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Acorn Computers Limited BBC A Hardware Manual

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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.
1

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  • Page 1: Hardware Guide

    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.
  • Page 2 Published in the United Kingdom by: Wise-Owl Publications, Hull Innovation Centre. Guildhall Road, Queens Gardens Kingston-upon-Hull, England. Copyright © 1983 Wise-Owl Publications. All rights reserved. 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.
  • Page 3 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.
  • Page 4 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:...

  • Page 5: Table Of Contents

    Contents CHAPTER 1 : INTRODUCTION. CHAPTER 2 : ATTITUDES TO WORK AND DISCIPLINES REQUIRED. Attitudes to Work and Working Environment. Anti-Static Precautions. Tools and equipment Required. Soldering. Desoldering. Defluxing. CHAPTER 3 : COMPLETE CIRCUIT DESCRIPTION The Power Supply Unit. The Microprocessor (MPU) The Clock Circuitry.
  • Page 6 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.

  • Page 7: Chapter 1 : Introduction

    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.
  • Page 8 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.
  • Page 9 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"...
  • Page 10 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.
  • Page 11 <p11 intentionally left blank for diagram>...
  • Page 12 <p12 intentionally left blank for diagram>...

  • Page 13: Chapter 2 : Attitudes To Work And Disciplines Required

    CHAPTER 2 :- ATTITUDES TO WORK AND DISCIPLINES REQUIRED. This chapter is intended to provide the less experienced reader with some of the information which he or she may require in order to carry out some of the more advanced upgrades and modifications described within this book.

  • Page 14: Anti-Static Precautions

    It is important to allow yourself plenty of time in which to carry out the work. Do not be tempted to rush because this usually results in poor quality workmanship. Also if you rush work of this type it is very likely that mistakes will be made and these could turn out to be costly.
  • Page 15 discharge is hardly worth considering recent research has proved otherwise. Recent investigations have revealed that damage caused to MOS and CMOS integrated circuits may not be immediately apparent, that is to say that the device may function perfectly satisfactorily but will fail after only a relatively short time in service.

  • Page 16: Tools And Equipment Required

    2.3 Tools and Equipment Required. In order to carry out many of the modifications described in subsequent chapters, certain basic tools are required. If possible, obtain good quality tools, as these should last a lifetime if treated with care. The following list of tools will allow the reader to carry out all of the hardware upgrades and modifications that are described in this book: No.2 cross-point screwdriver.

  • Page 17: Desoldering

    Before the soldering iron is used to make a joint, the heated tip should be thoroughly cleaned on a damp sponge and then "tinned" using rosin cored solder. For soldering delicate PCB assemblies, such as the main PCB of the BBC Microcomputer, a good quality, rosin flux cored, 60% tin, 40% lead solder of 22 SWG should be used for both tinning the bit and soldering the components.

  • Page 18: Defluxing

    2.6 Defluxing. After soldering, flux deposits will be left on the printed circuit board, which look unsightly as well as possibly causing high impedance tracks between soldered joints. These deposits can be easily removed, using a proprietary defluxing solvent (e.g. RS stock number 555-134), which should be applied using a stiff brush.

  • Page 19: Chapter 3 : Complete Circuit Description

    CHAPTER 3 :- COMPLETE CIRCUIT DESCRIPTION. 3.1 The Power Supply Unit. The power supply unit fitted to the BBC Microcomputer is a switch mode type. This unit produces +5 volts at 3.75 amps and -5 volts at 0.l amps, which are supplied to the main printed circuit board (main PCB) via seven flying leads.
  • Page 20 fed to one input of the cross-coupled NAND gates. Similarly, the 4MHz signal having been inverted by part of IC43 is coupled via a capacitor/resistor network (C41 and R94) to the other input of the cross- coupled NAND gates. This CR network delays the reset action of the bistable thus producing a modified 4MHz waveform (see figure 3.1).
  • Page 21 Figure 3.1 Generation of 6MHz Clock signal: <p21 left blank for diagram>...

