BBC Micro Model A/B

BBC Micro Model A/B

BBC Microcomputer Model B
Specifications
Manufacturer	Acorn Computers Ltd
Type	Home and education computer
Released	1 December 1981; most of the initial run delivered during 1982
Discontinued	Model A withdrawn after the Acorn Electron launch; Model B superseded by the Model B+ in 1985
Intro price	Launch: Model A £235, Model B £335; raised to £299 and £399
CPU	6502A @ 2 MHz
Memory	Model A 16 KB RAM, Model B 32 KB RAM
Storage	Cassette (300 or 1200 baud); 5.25" floppy via the optional disc interface
Display	Teletext mode 7; up to 640×256 in 2 colours or 160×256 in 16 logical colours
Sound	Texas Instruments SN76489: 3 tone channels + 1 noise channel, ADSR envelope control
OS / Firmware	Acorn MOS in ROM, with BBC BASIC in a separate 16 KB language ROM
Predecessor	Acorn Atom
Successor	BBC Micro Model B+
Codename	Proton
Model no.	ANA01, ANA02 (Model A); ANB01–ANB04 (Model B)

The BBC Microcomputer — the "Beeb" — was designed and built by Acorn Computers for the BBC's Computer Literacy Project and launched on 1 December 1981, though production problems pushed most of the initial run into 1982. It became the standard computer in British schools and one of the defining British machines of the 1980s.

Acorn supplied it at two levels of hardware provision, Model A and Model B, on the same circuit board: a Model A can be upgraded to a Model B.

Prices

The Model A launched at £235 and the Model B at £335. The prices rose almost immediately — the Model A to £299 and the Model B to £399 — because Acorn could not build the machine at the lower price.

Models and variants

Acorn's own model codes, which appear on the machine and in the service manual:

Code	Machine
ANA01	Model A, cassette only
ANA02	Model A with the Econet interface fitted
ANB01	Model B, cassette only
ANB02	Model B with Econet
ANB03	Model B with the 8271 floppy disc interface
ANB04	Model B with both disc and Econet interfaces

Early machines carried a "BBC Microcomputer" badge on the Perspex keyboard strip. A trademark dispute with Brown Boveri led to the badges being changed to read "British Broadcasting Corporation".

Processor and memory

The machine uses a 6502A at 2 MHz. Model A has 16 KB of RAM and Model B 32 KB, in 4816 DRAMs at IC53–IC68.

ROM is not a single 32 KB block. There is a 16 KB Machine Operating System ROM at IC51 and a separate 16 KB language ROM holding BBC BASIC at IC52, and up to four 16 KB "sideways" ROMs can be paged in at any time at sockets IC52, IC88, IC100 and IC101. Those paged sockets hold Pascal, word processors, CAD software, the disc and Econet filing systems or teletext acquisition software.

The Model A/B has no cartridge slot. Sideways ROMs are fitted internally, in sockets on the main board. The only cartridge interface Acorn offered for this machine came with the optional speech synthesis upgrade.

Custom chips

ULA stands for Uncommitted Logic Array, and this machine has two of them, not one. Acorn's component location table gives:

Position	Device	Function
IC1	6502A	Processor
IC2	6845	CRT controller
IC3	6522	Internal (system) VIA
IC4	6850	ACIA, serial and cassette
IC5	SAA5050	Teletext character generator
IC6	5C094	Video ULA
IC7	2C199	Serial ULA
IC18	SN76489	Sound generator
IC51 / IC52	27128	Operating system ROM / BASIC ROM
IC69	6522	User and printer VIA (Model B)
IC73	µPD7002	4-channel analogue-to-digital converter (Model B)
IC78	8271	Floppy disc controller (optional)

Later machines could be fitted with a 1770 disc interface daughter board in place of the 8271; Acorn documented that upgrade, and a 64 KB sideways RAM upgrade, in Section 3 of the service manual.

Display modes

The definitive mode list, from Acorn's own specification, with the screen memory each mode uses:

Mode	Resolution	Colours	Text	Screen RAM	Model A?
0	640 × 256	2	80 × 32	20K	no
1	320 × 256	4	40 × 32	20K	no
2	160 × 256	16	20 × 32	20K	no
3	text only	2	80 × 25	16K	no
4	320 × 256	2	40 × 32	10K	yes
5	160 × 256	4	20 × 32	10K	yes
6	text only	2	40 × 25	8K	yes
7	teletext	teletext	40 × 25	1K	yes

Two things are commonly got wrong here. Mode 2 has 16 logical colours, not 8 — eight steady and eight flashing. Modes 3 and 6 are text-only; they are not reduced-memory graphics modes. A Model A has only 16 KB of RAM and so can use modes 4, 5, 6 and 7 only.

