Acorn Computers Limited BBC A Montageanleitung

A Hardware Guide

For The BBC

Microcomputer.

Written By:

A. D. Derrick. B.Sc.

D. S. Harding. B.Sc.

S.D. Middleton.

M. P. Smith.

Edited and Published by:

Wise Owl Publications

Printed by Queenprint Ltd.; 64/74 Holderness Road, Hull HU9 lEQ. Telephone (0482) 224935.

Published in the United Kingdom by:

Wise-Owl Publications,

Hull Innovation Centre.

Guildhall Road,

Queens Gardens

Kingston-upon-Hull, England.

No part of this publication may be produced or transmitted, in any form or by any means, without the prior

written permission of the publishers.

First Published 1983.

The Authors would like to thank Dr. W. K. Donaldson, Ian Halstead, Instruments and Controls (Hull),

Bernadette, Catherine and Julie for their assistance in the production of this book.

The information contained within this book is given in good faith and is believed to be correct at the time

of writing. However, neither the Authors or the Publishers take any responsibility for omissions or errors.

No liability is assumed for any damages or injury whatsoever resulting from the use of information

contained within this publication.

All references to the BBC Microcomputer in this publication refer to the microcomputer produced for the

British Broadcasting Corporation by Acorn Computers Ltd.

CP/M is a registered trademark of Digital Research Corporation.

It should be noted that the terms "Econet" and "Tube" are registered trademarks of Acorn Computers Ltd.

The Authors thank the following manufacturers for granting permission to publish the data sheets given in

Appendix 1 of this book:

Hitachi

Intel Corporation

Motorola Ltd.

Mullard Ltd.

NEC Microcomputers Inc.

Rockwell International.

Texas Instruments Ltd.

RS Components Ltd.

The data sheets printed in Appendix 1 remain the copyright of the respective manufacturers.

Manufacturer’s disclaimers given within these data sheets should be strictly observed.

ISBN 0 946949 00 X

Preface

The BBC Microcomputer, manufactured by Acorn Computers Limited, was commissioned by the BBC as

part of its Computer Literacy Project. As such the machine is an educational tool, which is capable of

being expanded into a very sophisticated computer with computational powers to rival those of present

day mini-computers.

As with any complex machine it makes sense to understand something of how it works, what goes where

and how to obtain the best from it. A computer is no different and a working knowledge of the internal

functions and layout will boost your confidence in computing.

This Hardware Guide delves inside the computer to reveal some of the many modifications, upgrades

and circuit changes that can be performed. Some modifications only require extra components to be

inserted whilst others require careful work with a soldering iron on the main printed circuit board. For

those who prefer only to peek inside the lid, photographs and circuit layouts have been provided to

identify the various integrated circuits and work areas on the main board. Some handy tips, such as

getting rid of that annoying buzz found on many computers, introduce the beginner to electronics

stimulating his or her interest in both spheres of activity.

For the more advanced reader, system fault finding, further expansion and the inclusion of manufacturers

data sheets and detailed descriptions of complex circuitry make this manual a necessary adjunct to the

existing publications available.

SUPPLIERS ADDRESSES AND CODES.

Below are the names and addresses of suppliers, which are identified within the book by the following

codes. These are given as examples to assist the reader, not as recommendations.

Code A:

Acorn Computers Ltd.

Fulbourn Road, Cherry Hinton, Cambridge. CB1 4JN.

Tel: 0223 245200

Code B:

W. Broady & Son Ltd.

English Street, Kingston-Upon-Hull. HU3 2DU.

Tel: 0482 29894

Code F: Farnell Electronic Components Ltd.

Canal Road, Leeds L512 2TU.

Tel: 0532 636311

Code RR:

The Radio Resistor Co. Ltd.

St. Martins Way Industrial. Estate, Cambridge Road, Bedford. MK42 0LF.

Te1: 0234 47188

Code RS:

RS Components Ltd. (TRADE ONLY)

P. O. Box 427,

13/17 Epworth Street, London.

EC2P 2HA.

Tel: 0l 253 3040

0l 250 3131

Code T:

Technomatic Ltd.

17 Burnley Road, London.

NW10 1ED.

Tel: 0l 452 l500

01 450 6597

Code W:

Watford Electronics.

34/35 Cardiff Road,

Watford. Herts.

Tel: 0923 40588

Contents

CHAPTER 1 : INTRODUCTION.

CHAPTER 2 : ATTITUDES TO WORK AND DISCIPLINES REQUIRED.

2.1 Attitudes to Work and Working Environment.

2.2 Anti-Static Precautions.

2.3 Tools and equipment Required.

2.4 Soldering.

2.5 Desoldering.

2.6 Defluxing.

CHAPTER 3 : COMPLETE CIRCUIT DESCRIPTION

3.1 The Power Supply Unit.

3.2 The Microprocessor (MPU)

3.3 The Clock Circuitry.

3.4 Memory Devices.

3.5 Random Access Memory.

3.6 The Video Generating Circuitry.

3.7 The Video Driving Circuitry.

3.8 Cycle Stretching.

3.9 The "Econet" Circuitry.

3.l0 The Keyboard Circuitry.

