Embarking on an Altair 8800C build

[TheoAU]

I had similar troubles with one of my front-panel interface boards. Mine ended up being caused by one of the NOR gates in the 74LS02 chip in location "L."

If you socketed your chips and have spares, you might swap 'em one at a time and retest. You can always break out the 'scope too, but you may find it just by swapping parts.

Thanks Wayne, I did read about the issues you had in your thread. Yes, all my chips are socketed just in case I need to swap one out. But I'm happy to say I've resolved both the issues I was having. I'm not sure what happened with the FDC+ PROM, it seems to work fine now. I may have had it disabled when I thought it was enabled. But I've tested it today and everything is working fine. PROM and RAM work fine, and disks load fine too.

The issue I was having with the 882SIOJP is a little more interesting. I've found that it has issues depending on which slot it's in. It seems that if it's positioned after the CPU and Front Panel Interface it has issues. But if I position it in slot 1 it works fine. So, I currently have a fully working 8800C, with all new boards. The slot configuration that works is:

1: 882SIOJP
2: empty
3: CPU
4: Front Panel Interface
5, 6, 7, 8: empty
9: FDC+

I don't think it really matters which slot the FDC+ is in. I tried it in several different slots and it seemed to work consistently. Not so with the 882SIOJP. I tried that in the centre and it worked a little better, but not perfect. It seems to only work perfectly if positioned before the CPU.

It'd be interesting to work out why different slot position have an effect on it? This isn't an issue in my original Altair.

[BillO]

I too had some board position sensitivity that has all seemed to clear itself up. No idea what was causing it.

[Wayne Parham]

Yes, I thought I had a board sensitivity problem too. You may have noticed my saying that in my build thread. And I may have, but after I found out that I had other issues - namely the bad front-panel interface NOR chip - my focus shifted to that. I have all my boards in slots 1, 2, 3 and 4 and I haven't gone back to see if they work in other slots.

When I started building my system(s), I thought I would experiment with things like termination. And I may still yet. Slot sensitivity might be something to look at then too. But for now, I'm just running programs and watching the blinky lights. It's way too much fun.

[AltairClone]

Have you adjusted the CPU clock yet? That almost always needs tweaking on the Altair CPU board.

Mike

[AltairClone]

I thought I would experiment with things like termination. And I may still yet.

In general, you don't want to terminate an Altair front panel system. The address lines drive the front panel LEDs directly, so that is already a load on the address bus drivers. The data-in bus has 1K pull-ups to Vcc on the CPU board as well. I guess you might consider termination on just the data-out bus.

Mike

[Wayne Parham]

Got it. Thanks, Mike!

When I bought my 9-slot and 18-slot backpanels, I also bought terminators. On the 18-slotter, they're small boards. On the 9-slot backpanel, I populated all the components that provide termination supply voltages and I even set the trim pot. But I populated the actual terminator resistor strips with SIP sockets, and left the resistor packs off.

Now I see all of that was unnecessary. I'm certain you documented that somewhere - probably in numerous places - so it's undoubtedly a RTFM moment for me. But I guess it doesn't hurt anything either. :-)

[BillO]

It's way too much fun.

This!

[BillO]

Have you adjusted the CPU clock yet? That almost always needs tweaking on the Altair CPU board.

Mike

Now that you mention this (again) It was once my clock got sorted out that things just started to work better. That may have been my issue. My original clock was REALLY bad.

[TheoAU]

Have you adjusted the CPU clock yet? That almost always needs tweaking on the Altair CPU board.

Mike

Now that you mention this (again) It was once my clock got sorted out that things just started to work better. That may have been my issue. My original clock was REALLY bad.

I've just been reading this thread:
viewtopic.php?f=5&t=310&p=1645&hilit=cpu+clock#p1645

Sounds like you had exactly the same issue with the 882SIOJP that I'm having.

Is there a standard fix for the CPU clock issue, or is it a case by case basis? Because it seems that many of us have the same hardware configuration with the 9 slot motherboard.

[toml_12953]

Have you adjusted the CPU clock yet? That almost always needs tweaking on the Altair CPU board.

Mike

Now that you mention this (again) It was once my clock got sorted out that things just started to work better. That may have been my issue. My original clock was REALLY bad.

I've just been reading this thread:
viewtopic.php?f=5&t=310&p=1645&hilit=cpu+clock#p1645

Sounds like you had exactly the same issue with the 882SIOJP that I'm having.

Is there a standard fix for the CPU clock issue, or is it a case by case basis? Because it seems that many of us have the same hardware configuration with the 9 slot motherboard.

You have to tweak each system individually. In my case, I got lucky and simply swapping out the 8080a (even though both are genuine Intel chips) was enough to bring the system into spec and eliminate the front panel errors I had been getting.