Editing BBC Micro Model B+

{{Infobox computer
| name         = BBC Micro Model B+
| logo         = [[File:BBC_Micro_Logo.svg|200px]]
| image        = [[File:BBC_Micro_Front_Restored.jpg|260px|frameless]]
| caption      = BBC Micro (case and keyboard are externally indistinguishable between the Model B and the B+)
| manufacturer = [[Acorn Computers]] Ltd
| type         = 8-bit home / education microcomputer
| release_date = Mid 1985
| discontinued = Early 1986
| price        = B+64: £499; B+128: £599 (1985, inc. VAT)
| os           = Acorn MOS 2.00, BASIC II, DFS 2.10
| 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)
| display      = 8 modes incl. 80×25 teletext (SAA5050), HD6845 CRTC
| sound        = Texas Instruments SN76489 (3 tone + 1 noise channel)
| storage      = Cassette tape; 5.25"/3.5" floppy via on-board WD1770
| dimensions   = 40 cm × 34 cm × 8 cm
| weight       = 4.5 kg
| predecessor  = [[BBC Micro Model A/B]]
| successor    = [[BBC Master 128]]
| codename     = Acorn ICA-19
| model        = B+ (B+64), B+128
}}
The '''BBC Micro Model B+''' is an enhanced revision of the [[BBC Micro Model A/B|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 128|BBC Master]] launched later that year.<ref>[https://chrisacorns.computinghistory.org.uk/8bit_Computers/BBCBp.html "BBC Microcomputer Model B+", Chris's Acorns]</ref> 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).<ref>[https://chrisacorns.computinghistory.org.uk/8bit_Computers/BBCBp128.html "BBC Microcomputer Model B+128", Chris's Acorns]</ref>

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 [[BBC Micro Model A/B|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.<ref>[https://stardot.org.uk/forums/viewtopic.php?t=9421 "6512 vs 6502 in the BBC B+", Stardot Forums]</ref>

=== 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.<ref>[https://stardot.org.uk/forums/viewtopic.php?t=4502 "B+ WD1770 vs Model B 8271", Stardot Forums]</ref>

=== 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 ==

[[File:BBC Micro BPlus Block Diagram.png|center|800px|thumb|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 [[BBC Master 128|Master]].

== PCB Schematics and Service Manual ==

The B+ inherits the bulk of its main-board schematic from the [[BBC Micro Model A/B|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.

{| class="wikitable styled-table" style="width:80%; text-align:center;"
! Block Diagram !! Main Schematic
|-
| [[File:BBC Micro BPlus Block Diagram.png|400px]] || [[File:BBC Micro BPlus Main Schematic.png|400px]]
|-
! PSU Schematic !! Keyboard Schematic
|-
| [[File:BBC Micro BPlus PSU Schematic.png|400px]] || [[File:BBC Micro BPlus Keyboard Schematic.png|400px]]
|-
! PCB Layout !! PCB Silkscreen
|-
| [[File:BBC Micro BPlus PCB Layout.png|400px]] || [[File:BBC Micro BPlus PCB Silkscreen.png|400px]]
|-
! PCB Traces !! Assembly (Exploded)
|-
| [[File:BBC Micro BPlus PCB Traces.png|400px]] || [[File:BBC Micro BPlus Assembly Exploded.png|400px]]
|}

All schematics reproduced from the ''Acorn BBC Microcomputer Service Manual'', October 1985 (Sections 1 & 2), mirrored at the Chris's Acorns archive.<ref>[https://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_BBCMicroServiceManual.html Acorn BBC Microcomputer Service Manual, Chris's Acorns]</ref>

== Reception and Legacy ==

The B+ shipped at £499 for the 64 K version and £599 for the B+128.<ref>[https://www.computinghistory.org.uk/det/2516/Acorn-BBC-Micro-Model-B-plus/ "Acorn BBC Micro Model B+", The Centre for Computing History]</ref> 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|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]]

== References ==

<references />

[[Category:Acorn Computers]]
[[Category:BBC Micro]]