The UK101 is an 8-bit microcomputer based on the MOS Technologies 6502 chip. This is the same CPU as the Apple II, the Commodore PET and the BBC Micro. In common with most machines of its day, the UK101 had a BASIC interpreter in ROM. In fact, at the time, it was difficult to imagine a machine that didn't run BASIC.

Before the UK101 came the Ohio Scientific Superboard. The two machines were very similar, so much so that were was talk of legal action at Ohio Scientific. They weren't identical, though, because the UK101 worked at at UK video frequencies and had the 5-volt power regulator built-in. The layout of the PCBs for the two machines, however, is almost the same, as is the choice of chips for the “glue” logic.

In common with many microcomputers of the early 1980s, the UK101 had a memory-mapped text display, BASIC in ROM, a cassette interface and a switch-matrix keyboard. It didn't use the 6502's interrupts at all; I/O was done by polling. There was a crude expansion connector for additional memory and peripherals. The memory map was conventional for the 6502, with RAM at the lower addresses, I/O in the middle and ROM at the higher addresses.

Interfaces on the UK101 were quite limited, as on most microcomputers of the time. The video output was a 625-line, 50Hz signal modulated onto the UK UHF channel 36. A minor modification would allow the use of a composite-video monitor, if you were lucky enough to have one.

Data storage was via the 300 baud cassette tape interface, although there was no provision for controlling the motor of the cassette recorder. BASIC programs were recorded in ASCII source form, while machine code programs used a hex format. In practice, most machine-code programs initially loaded a checksum loader, which was then executed to read data in MOS Technology Paper Tape format. The Extended Monitor program would allow the user to write checksum format tapes, too.

Another minor modification would allow the use of an RS-232 interface for a printer. The baud rate was fixed at 300 baud, though, and there was no provision for handshaking. Practical Electronics published a more complex modification to generate 110 baud for a standard ASR-33 Teletype.

Most of the chips in the UK101 were absolutely standard 7400 series TTL parts, while others were obscure. All of them were packaged in standard DIL packages — surface-mount packages hadn't been introduced at the time. The 6502 CPU has become very well-known, and software for it is widely available. The memories in the UK101, 2114 NMOS static RAM chips, have a reputation for high power consumption and unreliability. The ACIA chip, the 6850, is fairly widely used in 6800 and 6502 designs. But there are a few chips in the UK101 that have disappeared into obscurity, such as the 8T28 bus buffers. The 8T28 is a four-bit bidirectional bus buffer chip, and the UK101 uses four of them, two in the VDU and two for the expansion interface.

In some cases, the original chips of the Ohio Scientific Superboard could have been replaced with more modern equivalents. The keyboard port uses a pair of 74LS125 tri-state buffer chips, which might have been replaced by a single 74LS244. Also in the keyboard port, a pair of 74LS75 latch chips could have been replaced by a 74LS374 or 74LS377. For RAM, the bank of 16 2114 static RAM chips could have been replaced by a much smaller row of four 6116 SRAMs (the 2114 is 1k x 4 bits; the 6116 is 2k x 8 bits). None of these changes would have required any software modifications.

The UK101 had a 40-pin DIL socket known as the “J1 Expansion Connector”. It was intended to be used to connect up a disk controller or other peripherals. In fact, the original Practical Electronics article claimed that it “Supports all Ohio Scientific expansions including floppy disc” and included a photograph of the disk unit plugged in.

Comp Shop planned a few add-on devices for the UK101, but the only one I ever saw was the colour video board. It consisted of a couple of 2114 memories, some multiplexers and an RGB output stage. I actually borrowed the prototype for a weekend to try it out! I don't think it was ever made into a product, but I built a similar circuit into my machine.

Other hobbyist suppliers at the time did produce expansion boards for the UK101. In fact, for a brief period there was competition for the best UK101 Monitor ROM. CEGMON was a popular monitor, produced by Mutek of Quarry Hill, Box, Wiltshire (click on advert, right, for a larger version). Chromasonics produced a colour video board, a sound board, a parallel printer interface and a memory expansion board. Several suppliers made cases and memory expansion boards.

Practical Electronics published some hardware projects based on the UK101, notably an EPROM programmer by Dr. A.A.Berk. A seven-part series appeared from January to July 1981 called “Interfacing Compukit”.