  • Page 22: Memory Devices

    3.4 Memory Devices. The 8-bit 6502A microprocessor can directly address 64 kilobytes of memory. In the BBC Microcomputer, 32 kilobytes of this memory is contained within Read Only Memories (ROM s). ROM is non-volatile; that is, it retains data when the power to it is removed. The Machine Operating System (MOS) of the microcomputer is contained within 15 kilobytes of this read-only memory on a 16-kilobyte device, (normally IC 51).

  • Page 23: Random Access Memory

    3.5 Random Access Memory. The Model A BBC Microcomputer is fitted with eight 16k by 1 bit random-access memory devices in IC positions 53 to 60 providing a total of 16 kilobytes. A further 16 kilobytes of RAM is fitted to the Model B machine in IC positions 61-68 inclusive, thus providing the full 32 kilobytes of RAM.
  • Page 24 The 6845 CRTC is used to generate the raster scanned video display and also to refresh the dynamic RAM (see previous section). The horizontal and vertical synchronisation signals required for the video display are generated by this device. These two signals are fed to the SAA5050 Teletext character generator (ICS) and are also combined by a NOR gate (part of IC41) to provide the composite synchronisation (CSYNC) signal.
  • Page 25 <Figure 3.2 to go here> PALETTE 64 BJT RAM (4x16) RGB FROM SAA5050 MULTIPLEXER CURSOR CONTROL RGB out FIGURE 3.2 RGB SYNTHESIS IN VIDEO ULA A feature of the BBC Microcomputer is its ability to perform a "hardware scroll". The start-address-register within the 6845 CRTC is used to refer to the start address of screen memory.

  • Page 26: The Video Driving Circuitry

    kilobytes, this leaves an excess of 12 kilobytes. This poses a problem in that if the start of screen memory were at &3000, (as would be the case in these screen modes) then as the active display area is scrolled upwards, the 20 kilobytes required by the screen would normally enter the area of the memory map which is set aside for ROM.

  • Page 27: Cycle Stretching

    It must be stressed that any monitor connected to either the R.G.B. or video B.N.C. sockets must be electrically isolated from the mains supply. The easiest way to check this is to consult the manufactures of the monitor in question. Colour television receivers usually require a UHF modulated PAL encoded signal.

  • Page 28: The "Econet" Circuitry

    inverted inputs on the Acorn circuit diagram), is taken low. This results in the output from the NAND gate going to logic 1 , which is used to inform the cycle stretch circuitry that a slow access device requires service. The logic 1 is first inverted by part of IC33 (74LS04) before being fed to the set input of a bistable (half of IC31) and also one input of a NOR gate (part of IC~9).
  • Page 29 the data carrier detect signal (DCD) into the ADLC, the other output activates the clear to send signal (CTS) via a dual input NAND Schmitt gate. By using this technique, network activity is suspended if the clock signal is interrupted. Because of the democratic nature of the Econet system it is possible for two or more devices to attempt simultaneously to transmit data on the network.

  • Page 30: The Keyboard Circuitry

    3.10 The Keyboard Circuitry. The keyboard consists of a ten by eight matrix of normally open contact switches, which are mounted on a metal plate. Connections to the contacts of these switches are made by a glass-fibre printed circuit board. This board also carries ICs 1, 2, 3 and 4, the loudspeaker, three LED s, the keyboard connector (to PL13) and the ROM cartridge socket (if fitted) The keyboard circuitry is based on the "walking zero 0 technique.
  • Page 31 outputs respectively. The RXD, DCD and RXC outputs of the serial ULA are fed into the RXD, DCD and RXC inputs of IC4, an Asynchronous Communication Interface Adapter (ACIA) type MC6850. The ACIA converts the data to a parallel form, which is then presented to the data bus of the microprocessor, on demand.

  • Page 32: The Floppy Disc Interface

    3.12 The Floppy Disc Interface. The BBC Microcomputer was designed for use with either one or two 5 1/4 inch or 8 inch floppy disc drive units. These drives may be single or double sided. In order to use floppy disc drives with the BBC Microcomputer, the disc interface circuitry must be fitted to the main PCB of the machine (see chapter 6).

