BBC Micro Model B+128
The BBC Micro Model B+ 128K is the BBC Micro Model B+ with an extra 64 KB of sideways RAM, announced by Acorn in November 1985 at £499. The extra memory is four 16 KB banks on a small daughter board that stands vertically at the right-hand end of the main board; existing B+ 64K owners could have their machines upgraded by an Acorn dealer, for £39.95 plus fitting according to Practical Computing.[1][2] Everything else, including the main board, power supply and keyboard, is the B+ 64K's, so the B+ guides apply: BBC Micro B+ General Maintenance, BBC Micro B+ Troubleshooting Guide and BBC Micro B+ Capacitor Replacement Guide.[3] The BBC Master 128 replaced it soon after.[4]
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| BBC Model B+ 128K, with the 128K labels on the case and keystrip (photo: Chris Whytehead, CC BY-SA 3.0) | |
| Specifications | |
|---|---|
| Manufacturer | Acorn Computers Ltd |
| Type | Home and education computer |
| Released | Late 1985 (announced November 1985)[1] |
| Intro price | £499 including VAT; 64K to 128K upgrade £39.95 plus fitting[2] |
| CPU | 6512 at 2 MHz |
| Memory | 128 KB: the B+ 64K's 64 KB plus 64 KB of sideways RAM on a daughter board |
| Storage | Cassette; 5.25-inch disc through the 1770 disc controller |
| Display | — |
| Sound | — |
| OS / Firmware | Acorn OS 2.00; BBC BASIC 2; 1770 DFS |
| Predecessor | BBC Micro Model B+ (64K) |
| Successor | BBC Master 128 |
The 128K upgrade
edit
Practical Computing explained why the B+ had looked unfinished: only 12 of the 16 sideways slots were implemented, and the board had an unused connector position on the right. The 128K board fills that position and maps its four 16 KB banks into the vacant sideways slots.[2] Acorn's fitting instructions are in Section 3 of the BBC Microcomputer Service Manual. The kit held the daughter board, a 64K Sideways RAM support disc, a new 1770 DFS ROM ("1770 IFS") and instructions, and the work was for Acorn dealers only.[5]
- Remove the main board. On its solder side, cut the track joining pins 13 and 14 of IC40, and the track to pin 10 of IC52.
- Clear the solder from position IC96 and solder the daughter board in vertically, with its solder side towards the case. If it leans towards the case side, the lid will not fit.
- Connect the ten flying leads. All except the black lead run round the back of the daughter board and under the main board to solder points on its solder side; the black lead crosses the component side to a connector.
- Replace the original 1770 DFS ROM with the one in the kit, which contains the sideways RAM utilities.
- Reassemble, then run the test on the support disc:
CHAIN "TEST", and when the prompt returns, press BREAK. The test ends with a pass or fail message.
| Wire | Signal | Length | Connects to |
|---|---|---|---|
| Brown | QA | 255 mm | IC45 pin 14 |
| Red | C/D (inverted) | 265 mm | IC46 pin 9 |
| Orange | 4M | 155 mm | IC53 pin 6 |
| Yellow | ENB (inverted) | 145 mm | IC40 pin 13 |
| Green | RSL | 125 mm | Via hole next to pins 10 and 11 of IC63 |
| Blue | 2M (inverted) | 175 mm | IC26 pin 6 |
| Violet | ENCAS | 165 mm | IC52 pin 10 |
| Grey | PGRAM (inverted) | 155 mm | IC40 pin 12 |
| White | A15 | 170 mm | IC40 pin 10 |
| Black | 0/1 (inverted) | 305 mm | North pin of link S13 |
Source: Acorn's fitting instructions.[5] Dave Curran describes the result as messy, with several of these wires soldered directly to IC pins.[3]
Using the extra memory
editAt switch-on the B+ 128K behaves as a standard 64K B+ with four banks of sideways RAM, which can hold languages and utilities loaded from disc.[1] BAS128, supplied on disc, is a 19 KB version of BASIC 2 that loads into the screen memory and uses the whole extra 64 KB for a BASIC program; Practical Computing gives PAGE as &10000 and HIMEM as &1FFFF, with shadow mode forced on and operation slower than normal BASIC.[1][2] Acorn's View programs cannot use the extra memory.[1]
