114 I.C. SOCKET D.I.L. 14 PIN 4 55,94,95,93 

117 800,120 I.C. SOCKET D.I.L. 20 PIN 1 IC14 

118 

119 800,128 I.C. SOCKET D.I.L. 28 PIN 11 IC6,7,51,52,73, 

89,98,99,100,101 

120 800,140 I.C. SOCKET D.I.L. 40 PIN 5 ICI-3,69 78 

121 740,038 INTEGRATED CIRCUIT 7438 3 IC27,79,80 

122 741,000 INTEGRATED CIRCUIT 74S00 1 IC40 

123 742,000 INTEGRATED CIRCUIT 74LS00 4 IC21,49,50,77 

124 742,002 INTEGRATED CIRCUIT 74LS02 1 IC41 

125 741,004 INTEGRATED CIRCUIT 74SO4 1 IC43 

126 742,004 INTEGRATED CIRCUIT 74LS04 2 IC33,37 







133' 742,074 INTEGRATED CIRCUIT 74LS74 5 IC30,31,34,44,97 

134 742,086 INTEGRATED CIRCUIT 74LS86 3 IC38,47,48 





139 742,244 INTEGRATED CIRCUIT 74LS244 3 IC70,71,96 




83


144 770,324 INTEGRATED CIRCUIT LM324 2 IC17,35 

145 770,386 INTEGRATED CIRCUIT LM386 1 IC19 

146 770,555 INTEGRATED CIRCUIT LM555 1 IC16 

147 738,095/097 INTEGRATED CIRCUIT 81 

LS95/97 2 IC12,13 

148 706,489 INTEGRATED CIRCUIT 76489 1 IC18 

149 738,095 INTEGRATED CIRCUIT 81LS95 4 IC8-11 


151 706,502 INTEGRATED CIRCUIT 6502A 1 IC1 

152 706,522 INTEGRATED CIRCUIT 6522 2 IC3,69 




156 705,050 INTEGRATED CIRCUIT SAA5050 1 IC5 

157 201,601/647 INTEGRATED CIRCUIT 

VIDEO PROCESSOR 1 IC6 

158 201,602/648 INTEGRATED CIRCUIT 

SERIAL PROCESSOR 1 IC7 


160 704,816 INTEGRATED CIRCUIT 4816 16 IC53-68 

161 201,629 INTEGRATED CIRCUIT 23128 1 IC51 (OPERATING 

SYSTEM) 

162 201,628 INTEGRATED CIRCUIT 23128 1 IC52 (BASIC) 

163 

164 800,004 SOCKET D.I.N. 5 WAY 1 SK7 

165 825,000 SOCKET UM1233-E36 1 SK1 (UHF 

MODULATOR) 


167 800,001 SOCKET D.I.N. 5 WAY DOMINO 1 SK4 


169 800,304 SOCKET 'D' TYPE 15 WAY 1 SK6 

170 

171 800,059 PLUG 17 WAY 1 PL13 (KEYBOARD) 

172 800,055 PLUG 10 WAY 1 PL14 (SERIAL ROM) 

173 800,050 PLUG 2 WAY 4 PL15 S21(2),39 

174 800,051 PLUG 3 WAY 7 S20,22,25,26, 

31-33 

175 800,070 SHUNT 16 S20,21(2),22,25, 

26,31-33,S11(7) 

176 800,054 PLUG 8 WAY 2 S11 

177 880,040 SLEEVING BLACK NEOPRENE A/R FIT TO R114 

178 201,029 HEATSINK 1 FIT T0 IC6 IF 

201,601 

179 870,420 WIRE TCW A/R 18(N),19(E) 

180 800,200 FASHION TAB 7 

181 800,006 CONNECTOR IDC 34 WAY 2 PL8,11 

182 800,008 CONNECTOR IDC 26 WAY 1 PL9 



185 





190 706,854 INTEGRATED CIRCUIT 68B54 1 IC89 

191 754,013 INTEGRATED CIRCUIT 4013 2 IC83,84 



194 770,319 INTEGRATED CIRCUIT LM319 2 IC94,95 

195 201,666 INTEGRATED CIRCUIT 27128* 1 IC88 (DNFS ROM) 

