Colin MK I

The initial system
The CPU card and the keyboard interface were the first two cards I bought, and once I received them along with their associated manuals, I set about building a rack, hand wire wrapping the bus along the back. For the +5VDC power supply, I managed to get a faulty unit from work and repair it. I designed and built my own power supply to feed the ±12VDC and -5VDC rails. The power units are housed behind the Verorack in the case.

The next card to arrive was a teletext output video card. I could see what I was typing, but had to enter a load of machine code into memory every time I switched on. The cassette interface card was next. This then allowed me to save whatever coding I was doing. Now that I had the basis of a computer, I bought an assembler / disassembler package called ZEN. It was cassette based and allowed me to write some real code, and save it !!.

Below are a few pictures of the case, power supplies and cards that formed the basic system.

Colin MK I with a full set of cards, and a 5¼" floppy in the space to the right.
Nothing spectacular about the back panel. It had a mains socet, fuse & on/off switch. Once removed, access to the power supplies was available.
The +5VDC PSU is a Gould Advance unit that I rescued from work and repaired. The toroidal transformer to the left of the main unit is for the power supply that I designed, which fed the rack with ±12VDC and -5VDC.
Looking further into the depths, reveals the wire wrapped backplane. 8 slots in all. Every connection hand wired.
The CPU card. 4MHz Z80 with full bus buffering. The front panel housed a reset switch & HALT LED.
The keyboard interface card. Operating system firmware is housed in a 2kB EPROM. The two empty sockets were for the onboard 1kB RAM. Mode 2 interrupt vectors are selected by the DIP switch at the bottom left.
The cassette interface card with a double width front panel housing the cassette ports, a printer port, and a BNC connector that could be used for basic frequency measurements.
Here's a rear view of the cassette interface front panel showing the two cassette DIN sockets, the printer 'Dee' socket and the frequency input BNC socket & wiring.
The video interface was based around a set of Teletext chips giving a 40 x 24 character colour display. The front panel facilitated connections to a TV, B/W monitor or colour Monitor.

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