BBC Micro Model A/B

BBC Micro Model A/B
	BBC Micro Model B
Specifications
Manufacturer	Acorn Computers Ltd
Type	Home Computer
Released	1 December 1981
Discontinued	1986
Intro price	Model A: £235, Model B: £335 (1981, inc. VAT)
CPU	MOS Technology 6502A @ 2 MHz
Memory	Model A: 16 KB RAM, Model B: 32 KB RAM; 32 KB ROM (BASIC, MOS, DFS)
Storage	Cassette tape, optional 5.25" floppy disk (DFS), ROM cartridge
Display	Teletext, 640×256 (monochrome), 320×256 (4 colours), 160×256 (8 colours), 80×25 text
Sound	Texas Instruments SN76489: 3 sound channels + 1 noise channel
Dimensions	40 cm × 34 cm × 8 cm
Weight	4.5 kg
OS / Firmware	Acorn MOS (Machine Operating System) in ROM
Predecessor	Acorn Atom
Successor	BBC Micro Model B+
Codename	Proton
Model no.	Model A, Model B, Model B+

The BBC Microcomputer System (commonly known as the BBC Micro) was a series of microcomputers designed and built by Acorn Computers Ltd for the BBC Computer Literacy Project. Launched in late 1981, it became a defining educational and home computer in the UK, renowned for its robust design, expandability, and role in British computing history.

Architecture and Processor

At its heart, the BBC Micro employed the MOS Technology 6502A 8-bit microprocessor, clocked at 2 MHz. The system architecture was designed for flexibility and performance, with a fast system bus and a highly modular layout supporting a wide range of expansion options.

Key features:

CPU: MOS 6502A @ 2 MHz
System bus: 8-bit data, 16-bit address (64 KB address space)
ROM: 32 KB (split between BASIC II, Acorn MOS, and optionally DFS or other utilities)
Custom ULA: Unified Logic Array for video, keyboard, and I/O timing

Memory and Storage

The BBC Micro was available in two main variants:

Model A: 16 KB RAM
Model B: 32 KB RAM (most popular)

Both models included 32 KB of ROM, containing:

Acorn MOS (operating system)
BBC BASIC II
Optional: Disc Filing System (DFS) or other ROMs in cartridge slots

Storage options:

Cassette tape interface (standard, 1200 baud)
5.25" floppy disk drive (optional, via DFS)
ROM cartridge slots for instant software loading

Display and Graphics

Graphics and video output were handled by a custom ULA and Motorola 6845 CRTC, supporting a range of text and graphics modes:

Mode	Resolution	Colours	Text Columns × Rows
0	640×256	2 (monochrome)	80 × 25
1	320×256	4	40 × 25
2	160×256	8	20 × 25
3	640×256	2 (monochrome)	80 × 25 (text only)
4	320×256	2	40 × 25
5	160×256	4	20 × 25
6	320×256	2	40 × 25 (reduced memory)
7	Teletext (Mode 7)	8 (Teletext)	40 × 25

Teletext Mode 7: Hardware support for the UK's teletext broadcast standard, enabling high-quality text and simple graphics.
Hardware scrolling, cursor, and palette controls.

Sound Capabilities

Audio was provided by the Texas Instruments SN76489 programmable sound generator, offering:

3 tone channels (independent pitch and volume)
1 noise channel
7-octave range
Internal speaker (mono), with line-level output

The BBC Micro's sound system was widely used in educational software, games, and music applications.

Input/Output and Expansion

The BBC Micro was designed for expandability, with a comprehensive set of I/O ports and internal connectors:

User port (8-bit parallel, TTL-level)
Printer port (Centronics parallel)
RS-423 serial port (for modems, printers)
Analogue port (joysticks, ADC)
RGB, composite, and UHF video outputs
Cassette interface
Econet network port (optional)
Tube interface (for second processor expansion, e.g., 6502, Z80, 32016, ARM)
1 MHz bus (for peripherals such as ADC, IEEE-488, etc.)
ROM cartridge sockets (up to 4 sideways ROMs)

The Model B included all ports as standard; Model A required upgrades for some interfaces.

PCB Schematics & Service Manual

The BBC Micro featured a robust motherboard design, with separate logic and power supply boards in most models.

Logic Board	Logic Board Schematic
BBC Micro Model B Logic Board	BBC Micro Model B Schematic

Full service manuals and schematics are available at BBC Microcomputer Service Manual October 1985

Gallery

BBC Micro Model B (front view)
Rear panel with I/O ports
BBC Micro Model B motherboard