BBC Micro Model B+

BBC Micro Model B+

BBC Micro (case and keyboard are externally indistinguishable between the Model B and the B+)
Specifications
Manufacturer	Acorn Computers Ltd
Type	8-bit home / education microcomputer
Released	Mid 1985
Discontinued	Early 1986
Intro price	B+64: £499; B+128: £599 (1985, inc. VAT)
CPU	Rockwell 6512A @ 2 MHz (NMOS, externally clocked)
Memory	B+64: 64 KB; B+128: 128 KB (32 KB main + 20 KB shadow + 12 KB / 76 KB sideways)
Storage	Cassette tape; 5.25"/3.5" floppy via on-board WD1770
Display	8 modes incl. 80×25 teletext (SAA5050), HD6845 CRTC
Sound	Texas Instruments SN76489 (3 tone + 1 noise channel)
Dimensions	40 cm × 34 cm × 8 cm
Weight	4.5 kg
OS / Firmware	Acorn MOS 2.00, BASIC II, DFS 2.10
Predecessor	BBC Micro Model A/B
Successor	BBC Master 128
Codename	Acorn ICA-19
Model no.	B+ (B+64), B+128

The BBC Micro Model B+ is an enhanced revision of the BBC Micro Model B released by Acorn Computers in mid-1985 as a stop-gap between the original Proton Model B board and the BBC Master launched later that year. The B+ retained software compatibility with the Model B while addressing the Model B's two biggest limitations — only 32 K of contiguous RAM and an Intel 8271 floppy controller — by adding paged shadow screen RAM, sideways workspace RAM and the Western Digital WD1770 disc controller as standard.

Variants

Two production variants were sold:

• BBC Model B+ (B+64) — 64 KB total: 32 KB main RAM + 20 KB shadow screen RAM + 12 KB "S-RAM" sideways workspace.
• BBC Model B+128 (B+128) — 128 KB total: as B+64 plus an additional 64 KB occupying two sideways RAM banks 4 and 6 (32 K each).

The B+128 was aimed at the schools market for users of large applications such as View and ViewSheet that benefited from extra paged RAM.

Hardware Architecture

Processor

The B+ uses the Rockwell 6512A, an NMOS variant of the 6502A that runs at 2 MHz and accepts an external clock. This is a notable change from the Model B, which used a 6502A with internal clock. The 6512 was selected so that the 1 MHz bus and the 2 MHz CPU clock could be derived cleanly from the ULA. Despite the part-number change, opcode behaviour is identical to a 6502A.

Memory Map

The B+ adds two new RAM blocks beyond the Model B's 32 K main DRAM:

• 20 KB shadow screen RAM — paged in on demand for the screen, leaving the entire 32 K main RAM available for the user program even in MODE 0 (which on the Model B consumed the top 20 K of RAM for the screen).
• 12 KB sideways "S-RAM" — accessed in the same paged 16 K window as sideways ROMs, used as workspace by filing systems and printer buffers.
• Optional 64 KB additional sideways RAM on the B+128, mapped as two 32 K banks at sideways ROM slots 4 and 6.

Crucially, the BBC OS keeps the user's program model identical to the Model B, with sideways paging handled by *PAGE-aware MOS calls — most Model B software runs unchanged on a B+.

Sideways ROM Sockets

The B+ has five sideways ROM sockets, repositioned to the top-left corner of the board next to the keyboard cable. (The Model B has four sockets along the front edge.) On the B+128, two of those sockets are populated with the additional sideways RAM banks, leaving three sockets for ROMs.

Floppy Controller

The B+ ships with the Western Digital WD1770 as standard, replacing the troublesome Intel 8271 found on early Model Bs. The WD1770 supports both single- and double-density operation, has fewer software workarounds, and reads/writes DFS-format disks with the standard DFS 2.10 filing system supplied in ROM.

Other Hardware

• CRTC — Hitachi HD6845 (same as Model B).
• Teletext / MODE 7 — Mullard SAA5050 character generator.
• Sound — Texas Instruments SN76489 three-voice tone + noise generator.
• VIAs — two 6522s (System VIA and User VIA) at the same addresses as the Model B.
• ADC — μPD7002 4-channel analogue-to-digital converter (used for joysticks).
• Serial ACIA — 68B54.
• Power supply — internal Astec linear PSU producing +5 V, +12 V and −5 V.

Block Diagram

BBC Micro Model B+ block diagram showing the 6512 microprocessor, 64 K DRAM, 32 K sideways ROM bank, video processor, teletext ROM, HD6845 CRTC, WD1770 disc controller, 6522 VIAs, ADC, SN76489 sound, and Econet interface.

Firmware

• Acorn MOS 2.00 (replaces MOS 1.20 of the Model B).
• BASIC II (Roger Wilson's revised interpreter; same as later Model B production).
• DFS 2.10 for the WD1770 floppy controller.
• No real-time clock — the B+ has no battery-backed RTC, unlike the Master.

PCB Schematics and Service Manual

The B+ inherits the bulk of its main-board schematic from the Model B (Acorn Service Manual Section 1) with B+-specific changes — shadow RAM paging logic, the WD1770 sub-circuit, and the relocated sideways ROM sockets — documented in Section 2.

Block Diagram	Main Schematic
PSU Schematic	Keyboard Schematic
PCB Layout	PCB Silkscreen
PCB Traces	Assembly (Exploded)

All schematics reproduced from the Acorn BBC Microcomputer Service Manual, October 1985 (Sections 1 & 2), mirrored at the Chris's Acorns archive.

Reception and Legacy

The B+ shipped at £499 for the 64 K version and £599 for the B+128. It sold modestly in the UK schools market for the second half of 1985 before being superseded by the substantially more capable BBC Master 128 in February 1986. The B+ remained the only BBC Micro that shipped with shadow RAM, the WD1770 and Acorn MOS 2.00 as the standard configuration; the Master's MOS 3.20 carried these features forward.

Related Pages

• BBC Micro Model A/B — predecessor
• BBC Master 128 — successor
• BBC Micro B+ General Maintenance
• BBC Micro B+ Troubleshooting Guide
• BBC Micro B+ Capacitor Replacement Guide