84

9.9 Glossary of abbreviations 

ACK ACKnowledge line on the printer port 

ACIA Asynchronous Communications Interface Adaptor - serial to 

parallel and parallel to serial converter (6850) 

ADC Analogue to Digital Converter 

ADLC Advanced Data Link Controller - Econet control IC (68B54) 

ADSR Attack, Decay, Sustain, Release - defining the envelope of 

a sound 

ASCII American Standard Code for Information Interchange - binary 

code for representing alphanumeric characters. 

BASIC Beginners All-purpose Symbolic Instruction Code 

BBC British Broadcasting Corporation 

BNC Bayonet-Neill-Concelman - the type of bayonet connector 

used for the video output 

CA1/2 Control lines associated with the PA port on a VIA 

CAS Column Address Strobe - control line :for the dynamic RAM 

CASO Refers to the area of RAM selected by the CASO line 

CAS1 Refers to the area of RAM selected by the CAS1 line 

CB1/2 Control lines associated with the PB port on a VIA 

CP/M Control Progam for Microcomputers - Z80 based operating 

system 

CPU Central Processor Unit (6502) 

CR Capacitor Resistor network 

CRT Cathode Ray Tube 

CRTC Cathode Ray Tube Controller IC (6845) 

CSYNC Composite SYNChronisation pulse from the CRTC 

CTS Clear To Send - control input on the RS423 port 

CUTS An American standard for frequency shift keying - ie using 

two different tones to represent logic levels 

DIN Connectors such as the cassette socket, RGB socket etc 

DRAM Dynamic RAM 

EPROM Erasable Programmable -Read Only Memory 

FIT Final Inspection Tester 

FDC Floppy Disc Controller (8271) 

IC Integrated Circuit 

ID IDentity - refers to the unique number of a given Econet 

station 

IDC Insulation Displacement Connectors - parallel cable 

connectors underneath the computer 

IEEE488 A parallel interface usually associated with automatically 

controlled test instruments 

I/O Input Output 

IRQ Interrupt ReQuest - control line on the 6502 processor 

LK PCB link 

MAO-13 Memory Access - control lines out of the CRTC 

MOS Machine Operating System 

MPU Microprocessor Unit 

NMI Non-Maskable Interrupt - control line on the 6502 processor 

PA Port A - One of the two ports of a VIA 

PAL Phase Alternation Line - coding method used for combining 

separate colour information into a single signal 

PB Port B - The other port of a VIA 

PCB Printed Circuit Board 

PET Progressive Establishment Tester 

PL Header plug 

PSU Power Supply Unit Q1 etc Transistor numbers 

85

QWERTY These are the upper left keys on the keyboard ie refers to 

the standard keyboard layout 

RA0-2 Row Address lines from the CRTC to access the RAM 

RAM Random Access read/write Memory 

RAS Row Address Strobe - Control line for the DRAM 

RC Resistor Capacitor network 

RGB Red Green Blue - individual colour signals for the VDU 

ROM Read Only Memory ROMSEL ROM SELect latch 

RS423C An internationally defined convention for serial 

transmission of data 

RTS Ready To Send - control output on RS423 port 

SK Socket 

SOT Select On Test 

SW South West 

Taw Tinned Copper Wire 

TTL Tranistor Transistor Logic a standard type of digital IC ( 

74- series) 

UHF Ultra High Frequency ` - signal for input to a TV aerial socket 

ULA Uncommitted Logic Array - semi-custom IC 

VDU Visual Display Unit 

VIA Versatile Interface Adaptor (6522) 

VR Variable .Resistor 

Z80 a' commonly used 8 bit microprocessor 

1MHz 1 MegaHertz - usually refers to the - interface bus running 

at that speed 

1MHzE Strobe to which the processor is synchronised when accessing " slow 

devices such as 6522 VIA and 1MHz bus 

2MHzF Strobe' to which the processor is synchronised when accessing " 

fast" devices such as ROM and DRAMs. 

86

BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

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