3.11 The Cassette and R5423 Serial Interfaces.

3.12 The Floppy Disc Interface.

3.13 The User and Printer Ports.

3.14 The Reset Circuitry.

3.15 The Analogue Port.

3.16 The Voice Synthesiser.

3.17 The Sound Generator and Audio Amplifier.

3.18 The 1MHz Bus Interface.

3.19 FRED.

3.20 Glitches.

3.21 Double Accessing.

3.22 NPGFD (JIM)

3.23 General Guidelines For Use of the 1MHz Bus.

CHAPTER 4 : LINK SETTINGS AND FUNCTIONS.

4.1 Keyboard PCB Removal.

4.2 Removing the Main PCB.

4.3 Re-Assembling the Machine.

4.4 Link Functions.

CHAPTER 5 : MACHINE UPGRADING.

5.10 Upgrading a Model "A" Machine to a Model "B" Machine.

5.11 Insertion of IC's Into Sockets.

5.12 Connectors.

5.13 Adding the User/Printer Port.

5.14 The RS423 Interface.

5.15 The Analogue to Digital Converter (ADC) Upgrade.

5.16 Testing the Analogue Input Port.

5.17 The RGB (Red, Green, Blue) Primary Colour Output.

5.18 The "Tube" Interface.

5.19 Memory Upgrade.

5.20 Sideways ROM Selection Upgrade.

5.21 The 1MHz Bus.

5.22 Further Expansions.

5.23 The Floppy Disc Expansion.

5.24 The Speech Expansion.

5.25 The "Econet" Expansion.

5.26 The Second Processor Add-Ons.

CHAPTER 6 : HINTS, TIPS AND MODIFICATIONS.

6.1 The Cassette Interface.

6.2 Sound Output From the BBC Microcomputer.

6.3 Video Modifications and Hints.

6.4 Speech Synthesiser Tips and Modifications.

6.5 Analogue Port Modifications.

6.6 Printer Modifications and Tips.

6.7 Keyboard Hints.

6.8 Sideways ROM Socket.

6.9 Disc Interface Modifications.

6.10 R5423 Hint.

INDEX.

APPENDIX 1: MANUFACTURERS DATA SHEETS FOR MAIN INTEGRATED

CIRCUIT DEVICES USED IN THE BBC MICROCOMPUTER.

CHAPTER 1 :- INTRODUCTION.

It is the intention of the authors to provide the reader with detailed information on the hardware aspects of

the BBC Microcomputer. Some of this information has appeared in magazines but these articles usually

just give sufficient information to explain a particular project. This book goes further, giving both the

enthusiast and advanced user a detailed understanding of the internal workings of the BBC

Microcomputer, thus making it a very useful reference manual, whether undertaking projects of your own

or elaborating on projects found in magazines. There is a comprehensive link survey and a large number

of hints, tips and modifications which can be undertaken.

Servicing is a task that is best entrusted to an approved Acorn service centre. It should be pointed out

that modifying the machine yourself could invalidate the manufacturers guarantee. Further, it is possible

that you may discover a fault outside the scope of this book, or one you prefer not to tackle yourself. In

either case it would be worth consulting your local Acorn dealer first who will probably have access to

diagnostic test equipment. This should be either a Progressive Establishment Tester (PET), or Final

Inspection Tester (FIT). Both of these diagnostic and test devices are produced by Acorn Computers Ltd.

and are presently only available to official Acorn dealers.

Until late 1983 the machine was available in two options, model A and model B, both having the same

main printed circuit board, case, and keyboard. The main differences being that the model A version had

fewer IC's and connectors fitted. The BBC Microcomputer has a maximum available on-board Random

Access Memory (RAM) of 32 Kilobytes, (one kilobyte = 1024 bytes). However only half of this RAM (16K)

is fitted to the model A machine. The model B machine has the full 32K.

There is a requirement for a minimum of two Read Only Memories (ROMs) in the Machine (both A and

B), one houses the Machine operating System, (MOS) the other houses the Basic Interpreter and 6502

Assembler. There are three additional internal sockets for other ROMs (e.g. Word Processors, Filing

systems and other languages etc.). A number of hardware add-ons are available to allow a further 13

ROMs to be installed in the machine.

The Computer has a UHF output, to connect to the aerial socket of a standard television set tuned into

channel 36. In addition to this there is a composite video output and a Red, Green and Blue, (RGB)

output (model B only), to drive a black and white, or colour monitor respectively. These outputs give a

much clearer and sharper display, since they eliminate the need to modulate and demodulate the signal

on to a UHF carrier.

The Computer has eight different formats for the screen layout these are called screen modes and are

numbered 0 to 7, each giving different resolutions and colour availability. The screen display is stored

within the machines RAM, the amount required being dependant upon which particular screen mode is

selected. The highest resolution modes require 20K of RAM. Mode 7 is unique, in that it uses a special

teletext character generator IC. The advantages of using this method of character generation are that

high quality text and easy to use graphics are available using only 1K of memory.

