Restoring a UK101

 

Yes children, computers looked like this once

This contraption is a Compukit UK101 built over 40 years ago by me from a kit. If I remember right, the kit cost £99 (so must have been one of the last ones) and was smuggled across the UK-Ireland customs border in my parents' car.

Its home is a case made out of aluminium by my brother in our school's metalworking shop. Until fairly recently it resided in my father's attic before being rescued in a clear-out.

An unusual feature (by modern standards anyway) was that its manual contained full schematic diagrams. This led to the proliferation of many third-party hardware modifications, in addition to the ones mentioned in the manual itself. I implemented several of the easier ones.

Under the hood.

Under the hood, it is quite a mess! This was a result of inexperience and lack of funds to "do things right". Looking at the board now, the modifications I seem to have made (or at least attempted) are:

• Switchable Monitor ROMs
• Double-speed CPU, to 2MHz
• Double height display, to 64x32 characters
• Double-speed tape interface, to 600 baud (or maybe even 1200?)
• RS-232 interface (used to communicate with a Sinclair QL)
• 8kB RAM expansion, from 1kB (below the UHF modulator and PSU)
• Extension ROMs 
• VIA-6522 board (above the keyboard, on the right)

Some time in the 1990s, I pulled all of the ROMs and read them to use as the basis for an emulator which has been described before here. This explains the empty sockets above the keyboard, on the left.

Whether it still works or not is anyone's guess but inspired by Usagi Electric's fine work on a Centurion mini-computer, I've decided it might be an interesting project. A rough outline of the steps involved might be:

• Verify the power-supply is working. Done!
• Remove all of the piggy-backed chips and other bodges; replace IC sockets where required; test RAM chips and leave 1kB system RAM and 1kB display RAM.
• Using a 16-channel logic analyser and the excellent 6502 decoder project, verify that the system boots by comparing the CPU trace to that from the UK101 emulator.
• Clean up the video circuitry by connecting to a video monitor, either by buying an old one second-hand, or using this adapter to connect to a modern (HDMI) one.

Once all that has been done, I can start to think about adding new hardware:

• 32kB static RAM chip (62256-80 seems to be suitable) and 16kB ROM (27C128). The former will occupy the lower half of the address space (0000-7FFFh) and the latter the next quarter (8000-BFFFh).
• Display modifications: double-height, colour.
• I/O board: disk controller, VIA

One very interesting possibility would be to build a "universal hardware module" out of an Arduino (e.g., an esp32). This would run a modified UK101 emulator and provide all of the missing hardware by listening on the address bus, data bus and CPU control lines. The CPU would initially have no RAM or ROM: it would all be provided by the emulator. (This idea is reminiscent of the ROMulator as featured on Adrian's Digital Basement but would be more flexible.)

Quite a laundry list there: stay tuned!