Why IMSAI Here?

I know some folks are wondering why I am discussing a pair of IMSAI computers on a blog primarily about restoring Computer Automation LSI/2 16 bit mini-computers…

Well, here is my logic – both types of systems have “programmer’s” front panels that show what is happening inside the machine, and allow you to stop, start, debug, single step, and input data. Also, the Northstar Horizon and IMSAI 8080 S-100 computers were my second and third computers, in my house, (computers that I actually owned, instead of leased ). The Alpha 16 and LSI/2 minicomputers were the first computers I had possession of – even though they were leased from Computer Automation.

The second reason is that the LSI/2 and IMSAI S100 computers are actually quite similar – except for early S100 systems having an 8 bit data bus and CPU. Therefore, easing back into assembly programming is actually easier on the IMSAI 8080 and Z80 systems because there are a large number of more advanced programming tools – assemblers, debuggers, emulators, etc. that can be run on the hardware, or emulated on a more modern PC system.

Plus, both types of systems were primarily programmed in assembly language. I know, a lot of S100 systems with CP/M ran MBASIC-80 or CBASIC, but for systems programming, it was done in assembly – look at WordStar, Turbo Pascal, MBASIC-80, Modem7, Kermit, Xmodem, and other utilities. All were written in 8080 assembly.

Another bonus is that the 8080 and Z80 systems have tons more documentation readily available on the internet.

Finally, the 8080 and Z80 systems have the capability to run an Operating System – CP/M – on floppy disks or even flash or SD cards. This means that I can relearn my skills with less friction and pain, then apply them to the LSI/2 which has more a primitive work environment, at least until I can get some floppies working and disks rebuilt.

So am I backing out of the Computer Automation LSI/2 restoration? NO! In fact, since I am moving soon, I have decided I need a new place with enough space to set up all of my vintage systems and be able work on them in parallel – including the THREE LSI/2 systems I own.

IMSAI Restore continued…

So how are the IMSAI’s doing? Well, after replacing the Z80 in my Fulcrum I8080 with the Jade Z80 CPU, I was able to see some functionality from the CPU. However, depositing data and viewing memory still didn’t work correctly. However, now I could read the boot EPROM in the Jade board correctly.

Next step was to see if it was the CPU, the memory, or the CPA front panel causing issues.

I built a replica IMSAI CPU-A board with an 8080-A CPU as the simplest CPU card that should absolutely work with the front panel. It mostly ran, but I still could not write to memory. So I decided to re-chip the front panel.

By the way, it is a Fulcrum replica version of the IMSAI CPA front panel PCB in the Fulcrum branded IMSAI. This version was re-laid out (with tape) from negatives of a bare original CPA, with all of the IMSAI fixes applied. I should know – I did the tape up myself at Fulcrum in 1981. I was hoping that either corrosion or bad flip-flops or one shots was the cause of the write problem. But no luck. Time to break out the scope.

I also got some advice from the retro forums about some S100 lines that might be grounded on the Jade CPU or CompuPro memory boards that could prevent memory writes. At this point I am fairly close to being able to run code on the system. But, I had to stop work because of the pending move to a new home. More coming soon.

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.