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Old 10-07-2007, 10:54 PM
Geoff Harrison Geoff Harrison is offline
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Join Date: Feb 2007
Posts: 104
Talking It works!

Well, after a couple of days hunched over my kenbak-1 motherboard, it's now working fine

And the culprit was:

If you want to know the nitty-gritty read on, otherwise just skip the rest.

When I tried to run the test program in Ex.2-3 in the manual, it didn't work. The front panel worked fine, but programs wouldn't run. I entered various little single command programs (LOAD A, #100 ... HALT) and found that some commands seemed to work OK, but others failed. Eventually, I hooked the board up to a logic analyzer and tracked the execution states that are documented in the manual while I repeatedly ran STORE B, @0100. It turned out to be running the command as though it was using indirect addressing instead of direct. Further experimentation showed that the machine was inserting an extra level of indirection into all 2 byte instructions. Immediate addressing became direct, direct became indirect.

It took me most of the two days to pour over the circuit diagrams trying to understand how the state transitions were implemented. I still don't have a deep understanding of it, but I was able to find the section that was inserting the extra indirection. A little probing around with a logic probe showed me that the output of the NAND gate @IC21 was never going high. It wasn't tied to ground either, it was just floating. I swapped the chip, but that didn't solve it. Then it dawned on me that it never went high because it probably wasn't receiving power to drive it high, and sure enough, pin 14 (Vcc) was not soldered.

I suppose I should have been able to find that with a close inspection, but I went over it twice after I built it and didn't find that unsoldered pin.

Oh well, at least I have a better understanding now of how the Kenbak-1 was designed. It's quite fascinating, John Blankenbaker put together one interesting machine there.
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Old 10-07-2007, 11:11 PM
Geoff Harrison Geoff Harrison is offline
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Join Date: Feb 2007
Posts: 104
Default Thanks, Grant

I just wanted to take the time to thank Grant for putting together another great kit. It's got the same beautiful quality as the Altair 8800, and the time Grant put into building the manual makes assembly much easier that it could have been.

Nice job, Grant. Thanks again.
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