Identification
editChris Whytehead's example has 128K labels on the case top, on the keystrip and at the right front, and its serial number is 01-ANB55-7504545 on a 233,000 issue 1 main board. It reports OS 2.00 and DFS 2.23.[4] Inside, the daughter board standing at IC96 with its flying leads identifies the 128K machine.[5][4]
Faults
editTreat the machine as a B+ first, using the BBC Micro B+ Troubleshooting Guide. For the extra memory, Acorn's instructions give the checks: the daughter board vertical and soldered at IC96, the two tracks cut (IC40 pins 13–14, IC52 pin 10), the ten flying leads at the points in the table above, and the DFS supplied with the upgrade fitted, because it carries the sideways RAM utilities. The test program on the support disc then reports pass or fail.[5]
Related pages
editReferences
edit- ↑ 1.0 1.1 1.2 1.3 1.4 "Enter the 128k Beeb", Acorn User, November 1985, news, PDF at Chris's Acorns (retrieved 2026-10-02). The 128K B+ announced after the 64K B+, due in the shops for Christmas at £499; existing B+ owners upgraded by Acorn approved dealers; the extra 64K as four 16K sideways RAM banks for languages and utilities loaded from disc; BAS128, a 19K version of BASIC 2 that loads into screen memory and uses the extra memory as one block; at switch-on the machine is configured as a standard 64K B+ with four banks of sideways RAM; the View family cannot use the extra memory.
- ↑ 2.0 2.1 2.2 2.3 Roger Cullis, "BBC plug-ins: life in the old dog yet", Practical Computing, December 1985, PDF at Chris's Acorns (retrieved 2026-10-02). Specification box: BBC Model B+ 128K, 6512-based, 128K RAM, £499 including VAT; 64K to 128K upgrade £39.95 plus fitting. Text: the B+ had only 12 logical sideways ROMs implemented and provision for a connector on the right of the board; the daughter board occupies that position and adds four 16K RAM banks mapped into the vacant sideways pages; a disc-based utility configures them as contiguous memory with PAGE at &10000 and HIMEM at &1FFFF, forcing shadow mode on and loading a relocated BASIC 2 at &3000; operation is slower.
- ↑ 3.0 3.1 Dave Curran, "BBC Micro B Plus 64K and 128K", Tynemouth Software blog, July 2019 (retrieved 2026-10-02). The B+ uses the same power supply and keyboard as the Model B with a rearranged main board; the 128K version's daughter board for the extra 64K plugs into a row of pins at the side of the board but also needs ten extra wires connected around the board, several soldered directly to IC pins. Secondary source.
- ↑ 4.0 4.1 4.2 Whytehead, Chris. "BBC Microcomputer Model B+ 128K", Chris's Acorns (retrieved 2026-10-02). The B+ 128K as a B+ 64K with a further 64K of sideways RAM that can hold ROM images; the vertical 64K RAM card at the extreme right of the board; the 128K labels on the case top, keystrip and right front; soon replaced by the Master; his machine: serial 01-ANB55-7504545, main board 233,000 issue 1, Rockwell R6512PE at 2 MHz, 48 KB of ROM fitted (192 KB maximum), WD1770PH, DFS 2.23, OS 2.00.
- ↑ 5.0 5.1 5.2 5.3 Acorn Computers, BBC Microcomputer Service Manual October 1985 Section 3 (BBC Microcomputer Service Manual part no. 0433,001, issue 1, October 1985), "64K sideways RAM daughter board fitting instructions", PDF pp. 3–4: kit contents; cut the track between pins 13 and 14 of IC40 and the track to pin 10 of IC52; solder the board vertically into position IC96, solder side towards the case; the ten flying leads in the wiring configuration table; replace the 1770 DFS with the one supplied, which carries the RAM utilities; test with CHAIN "TEST" then BREAK.