I started this near the end of December 2022.

 Zbigniew Stachniac, at York University in Toronto, created an MCM/70 emulator which runs the actual machine code from an MCM/70 and which does an excellent job of emulating the experience of using an MCM/70. Looking through my correspondence with Zbigniew got me thinking about the development of the MCM/70. That involved the "rack" machine which was a hand built, wire-wrapped version of the MCM/70 with RAM in place of PROMs. Which got me thinking that it would be fun to build a replica of the rack machine using an actual 8008-1 chip and the necessary hardware to make it go. The goal isn't to make a clone of the old rack machine, that would be way beyond my level of ambition, just to make a machine that is electrically similar, connects to similar or identical peripherals, and could thus be used much like the original rack machine. This is not a small project.

So here is a photo - direct from the 1970's - of a very young me sitting in the machine room, with the original rack machine to my left.

Top to bottom we have:

To the right of the machine is a punched card reader. Yup, back in the day all the programs were done on punched cards and all the programmers time shared the one IBM 029 keypunch machine in the office. To the left of the rack machine, not shown in the photo, is a Diablo HyType daisy wheel printer and a keyboard.

A brief introduction to the architecture of the MCM/70

The 8008 has only 14 address bits, allowing it to address 16K of memory. That wasn't enough for what MCM was trying to do, which was to build a personal computer that ran the APL language. The solution was bank switching. So, in a production MCM/70, the memory  space was laid out thus:

In the rack machine the low 2K was 1702 EPROMS while the rest was RAM. What was in the EPROMS was the low 2K of the APL interpreter along with a little rinky-dink loader which could  read object code from a tape and branch to it. After that you were on your own.

A brief introduction to the Rack8 replica

The goal for the Rack8 is to have a machine which is functionally the same or very similar to the MCM rack machine. Toward that end it should have these characteristics:

Some challenges

Some things are more available than others. Intel 8008-1 CPU's are still available on eBay. I've got enough RAM and EPROMS in my parts drawers. A keyboard can be made. But Burroughs Self-Scans and those MFE-250 tape drives seem to be unobtanium, even on eBay. But the plan is to make the connectors compatible so, if I ever do manage to score a Self-Scan or a tape drive, I can plug them in and they should work. Should.

But in the meantime I need to do something. For the display I plan to make a simulated Self-Scan using a "cellphone size" display controlled by an Arduino which will be plug-compatible with an actual Self-Scan, other than not needing the +250 Volt supply. I will omit that supply until faced with an actual Self-Scan. The tape drives are more difficult. I'm game to try rolling my own, but no promises expressed or implied. I've already sourced some beefy motors with digital encoders and some tape head assemblies so I at least have some items to experiment with.