Acorn Computers Limited BBC A, Hardware Manual, Page: 7 / 47

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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.