Upgrading an Acorn BBC Model B

I recently won a charity auction for a BBC Model B bundle on eBay (see my original blog post here, and CRT restoration here). The Model B was a common sight in British schools during the 1980s thanks to the BBC’s Computer Literacy Project.

Acorn BBC Model B, signed by Stephen Furber.

It’s common for certain components in vintage electronics to deteriorate with time, and the BBC Model B is no exception, so these units require servicing before use following long periods in storage.

In the case of the Model B, the internal power supply contains paper X and Y mains filtering capacitors, usually RIFA brand, which are known to fail catastrophically during use and release lots of nasty, acrid smoke – this usually doesn’t cause damage, but it’s still not fun.

Luckily, this unit had already been serviced by RetroClinic before I purchased it, which included: the replacement of all the electrolytic capacitors in the PSU and on the mainboard; the replacement of the X and Y mains filter capacitors in the PSU; some miscellaneous repairs and upgrades, including a new RAM IC, a new 6522 VIA IC, a new keyboard cable, new keyboard LEDs, new key switches, and a ULA heatsink.

No worries there, then – I’m used to buying systems in sold-as-seen / untested condition (i.e. well broken), so it was refreshing to get one that had already been sorted.

Because I enjoy tinkering and I like to put my mark on all my machines, I thought I’d perform some practical modern upgrades to the machine, including:
#1 – An SD2BBC SD card interface, from The Future Was 8bit.
#2 – A 177X DFS kit, from RetroClinic.
#3 – An internal Raspberry Pi co-processor, from RetroClinic.

#1: SD2BBC

The SD2BBC is a very neat piece of kit, which plugs into the user port on the underside of the Model B and allows you to load disk images (which can be downloaded for free online) from an SD card. It comes with a pre-burned SmartSPI ROM which needs to be installed in a spare ROM socket inside the machine.

SD2BBC file loader screen.

177X DFS

The 177X DFS upgrade kit allows the use of double-density disks. It replaces the original 8271 drive controller IC (if fitted), and requires the installation of the provided DFS ROM into a spare ROM socket inside the machine, plus some miscellaneous modifications including glue logic IC swaps/removal and jumper fitting/removal.

I serviced the Opus 5.25″ FDD included in the bundle, and sure enough it reads and writes disks in both 40-track and 80-track mode via DFS, even in the fast 1772 mode.

Formatting an 80-track DSDD 48TPI 5.25″ floppy disk using DFS and the Opus 5802 FDD.

Raspberry Pi Co-Processor

The Raspberry Pi internal co-processor is a neat upgrade which takes advantage of the Tube(R) port installed in the BBC Model B, designed for use with high-speed peripherals, and can emulate many of the original co-processors which were developed by Acorn (i.e. a secondary 6502, a Z80 for CP/M, an 80286 for DOS, a 32016, a 6809, etc).

The Raspberry Pi that I purchased (a 3A+ with a 1.4GHz quad-core ARM CPU) came pre-loaded with the required firmware, so just needed to be plugged into the Tube(R) port at the back of the Model B. I mounted the board to the upper case using the provided adhesive standoffs, then ran the parallel cable underneath the mainboard to the port.

Overview

Machine mainboard before upgrade installations.
Machine mainboard after upgrade installations.

With the system reassembled, everything seems to be working great!

Testing the completed system.

Published by themightymadman

A conscientious, intelligent and committed graduate engineer, with excellent interpersonal skills, an eye for detail and a keen interest in hardware design, mathematics, and software development.

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