The Keyboard - Part 1

A project log for Rack8 MCM/70 rack machine replica

Goal: Build a replica, using an actual 8008-1 chip, of the "rack" machine used at Micro Computer Machines when developing the MCM/70.

camfarnellcamfarnell 03/15/2023 at 16:200 Comments

I wasn't entirely sure what I was going to do for the keyboard, but I got a lot of assistance from friendly folks along the way.

The MCM/70 and the rack machine used an unencoded keyboard - the processor was connected directly to the keyboard and it figured out what key has been pressed by probing the XY matrix of the keyboard. I thought about buying a commercial keyboard and hacking as needed. Then there was the matter of the key caps. The MCM machines ran the APL language exclusively and had key caps showing all the special APL symbols.

From Zbigniew I got a diagram showing the XY matrix layout of the MCM/70 keyboard, but that was just the matrix. It didn't show how the matrix was wired at the connector.

I wanted to make my keyboard pin compatible with the MCM/70 keyboard, so the next task was to figure out that wiring. Santo Nucifora of has an actual MCM/70 and on his web site has many excellent photos of the MCM/70 including all the PCBs that make it up, including the keyboard. By inspection I was able to figure out about half of the connections, but of course the traces on the one side of the PCB are obscured by the keys themselves. Santo was kind enough - and a big shout-out of thanks here - to take the keyboard out of his MCM/70 and trace out the lines for me. With that in hand I had enough information to design a plug compatible replica.

Then I came across Mike Gardi's MCM/70 Hackaday project. He has made an MCM/70 replica which looks remarkably like an actual MCM/70 while under the hood is running MCM/APL on a raspberry-pi which in turn is running Zbigniew's emulator. It's a very nice piece of work.

Mike, having made replicas of several vintage personal computers, knew where to go to get a custom keyboard and custom keycaps. For the keyboard I was directed to the Unified Retro Keyboard project, the goal of which is to make available open-source designs for retro keyboards. None of the existing retro keyboards (Apple 2, OSI etc) were what I needed but I was able to load the basic retro keyboard design into Kicad, make the necessary modifications, and send it off for production. All with the patient help of Dave, last name unknown, from the Unified Retro Keyboard project. Per Mike's suggestion the actual Futaba keys were ordered from Electronic Surplus. And for the key caps Mike wanted some more and I wanted two sets, one for myself and one for future consideration, so we did a group buy which went well.

The keyboard isn't assembled yet. There are a great many aspects of this project that need work, and only one person working on it. So the keyboard will have to wait until a few other things are done. Stand by for part-2. The photo below shows the keyboard PCB, a set of MCM style key caps and a two of the Futaba switches.