BBC Microcomputer System Information Sheet G2

BBC Microcomputer 

System

BBC Microcomputer System Plan 

*Options requiring additional 

hardware within BBC Microcomputer

■ Processor 2 MHz 6502A 

■ Memory 16K ROM BASIC 

16K ROM Machine Operating System 

32K RAM (16K on Model A) 

■ Keyboard 73 key full OWERTY layout, including 10 user definable 

function keys, 4 cursor control keys, two key rollover 

and auto repeat. 

■ Display Mixed high resolution graphics and upper and lower 

case text. 

Full colour on all 8 display modes: 

0 640x 256 2 colour graphics and 80x 32 text 

1 320x 256 4 colour graphics and 40 x32 text 

2 160 x 256 16 colour graphics and 20 x 32 text 

3 80 x25 2 colour text 

4 320x256 2 colour graphics and 40x32 text 


6 40 x25 2 colour text 

7 40x25 Teletext display 

UHF, composite video and separate RGB and Sync 

outputs. 

■ External Storage 300 baud and 1200 baud interface (with motor control) 

for standard cassette recorders. 

■ Tone Generation Three-voice music synthesis with full envelope control 

feeding internal loudspeaker. 

■ Printer Interfaces 1 8 bit plus full two-line handshake "Centronics" port. 

(Model B Only) 2 75 baud to 9600 baud software selectable serial 

port to drive RS232 devices with full two-line 

handshake. 

■ User Input/Output 8 bit parallel input/output port. 

(Model B Only) 

■ Analogue Inputs Four analogue inputs for games, paddles or control 

(Model B Only) applications. 

■ Expansion Within the computer (at extra cost) 

Capabilities Floppy Disk Interface 

Econet Interface 

Speech Synthesis 

Cartridge ROM Pack Interface 

"Tube" connector for second processor option. 

■ Second Processor The Computer is designed so that it can be expanded 

Option to run with a second processor and considerably 

expanded memory. 

Planned expansion units which are connected via the 

"tube" include: 

3 MHz 6502 with 60K RAM 

Z80 with 60K RAM running CP/M 

16 bit processor with 128K RAM. 

3

Model A BBC Microcomputer System 

A fast, powerful self-contained computer system generating high resolution 

colour graphics and capable of synthesising 3 part music. 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 with 10 user definable function keys. The 

keyboard has two key rollover and auto repeat. 

■ Internal 240/115V power supply is fully encased and made to BS Class 1. 

■ The internal loudspeaker is driven from a 3-voice music synthesis circuit with 

full ADSR envelope control. 

■ A fully modulated PAL colour television signal, for connection to a normal 

domestic television aerial socket, is available through a phono connector. 

■ 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 at 300 or 1200 baud using the CUTS 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 6502 and includes 16K of Random Access Memory. 

■ A 16K Read Only Memory (ROM) integrated circuit contains an extensive and 

powerful Machine Operating System designed to interface easily to high level 

languages. 

■ A further 16K "Language ROM" contains an extremely powerful and 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 Language 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, Disk and Econet routines or Teletext 

acquisition software. 

■ The standard television output is 625 line 50 Hz, interlaced, fully encoded P"' 

modulated on UHF channel 36. Other standards are available. 

■ The full-colour Teletext display of 40 characters by 25 lines has full character 

rounding with double height, flashing, coloured background and text— all to the 

Teletext standard. 

■ The non Teletext display modes 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 40x32 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 and scrolled 

separately. 

■ The Model A is able to support the following modes: 



6 40x 25 2 colour text 

7 40x 25 Teletext display

■ All graphics access is "transiparent" resulting in a fast snow-free display. 

■ Extensive support is provided in the Machine Operating System for the 

graphics facilities, and this is fully reflected in the BASIC interpreter. These 

facilities include the ability to rapidly draw lines and to fill large areas of colour. 

In addition, very rapid changes of areas of colour can be effected. 

■ The Model A BBC Microcomputer System can be expanded at any time to the 

Model B System. In addition, or as an alternative, other facilities such as the 

Econet, may be fitted within the computer systems. 

Model B BBC Microcomputer System 

The Model B BBC Microcomputer System is an enhanced version of the Model A 

Microcomputer but with the following differences: 

■ 32K Random Access Memory (RAM). This enables all the graphics modes to be 

used 

0 640x256 2 colour graphics and 80x32 text (20K) 

1 320x 256 4 colour graphics and 40x32 text (20K) 

2 160x 256 16 colour graphics and 20x32 text (20K) 

3 80x 25 2 colour text (16K) 



6 40x 25 2 colour text (8K) 

7 40x 25 Teletext display (1 K) 

■ The installed RAM is divided between the high resolution graphics display, the 

user's program and Machine Operating System variables. The 'Machine 

Operating System requires about 3 1/4K of RAM in the Model A. If higher 

resolutions are required with large programmes then the second processor 

option may be fitted. 

■ 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. Connection to the machine is made via a 

5 way "diamond" DIN connector and various interconnecting plugs are 

available for the various standard 25 way D type circuits. 

■ An 8 bit "Centronics type" parallel printer port is provided with "Strobe" and 

"Acknowledge" lines. 

■ An 8 bit input/output port is also provided. 

■ 4 12 bit analogue input channels are provided. Each channel has an input 

voltage range of 0-2.5V and the internal converter provides a number in the 

range 0 to 4095. The conversion time for each channel is 10 milliseconds and 

when the conversion is complete, the processor is interrupted and the value 

stored in a memory location for later access. These analogue inputs can be 

used not only in laboratory control situations, but also as inputs for 

games—paddles or joysticks. 

