IMSAI Updates

June 23, 2021

IMSAI Systems Restoration

As I mentioned in my previous post, I recently acquired two IMSAI 8080 computers – one original and one Fulcrum i-8080 vintage 2nd generation. My goal was to get them running and then have CP/M running on both of them. The original IMSAI came with Micropolis 5.25 inch floppy disk drives and controller and may boot when I get it working. The Fulcrum I-8080 came with a Jade Z80 S100 cpu and Morrow Disk Jockey 2D/B floppy controller, and a Morrow Thinker Toys Wunderbus motherboard. It also had a Fulcrum VIO-X video card.

I started working on the Fulcrum I-8080. The first issue was there was no memory in the system. Dusting of my ancient S100 memories, I decided to try to find static RAM boards for the system on eBay. There is more S100 memory than any other board type on eBay, but a lot of it is dynamic or off brand stuff. I found some CompuPro RAM 16 which I thought was a good deal – 32KB boards using 6116 static memory. I ordered a couple. Then as I was doing more research I discovered that many CompuPro RAM boards were difficult to use in older S100 systems like the IMSAI. They were compliant with the IEEE S100 standard while older system boards may be “pre-S100 standard” IMSAI or Altair bus interfaces with some bus lines used differently. The IMSAI front panel could also be an issue with some memory. The Vintage/Retro Computer Forums had notes about how to modify the RAM 16 to work with IMSAI style boards but it is a pain. So I found some CompuPro RAM 20 (RAM XX version) boards that have more options exposed and are easier to configure. I also got a replica Jade Bus Probe board and an S100 extender card to help with debugging.

Now I had the boards for a running system (if they worked!!).  I tested the Fulcrum power supply and the voltages looked good. In the S100 system the linear unregulated power supply provides high current DC voltages to each board and the board is responsible for regulating the voltages to +/- 5 VDC and +/- 12 VDC as needed. This uses a lot of power and generates some heat. Vintage S100 RAM boards are especially power hungry. The RAM 20 has FIVE 7805 regulators on the board.

I installed the Jade Z80 CPU, the Bus Probe,  and a memory card. When I applied power, the bus probe and front panel lit up. However the front panel switches did not have the expected results. The computer did not enter run regardless of switch inputs. I used an extender card and my Sigilent SDS1104X-E 4 channel oscilloscope. The M1 signal was not pulsing. The CPU clock oscillator was running. Reset was working. I decided to replace the Z80 CPU first. Voila! The CPU runs now. There are still other issues, but this is a good start.

One of the blue panel switches  (C&K 7113) paddle is missing, but I was able to find a couple of new matching replacements in Australia. These switches and paddles have not been manufactured for many years and are one of the hardest parts for restorers to find.  Fulcrum could not get the darker blue switch paddles that IMSAI used (they were custom made for IMSAI) and Fulcrum’s blue paddles were lighter as you can see in the photos in my original post. Unfortunately, these new paddles are lighter blue than Fulcrum’s blue paddles and do not match the other switches. It works but I will have to keep looking.

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