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 that 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.
HU1 1HJ.
FIGURE 1.1 THE BBC MICROCOMPUTER SYSTEM