  • Page 33: The User And Printer Ports

    3.13 The User and Printer Ports. IC69 is a 6522 Versatile Interface Adapter (VIA) which is used to provide both a standard Centronics parallel printer interface and the user port on the BBC Microcomputer. The printer port uses lines PA~ to PA7 of this VIA, which are buffered by an octal 3-state driver (IC70).

  • Page 34: The Voice Synthesiser

    Connection to the inputs of the ADC is by a printed circuit board mounted, fifteen way, "D-type" connector socket (SK6). This socket also carries the connections to the light pen strobe input of the 6845 cathode ray tube controller (IC2) and the joystick "fire" buttons. 3.16 The Voice Synthesiser.
  • Page 35 (IC19). The resulting analogue signal is fed to PL16, which is the low-level (DIN) audio output connector. The same signal is also fed to a pre-set potentiometer (VR1), which acts as a volume control for the internal power amplifier of the computer. This power amplifier is a low supply voltage device, type LM386 (IC19).

  • Page 37: The 1Mhz Bus Interface

    3.18 The 1MHz Bus Interface. In addition to the high speed "Tube” interface the BBC Microcomputer also has a 1MHz extension bus. Connection to this interface is through PL11 on the underside of the machine. The 1MHZ extension bus provides buffered address lines A0 to A7 (IC71), buffered bi-directional data bus lines D0 to D7 (IC72) and numerous control lines.

  • Page 38: Glitches

    Normally devices attached to the 1MHz bus should decode the address lines A~ to A7 and also the NPGFC line (FRED). In most cases the signal on the NPGFC line will need to be "cleaned up" by one of the circuits shown in figure 3.5 below. The reason that the NPGFC and NPGFD signals need to be cleaned up is because of the difference in the speed that the 1MHz bus is running at, compared to the speed at which the machines internal 65~2A processor is running.
  • Page 39 glitches are marked X and Y in figure 3.4. The glitches which occur when the 1MHz E clock is low (marked Y in figure 3.4) are normally of no consequence but the glitches occurring when 1MHz E is high (marked X in the diagram) may cause the chip select inputs of the various IC s on the 1MHz peripheral device to be spuriously activated.

  • Page 40: Double Accessing

    3.21 Double Accessing. It is possible for a peripheral device, which is attached to the 1MHz bus, to be accessed twice by the CPU. This happens as a result of the way in which the pulse stretching circuitry within the microcomputer works.
  • Page 41 <p41: fig 3.5B and 3.5C>...

  • Page 43: General Guidelines For Use Of The 1Mhz Bus

    3.23 General Guidelines For Use Of The 1MHZ Bus. Acorn Computers Ltd. have issued certain guidelines which should be adhered to when designing devices to attach to the 1MHz bus, these are summarised as follows; i) The maximum permissible loading on any one line of the 1MHz bus is 1 low power Schottky TTL load. In practise this will mean that all lines will have to be buffered by the peripheral device.

  • Page 44: Keyboard Pcb Removal

    Depending on the particular links that are to be modified it may be necessary to remove the keyboard in order to gain access. Links S4, 58, S10, S14, S17, S18, and S30 are those in question on boards up to issue 3.

  • Page 45: Re-Assembling The Machine

    All that remains now is to remove the four screws that secure the main PCB to the base with a No.2 cross-point screwdriver. The positions of these screws are shown in the photographs at the end of chapter 5. The PCB can now be removed from the base and carefully placed on a work surface which should be clean and free from static charge.

  • Page 46: Link Functions

    iv) Conduct a detailed visual inspection of the PCB s paying particular attention to those areas which have a high population of discrete components such as resistors and capacitors. These areas are mainly around the UM1233 video modulator in the NORTH EAST corner of the main PCB, the DIN sockets at the rear of the machine and the "Econet"...
  • Page 47 value of 300 ns and a maximum value of 0. Any attached peripheral device should, therefore, be able to cope with this. iv) The recommended method of connecting the peripheral device to the 1MHz bus is via a 600 mm length of 34 way ribbon cable terminated at each end with a 34 way IDC header socket i.e.

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