Colin MK I


My interest in computing started in the late 70's when I was at Newcastle Polytechnic, now grandly known as Northumbria University. As part of my HND course in Electrical and Electronic Engineering, we were introduced to 'computer studies'.

Moving on from Newcastle Poly and into the real world of work, I became involved in the 'professional' business of computers. In my first three years of work, I worked in both the software and hardware sides of computing, writing bespoke software packages and repairing systems to component level.

Working with computers, and having developed an interest in them at Newcastle Poly, I decided to look around to find out how I could make & own, a computer. To start with, I couldn't just invent one, not right away, so I decided to buy the minimum hardware possible to get a computer up and running, and then start to design my own stuff.

At the time, there were some computer designs in magazines like Byte & Electronics Today International, but I wanted something I could get going quickly without having to wait for "next month's article".

Looking in the electronics magazines, I found a company called Greenbank Electronics in Merseyside, UK. They were selling kits of parts for two computer systems, both of them were rack mounted multi card assemblies. One was the 'Interak' system. The second was the 'Custom 80' system. I decided to build the Custom 80 system.

The Custom 80 computer system was designed by 'Custom Design Associates'. Each card in the system came with a double sided PCB, full kit of components, blank front panel, and a very comprehensive user manual which described the circuits for the particular card in detail.

I bought what I considered to be the minimum to get a computer going, which was :-

CPU card Z80A Processor - 4MHz clock
Full bus & signal buffering
Custom 80 Micro Bus
Keyboard interface 7 bit ASCII parallel input
2kB firmware ROM
1kB static RAM
Video card 40 x 24 character colour Teletext display
Cassette / Printer interface Dual channel cassette ports
Printer output - RS232 or 20mA
Optional BNC input for Frequency measurement

In the beginning ...
Thus began the birth of Colin MKI.
Why Colin, I hear you ask? - Why not? I reply.

When I was at Newcastle Poly, a group of us used to name objects, as you do! After leaving Poly, I still persisted in this habit for quite a while, and Colin Computer seemed appropriate. Let's face it, naming computers became something of a cult in later years, probably a trend that I started! The Amstrad PCW was famously known as 'Arnold'. Allegedly a connection with Arnold Weinstock of GEC, a deliberate ruse to confuse.

The picture below shows Colin MKI in all his glory with a full set of cards, some original, and some designed by me. This is before new cards were developed post 2010.

He's sitting on, and connected to, the dual 8" floppy drive unit which was the primary storage until I managed to acquire a full height 5¼" drive. More of that later.

Colin MK I

From left to right, the cards in the chassis are:

Z80A CPU card
Keyboard interface & monitor EPROM
Dual cassette interface
Sound and speech synthesis card
Monitor / TV Interface
Dual port RS232 card
3¼"/5½"/8" Floppy disk interface

Note: A 64kB dynamic RAM card is hidden behind the cassette interface in a vacant slot.

Input is via a 7-bit, parallel ASCII keyboard; with output displayed on a Zenith Data Systems, 12" green screen.