Colin MK I

System developments
After a few months of writing small assembly language programs in HEX and saving them to cassette, I got somewhat bored and wanted to progess to some real programs. Hand coding wasn't a problem, as that was how I wrote short diagnostic programs for the process control computers I worked on, but I wanted more!! Having decided I knew enough to start designing my own parts for the computer, the most obvious expansion was going to be memory.

The buttons below the picture gallery give brief descriptions of the developments to Colin MK I, some of which were designed by me.

Below are a few pictures the updates to Colin MK I, including some of my original design drawings.

Colin MK I with all the cards installed. The RAM card is located behind the double width cassette interface card.
128k DRAM card, populated with the 1st 64k of memory .
Original design drawing for the DRAM card. The design used a Texas Instruments TMS4500A DRAM controller chip.
Original layout drawing of the DRAM card.
The floppy disk control card that I bought. This came with a modified operating system EPROM to allow booting from disk. Custom 80 logo is at the top right of the card.
This is a top view of the dual 8in floppy drive unit that I managed to acquire from work. Compact storage eh? NOT !!
Dual channel RS232 card with full hardware handshaking on both ports, based on the Z80 SIO. Baud rate timing provided by an 8253 counter / timer chip. Utilised mode 2 interrupts.
The sound card. Stereo provided by a pair of General Instrument's AY-3-8910 chips. Speech synthesis through a GI chip, the SPO256-AL2.
A very basic SRAM card giving 64k of memory, using 2 x 32kB SRAM chips.


At the time I started the quest for more memory, dynamic RAM was cheaper than static and therefore, obviously the way to go. Unfortunately, it was more complex to use. A couple of buffers & a bit of static RAM and you were sorted. With dynamic RAM, you had to consider refreshing the memory if you didn't want to lose your data. The Z80 did generate a refresh signal, but there were limitations to it's usefulness.

   It only provided a 7 bit refresh address which was no use for 64k dynamic rams. I wanted to use 64k dynamic rams.

   The refresh signal was not produced during DMA situations. The external source which had been granted the address bus was responsible for refreshing the memory when it was in control. No external refresh - No memory retention.

Enter the Dragon 'Texas Instruments' and the 'TMS4500A' dynamic RAM controller. Armed with the chip and the application guide, I set about implementing a memory upgrade as shown in the TMS4500 booklet. Sorted !!. As I was only using a Z80 at the time, I decided not to populate the upper 64k of memory, but to include sockets for it.

Loading and saving code to a cassette recorder was fine, but soooo slow, so I started to investigate what would be required to add floppy disk storage to my computer. I had access to loads of data sheets at work and liked Western Digital kit. While reading up on floppy disk controllers, the Custom 80 system on which my computer was based acquired a floppy disk card.

Keen to try floppies, I decided to buy the card to shortcut development time. The card came with an updated EPROM for the system and I managed to get hold of a Shugart dual 8" floppy drive from work. Also through contacts, I acquired a copy of CP/M 2.2 on single sided 8" media. Once I had customised CP/M as per the instructions in my CP/M guide, I had a fully working system that I could start to get to grips with.

Serial stuff

EPROM stuff

Centronics interface

Sound card

64kB of static RAM. A very basic card with no buffering. As I was looking to the future and the design of my new computer, Colin MK II, this was a simple trial using static RAM as I did not intend to use dynamic RAM in my new design.

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