Mode 7 is a hardware teletext display to the UK broadcast standard, with character rounding, double height, flashing and coloured backgrounds, and it costs only 1 KB of RAM. In the non-teletext modes, characters are user-definable and can be positioned anywhere, not only on the character grid, and text and graphics windows can overlap.

Sound

The SN76489 at IC18 gives three tone channels and one noise channel with full ADSR envelope control, driving the internal loudspeaker through IC19 (an LM386).

Video outputs

• UHF on a phono connector: 625-line, 50 Hz, interlaced, fully encoded PAL, modulated on channel 36.
• Composite video on a BNC connector, for a monochrome or PAL colour monitor.
• RGB on a 6-pin DIN: separate RGB and sync at TTL levels, RGB high true, sync link-selectable high or low true, pulse duration 4.7 µs.

Interfaces

Model A has the cassette interface and, optionally, Econet. Everything else below is Model B — the Model A board is missing the connectors and some of the components.

• Cassette: 7-pin DIN, 300 or 1200 baud to the Computer Users Tape Standard, with full automatic motor control.
• RS423 serial: software-selectable between 75 and 9600 baud, with two-way RTS/CTS handshaking. The software to drive it is only present from MOS 1.2 onwards.
• Parallel printer (Centronics).
• User port: 8-bit input/output with 2 control bits, on the IC69 VIA.
• Analogue port: four channels, input range 0 to 1.8 V, 10 ms conversion time per channel. The µPD7002 is a 12-bit converter but only 9 or 10 bits are significant; with averaging the full 12 bits can be approached.
• 1 MHz bus: buffered extension bus for a teletext acquisition unit, IEEE-488 interface, Winchester disc and similar.
• Tube: high-speed connection to a second processor. Acorn's own list at the time was a 3 MHz 6502 with 64 KB and a Z80 with 64 KB running CP/M; a 32016 second processor followed, and the ARM1 first appeared in the ARM Evaluation System, itself a BBC second processor in the standard "cheese box".
• Econet: optional network interface.
• Floppy disc interface: optional; drives ranged from single-sided single-density 100 KB to dual double-sided double-track-density 800 KB.

Keyboard

73 full-travel QWERTY keys including 10 user-definable function keys, with two-key rollover and auto-repeat, and a clear plastic strip above the function keys for a legend card.

Power supply

Most production machines have a switch-mode supply; a small number of early machines have a linear supply with a conventional mains transformer and regulators. On the linear unit the three outputs come from separate regulators, so it is possible for power to appear on two of the three output pairs and not the third — a useful diagnostic. The internal supply is built to BS 415 Class 1 and the mains fuse specified is 3 A to BS 1362.

Mains voltage is present on the primary side and the bulk capacitors hold a charge after switch-off. See Capacitor Failure Symptoms before working on either type.

Board issues

The main board went through issues 1 to 7, and Acorn's service manual documents the circuit changes between issue 3 and 4, and between issue 4 and 7, alongside a full link list with each link's X-Y position in millimetres from the south-west corner of the board. Links 18 and 19 control fast or slow access to the paged ROM sockets — worth checking when a sideways ROM behaves oddly.

Documentation

Logic board	Logic board schematic
BBC Micro Model B logic board	BBC Micro Model B schematic

The full service manual — Acorn part 0433,001, Issue 1, October 1985 — is hosted here as BBC Microcomputer Service Manual October 1985, covering the technical specification, general circuit description, upgrading the PCB, the link list, servicing and fault-finding, the interfacing survey, the component location tables and the circuit diagrams.

The user documentation hosted on this wiki includes the BBC User Guide, the BBC Micro Advanced User Guide, the BBC Micro BASIC ROM User Guide and BBC Micro Assembly Language Programming.

Gallery

• 
  BBC Microcomputer Model B
• 
  BBC Micro Model B main board

Related pages

• BBC Micro Model A/B General Maintenance
• BBC Micro Model A/B Troubleshooting Guide
• BBC Micro Model A/B Capacitor Replacement Guide
• BBC Micro Model B+
• Acorn Electron
• Recommended Tools