■ A 1 MHz buffered extension bus is provided for connection to PRESTEL, 

Teletext or various other expansion units. 

■ All interface sockets to external peripherals, are fitted to the Model B. These 

include R/G/B/Sync for colour monitors, Econet, serial interface, parallel printer, 

disk and tube. Having the interface sockets fitted enables the internal 

expansions mentioned above to be fitted without further soldering. 

5

Both Model A and Model B may have the following expansion options fitted 

internally at purchase, or by Dealers at a later date. 

■ Floppy disk interface 

■ Econet network interface (separate leaflet available) 

■ Voice synthesis circuits 

■ Cartridge ROM pack interface 

■ Various alternative high-level languages in ROM 

External options which plug directly into the machine include: 

■ Paddles 

■ Cassette Recorder 

■ Black and White and colour monitors and televisions 

■ 5 1/4" single-sided disk drive (100K) 

■ 51/4" dual double-sided double track density disk drives (800K) 

■ 80 column dot-matrix printer 

■ Daisy Wheel Printer 

■ Teletext acquisition unit t both of these enable Telesoftware to be 

■ Prestel acquisition unit f downloaded into the BBC Computer as well as 

providing access to the normal 

Teletext/Prestel services. Pages may be 

"grabbed" and stored for later use 

■ 3MHz 6502 second processor with 60K of RAM 

■ Z80 second processor with 60K of RAM and a fully CP/M-compatible operating 

system 

Considerable attention has been paid to the overall design of the system and 

application software. A modular approach has been adopted specifically to ease 

the interfacing of various high-level languages (such as BASIC and Pascal) to the 

operating system. 

■ Machine Operating System (MOS) 

A 16K ROM is used for the MOS. This software controls all input/output device , 

using a well defined interface. The MOS supports the following interrupts: 

❑ Event Timer (used as elapsed time clock) 

❑ 4 channel analogue to digital converter 

❑ Vertical sync 

❑ Keyboard and keyboard buffer 

❑ Tube byte transfer 

❑ Music tone generation and buffer 

❑ Serial interface, input and output, and buffers 

❑ Parallel input/output port 

and "hooks" are provided to support other devices such as: 

❑ Teletext 

❑ Prestel 

❑ Econet file system 

❑ Disk file system 

Many of the operating system calls are vectored to enable the user to change 

them if required at a future date.

■ BASIC 

The BASIC interpreter is an extremely fast implementation, very close to the 

Microsoft standard but with numerous powerful extensions: 

❑ Long variable names 

❑ Integer, floating point and string variables 

❑ Multi-dimension integer, floating point and string arrays 

❑ Extensive supportfor string handling 

❑ IF ... THEN ... ELSE 

❑ REPEAT... UNTIL 

❑ Multi-line integer, floating point and string functions 

❑ Procedures 

❑ Local variables 

❑ Full recursion on all functions and procedures 

❑ Effective error trapping and handling 

❑ Cassette loading and saving of programs and data 

❑ Full support for the extensive colour graphics facilities 

❑ Easy control of the built-in music generation circuits 

❑ Built-in 6502 mnemonic assembler enabling BASIC and assembler to be mixed, 

or pure assembly language programs to be produced. 

■ Other High Level Software 

A range of other high level software is presently under development or being 

commissioned. It is envisaged that a number of the larger programs will be 

available on ROMs which can be left in the machine and used as needed. It is 

hoped that these will include Pascal, Computer Aided Design package, and Word 

Processing. Details will be announced. 

■ Applications Software 

A range of applications packages is being developed 

atthe same time as the computer equipment. Each computer will be 

accompanied by a substantial, free 'welcome' cassette which 

will illustrate the wide range of things the computer is 

capable of doing. Other packages within the initial inventory, 

to be ready late 1981, include: 

❑ A home finance package 

❑ A computer aided design 'starter' package 

❑ A music package 

❑ Games packages 

❑ A home data-base package 

❑ Various programsto develop skills in primary school Maths and English, 

and in pattern matching 

❑ Scientific and business simulations 

❑ Programs which 'learn' 

At a later date (early 1982 onwards) more substantial programs which are 

envisaged include word processing, enhanced graphics packages, 

engineering packages, a VAT. package, a high level language utility, energy saving 

calculations, a more advanced music package, an astronomy pack, etc. Details 

of the full BBC software library will be announced as they become available.

The Econet communications network enables a number of computers to share 

expensive resources such as a printer and a disk "file server". The system is 

primarily intended for schools and colleges but also lends itself to many office and 

business applications. 

■ Up to 254 stations may share the network facilities 

■ Connection between stations is by cheap 4 wire "telephone" cable 

■ The network may be up to 1 kilometre from end to end 

■ Very low interface cost on each computer 

■ More than one printer or file server may be on the net 

■ Any station can "VIEW" any other similar station's screen 

■ Messages may be passed between any machines 

■ All inter-machine interaction can be barred to any specific machine or group of 

machines 

Because of the low costs involved, it is possible to dedicate one computer as art re 

server and one as a printer server. However, once pupils have loaded files from 

the file server, there is no reason why the file server computer should not be used 

as a normal disk computer until it is again required to act as a file server. The same 

flexibility applies to the printer server. 

Technical support and service can be obtained from a number of sources: 

■ Pre-sale advice (please send a large S.A.E.) from 

BBC Microcomputer System 

PO Box 7 

London W3 6XJ 

■ After sales service and advice by a National Dealer Network 

■ Warranty repairs by Currys Ltd 

This description and specification is subject to change without notice. 

September 1981 Information Sheet G2

BBC Microcomputer System Information Sheet G2

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