The two most common mass storage mediums used with the microcomputer are an audiocassette

system, (models A and B), or a floppy disc system (with the relevant upgrade). The cassette interface is

very tolerant with regard to the quality of the signal and the cassette storage system. Although the

cassette interface used within the BBC Microcomputer is very reliable it still only represents a good

compromise for a data storage and retrieval system. If you intend to use the machine for any serious

work then you will probably find the cassette system rather inadequate. A floppy disc or "Econet" system

will provide a much more suitable solution.

A single floppy disc can store between 100 and 300 Kilobytes of information with a 100-fold decrease in

access time over a cassette based system.

The BBC Microcomputer has an on-board sound generator which provides four independent sound

channels. Three of these channels can be used for music synthesis each one having full envelope

control, attack, sustain, decay and release. The fourth channel is a white noise generator and uses the

same envelope control features as the other three channels.

The machine also has the capability to interface to a great variety of other equipment, through its

numerous Ports, the connectors for which are mainly situated on the underside of the case. These are as

follows: -

Floppy Disc Interface (5 ¼ and 8 inch).

A Centronics type parallel printer interface.

1 MHz Bus Extension I/O.

The Acorn "Tube" second processor port.

An 8-bit parallel "User Port".

The Cartridge ROM system.

The Acorn "Econet" local area network system.

The analogue-to-digital converter.

RS423 (RS232C compatible) serial interface.

The parallel printer port can interface with a large number of different printers, including Epson, NEC, and

Seikosha.

The 1MHz bus is used to interface the microcomputer to a whole range of slower peripheral devices such

as EPROM programmers, input output rack systems, data acquisition systems etc.

The "User Port" provides the machine with an eight bit wide input/output connection which can be used to

control external equipment such as robots and relays etc.

The Acorn "Tube" interface is used to allow the BBC Microcomputer to access a second high-speed

microcomputer called a second processor. By adding a second processor to the standard machine the

overall computing power can be greatly enhanced. The resulting improvement will depend upon the

particular second processor. At the present time there are only two second-processor boards available

these are the Z80 and a 3MHz version of the 6502. The Z80 processor will enable the BBC

Microcomputer to use the C/PM operating system and thus enable the machine to run an extremely wide

range of currently available software.

The Acorn "Econet" network feature enables up to 255 BBC Microcomputers to be connected together

and each microcomputer can have access to a central information storage and retrieval device. This

system enables a large number of users to share expensive peripheral devices and also provides each

one with the facility to communicate with any other user in the network. The "Econet" system finds most

use in schools and colleges.

The analogue port, which is located at the rear of the machine, (model B only) allows the user to connect

up to four analogue signals into the computer. These signals could be derived from all manner of

transducers thus enabling the machine to measure and record real physical quantities. This feature will

enable the machine to be used for monitoring signals in experimental or perhaps industrial applications.

The RS423 port situated at the rear of the machine (model B only) enables the microcomputer to

communicate with other pieces of equipment. This equipment could be peripheral devices such as serial

printers or perhaps other computers. In most cases the RS423 port can interface with other equipment

which uses the more common RS232C (V24) serial interface standard, although this is not always the

case. The BBC Microcomputer does not provide all of the signals necessary for a full implementation of

the RS423 standard but the main signal lines are provided. The machine’s operating system enables the

port to be configured for a whole range of baud rates ranging from 75 to 19200 although the integrity of

the data is not guaranteed at 19200 baud.

To complement the impressive graphics features of the BBC Microcomputer the model B machine is fitted

with a primary colour drive RGB output. This can be used to connect the machine directly to primary

colour drive monitors. The display produced on good quality RGB monitors is far superior to that

produced by connecting the machine to a domestic television receiver.

Figure 1.1 is an overview of the BBC Microcomputer system showing the numerous expansions that can

be added to the machine. It can clearly be seen from this diagram that the machine is capable of being

considerably expanded.

The authors of this book have endeavoured to cover all of the hardware aspects of the machine in the

hope of assisting people to understand the inner workings of what otherwise might be considered as a

“magic black box" (cream box ?). It has been our aim throughout the book to simplify, wherever possible,

the descriptions and explanations of the internal circuitry of the machine. In some instances it has been

extremely difficult to describe in simple terms what is in fact a very complex process and the casual

reader may, therefore, find certain sections difficult to understand. The manufacturers data sheets

included in appendix 1 may be of help in such instances.

Wise Owl Publications have produced a further book on the hardware aspects of the BBC

Microcomputer. This book contains a full set of manufacturers data sheets on all of the TTL 7400 and

CMOS 4000 series devices used in the machine together with data sheets on all of the devices that are

not given in appendix 1. In addition to this book Wise Owl also produce a range of hardware "add-ons" for

the BBC Microcomputer including digital-to-analogue converters and an automatic stop/start scroller

called the "Owl-Scroll". For details on any of the above items send an S.A.E. to:

Wise-Owl

Hull Innovation Centre,

Guildhall Road,

Queens Gardens,

Hull.

HUl 1HJ.

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