Cromemco Z-1

[Really bad picture of the Cromemco Z-1]


RAM  128k
ROM  4k

Way back in the mid 1970's before I was born, when the closest thing to a personal computer was the PDP-8/e, Ed Roberts of MITS created a computer kit known as the Altair 8800 based around the Intel 8080 CPU. In the process he created a computer bus based upon the availability of inexpensive surplus 100-pin edge connectors. This bus was quickly adopted by the DIY community and later became known as the S-100 bus.

And the MITS Altair 8800 begat the IMSAI 8080; and the IMSAI 8080 begat the Cromemco Z-1; and that is where I'll stop...

I got this machine while working at an electronics store around early 2001. I had left an 8" floppy on the counter and a customer saw it. He mentioned that he had an old machine sitting in his garage that used them. I asked him what it was and his reply was something to the effect of "I don't remember but it has a lot of lights and switches on the front".

Those magic words again: LIGHTS AND SWITCHES!!!

I asked if he were willing to sell it and he said yes. He brought it to the shop the next day and offered it to me at $50 which I gladly accepted.

When I first got it, the leftmost switch paddle was missing but it has since been replaced. Most of the photographs, except the one above, were taken before I got the switch paddle.

Basically this machine is an IMSAI 8080 with a Cromemco backplane and cards, and different front panel artwork. There is an incompatibility between the front panel and the Cromemco RAM boards so I am unable to program it via the binary switches. This is due to using different signals for memory write which were not standardised at the time the front panel was manufactured. I made a few modifications to the panel to bring it in line with the IEEE standard but not the way it writes to RAM.

Before I left Melbourne, I purchased a couple of S-100 wire wrap boards. I designed a static RAM board with banking and active bus termination but didn't get around to building it as I found myself needing to go to Scotland.

Two years after returning to Melbourne, I picked up where I left off and decided to restore this machine beyond its former glory and scoured eBay for S-100 cards to beef this little puppy up. Fun, fun, fun...

One of the first things I added to this machine was a 32k Static RAM board. I purchased a PCB for the Salota Mixed Memory card from eBay. It was designed to take sixteen 2k static RAM chips but I modified it to take one 32k chip on a daughter board. This was useful for testing the front panel. It doesn't support the Cromemco RAM banking scheme so its installation was temporary.

I next built an Ithaca Audio Display Card from a kit I purchased from eBay. It worked as advertised but is not supported by CDOS so I put it aside for future use.

I have since purchased a second Cromemco 64KZ-II RAM board bringing up the total RAM to 128k. This will be useful when I figure out how to set up Cromix. I also have a Cromemco D+7A I/O board installed which provides digital-to-analogue and analogue-to-digital conversion. The seller provided me with Z80 source code to implement it as a frequency shifter or echo/delay effect. I really have to play with this board.


For further reading on the subject of S-100 machines, I recommend checking out the S-100 Computers website as it has a wealth of information and photographs.

More pictures:

The Z-1 before the new switch paddle was purchased
ZPU - Z80 processor board
64KZ-II - 64k dynamic RAM board
TU-ART - Dual serial and parallel I/O board
16FDC - Floppy disk controller
The backplane, card cage, and power supply
The Z-1 with the lid off
Close-up of the IMSAI copyright on the front panel
Modified Salota Mixed Memory board