View Full Version : IMSAI - the Fix Pt4 : Finally (for me too) a success story

May 28th, 2015, 08:34 PM
I’ve now had my IMSAI up and running for over a week.
I had also previously tried a few different hardware configs, including trying to set up up the system as a Processor Tech Subsystem B minus the GPM board...FAIL!!!

Previously, if you recall after some struggles, I had basically got the hardware running, but I only had one working CP/M 1.4 disk, and I was struggling to make a copy with the Shugart 8" drives that came with the system.
I spent a long time typing in code from the Versafloppy 1 manual, and was trying to use the DIAGNOSIS and CBIOS program listed to format the disk, but I could not get it to work. When trying to format the disk, the head would drop, but nothing more would happen.

Enter the SOL-20.
I decided to put my SOL-20 with Morrow DJ floppy controller to work and 8" NEC drives.
I was able to run the #FORMT.COM program to format a blank floppy at 128 bytes.sector setting. Then I was able to use DJCOPY.COM to copy track 0 and 1 from my working IMSAI CP/M boot floppy to the newly formatted disk. Initially I had trouble which I believe was fixed by cleaning the NEC drive heads (I had been using a lot of old disks recently that probably soiled the heads). After the clean I was able to make a good copy of the boot disk, and I was able to confirm that it booted perfectly on the IMSAI. Thanks SOL-20!

As an added bonus I was also able to copy a CP/M 2.2 disk. I had previously assumed that this didn’t work, but after a few attempts at copying I was able to get a good read and a good backup.

So now I have both CP/M 1.4 and 2.2 working!

Although it didn’t take years like smp’s fix, it did take about 4 months of solid work.

The entire fix is:

1) A complete clean

2) Fix a dead Line Delay, by building an adaptor for a modern part.

3) Replaced the MIO with a 3P+S card set up to mimic a 2SIO card. I was having issues with the MIO talking to my Mac via a USB/Serial adaptor. It did work flawlessly with my old PC laptop though.

4) Replaced the 8k RAM card with a PSS 16k card and rejigged the memory map.

5) Finally for this weekend I’m going to reburn all my EPROMs and put them at memory addresses that are more appropriate for my setup.

So, overall, the fix was relatively simple, but getting there was a steep learning curve, and at time a little panicked when I realised I only had one working boot disk.

Much much much thanks for everyone for their support, help, advice and words of encouragement.


May 29th, 2015, 01:50 AM
Here she is...
The original white ceramic 8080 is safe in a box. I didn't want to accidentally kill an original CPU while I was getting the hardware working, so I replaced it with this later model plastic version sourced from eBay.

The screen capture from my terminal emulator (Coolterm) on my Mac shows me toggling different jump addresses in ROM:

- The monitor from Erik Klein at F800h. It's a nice little monitor that can transfer in intel hex.
- Then to MON-88 at FC00h, which I believe is a modified version of the original Altair turnkey monitor. You can see the '#' prompt. Does everything in octal.
- CP/M boot from address F000h. Doing a directory listing shows various programs I've been able to salvage from the crusty original floppies, and some, like PIP, ASM, and DDT, I installed recently.


May 29th, 2015, 02:51 AM
Congratulations, Phil! I know the thrill of *finally* getting my machine into a good, solid, stable state.

I know my journey spanned "years," but it was not so dramatic as it sounds. It was a few intense sessions spread over that time, and quite a few other adventures with other machines in the meantime. I'm certain that you know what I mean.

I'm behind you a little bit, as I have not yet had success with CP/M, but I'm still going for it.


May 29th, 2015, 10:05 AM
Heartening to see all of these success stories here. I will be posting my own progress report here in the near future. I'm at the beginning of journey, just now to the point of getting keyed-in programs and serial I/O working, but I've already (re)learned more about computer architecture as part of this exercise than anything I've done since my CS courses back in college.