Date   : Sun, 14 Mar 1999 23:14:21 +0100
From   : "Mark Usher" <marku@...>
Subject: Model A & Model B...

Ok, about time I came in on this one. Here are the differences according to
the BBC Service manual.

Mark
BBC Documentation Project
http://members.magnet.at/marku/bbc.htm
ICQ: 21008976


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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 synthesisinq 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 l.

* 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.

* 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 alpha-numeric
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
user-defined 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 5: 160 x 256, 4 colour graphics and 20 x 32 text (10K)
Mode 6: 40 x 25, 2 colour text only (8K)
Mode 7: 40 x 25, Taletext 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.

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 0: 640 x 256, 2 colour graphics and 80 x 32 text (20K)
Mode 1: 320 x 256, 4 colour graphics and 40 x 32 text (20K)
Mode 2: 160 x 256, 16 colour graphics and 20 x 32 text (20K)
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-paddies 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.

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