View Full Version : IMSAI newbie

April 8th, 2015, 05:15 PM
A very nice looking IMSAI 8080 arrived today in the mail. Packed along with it were the following:

Ithaca IA-2000 Z80 CPU card
TDL SMB system monitor board
Godbout/CompuPro Econoram XIIIa RAM board (32K). Haven't figured out what this is mapped to yet (I guess I should read the fine manual).
TDL Z16 RAM board (16K). The jumpers imply this occupies $0000-$3FFF.

Now, I should preface my stupid questions by saying this is exactly the second S-100 system I've ever owned, and exactly the first that was in any semblance of working order. My first was a questionably preserved Alpha Micro S-100 box which was more for historical interest than functionality. But I'd like to get this one to live.

I carefully checked everything before powering it on with no cards, and the front panel appears to light (but does weird things, to be mentioned).

Next I tried installing the CPU board. The Ithaca board has a 20-pin header for the front panel, but the panel connector has 16 pins. I guessed that it went in the rightmost 16 and it resets, runs and stops, at least. I powered it on with all the address toggles on zero. If I press EXAMINE NEXT, it advances its address bus lights. I didn't try to do anything else at this point.

Next I tried installing the Z16 board since I had a good guess that mapped to the bottom of memory. Here's where it gets weird. Going through memory with EXAMINE NEXT, data bits 0 and 6 light alternately. If I try to store with DEPOSIT, nothing is stored. Plus, some of the address lines now stick high, which may be why I can't store anything since they're indicating addresses outside of the Z16 addressing range. Toggling the switch makes no difference.

Am I an idiot, or is something wrong? What would be the next suggestion to try investigating?

April 8th, 2015, 05:41 PM
I bought a TDL Z16 once for my Altair it turned out it to be faulty, not saying yours is. It had the same issues as you are seeing, certain data bits were stuck high. You address the Z16 using the jumper wires on the front. And from what I remember there is a switch block to enable each of the four k blocks. The first thing I would check is what location it's addressed at. You can download the manual from s100computers.com:


April 8th, 2015, 10:18 PM
It's not the data lines that are stuck high, it's certain *address* lines. The data lines just seem to cycle.

The board is jumpered to $0000-$3FFF (see above). I do have the manual, but thank you for the link.

April 9th, 2015, 02:59 AM
The pattern you are seeing could be the normal state of the ram board when powered on but the front panel is not compat. with the write mem line so it looks stuck. check card jumpers for front panel compatibility. the docs for the ram card may have a "for front panel systems you should..."

I could be wrong, because if I am right to suspect this it poses the question why is the card jumpered this way to begin with? Did the seller throw in a RAM card to make the system appear complete but orig this card came from someplace else?

April 9th, 2015, 04:07 PM
What would you expect it to be jumpered to? (serious question, I really have no idea).

Any idea why address lines would get stuck like that?

April 9th, 2015, 04:48 PM
I read the manual. I don't think you need to jumper the card to be compatible with a front panel system. You could check 1000 2000 3000 blocks to see if you have an issue with the entire card or just one block of RAM. If the whole card is not working remove the card and test the EconoRAM card.

Does the Godbout card allow you to deposit values into memory and retrieve them? If so then that means you have a good front panel and CPU card. You should make sure. The manuals for the version of the EconoRAM are online, but be sure you get the correct one. I am guessing the EconoRAM is jumpered to start at 8000 (left-most paddle up, everything else down.). Confirm the jumpers. Then examine/examine next to see if you have garbled memory, then attempt to deposit into 8000, 8001, 8002, etc. (or where ever it starts)

If that's also screwed up, you may have to return to the CPU card and check that out. Do you have an S100 riser card? You can use to mount the card you're testing above the chassis to easily get at it with a voltmeter, logic probe, or scope.

Not sure if you have done that kind of work before, but here you go...IMSAIs and Altairs are never just turn on a go :-).

Circling back to the Z16 if you have some troubleshooting experience here is what I'd start with....check U2 on the Z16 card for the signals described in the manual. The manual has a pretty good walk through of the how the card works. Check for hot chips on the Z16. Turn the switches off and on a few times to be sure they're really set as they look to be, the switch may be marginal and not actually set to 0000. You can try other memory locations just to see if you have different results.

Lastly - I assume you don't have a PROM card or other card interfering with 0000.


April 11th, 2015, 09:55 AM
I confirmed the jumpers on the EconoRAM and installed just that and the Z80 card and set the address to $8000. It looks like, happily, the stuck address line are just sticky front panel switches. I cranked them up and down a few times and was able to get them to settle down, and stored several random patterns in sequential locations on the EconoRAM and was able to read them back accurately.

I haven't tried running a sample program, but is there an easy way to clean those switches before I continue? It seems like a little TLC on the front panel would save me a lot of grief when I try to get the other two cards working.

Thanks for your help, Bill.

April 12th, 2015, 07:30 AM
First vaccum and or blow out any dust first depending on the status of the item, then clean with mild soap, dry thoroughly then use a product like DeOxit to remove anything that's left. if there is rust, use a little CLR in the soap..

April 12th, 2015, 04:11 PM
Should I take the front panel off to do that? If so, what's the easiest way to do it? Just undo the screws embedded in the acrylic?

April 12th, 2015, 04:42 PM
Don't use a vacuum, whatever you do - they have a tendency to build up large amounts of static electricity, and that's just begging for a sad, sad accident when combined with computer parts. A blow dryer might work, but I'd just use a can of compressed air instead.

April 12th, 2015, 04:57 PM
Take good notes, the front panels of the IMSAI 8080 are not easy to re-assemble. Take your time.



May 25th, 2015, 06:47 PM
Pleased to report that the switches are now mostly functional. They still stick a little now and then, but it was able to run its first program on the RAM board at $8000 cycling the output lights tonight:


The next thing to do when I get a round tuit is to get the other RAM board and the SMB board working, but this is very encouraging.