IMSAI (moving along...)

From: Bill Sudbrink <bill_at_chipware.com>
Date: Wed Jul 7 18:56:52 1999

> If this is a dynamic memory board it may not work.

No it is 8K of 2102 static.

> None of the dynamic boards, I tried, seemed to work
> from the front panel.

This front panel has the DRAM modification (IMSAI number
ECN 77-0039) which apparently changes how S-100 pin 71
is interpreted.

> I still don't know how a ls04 could have caused a
> swap of your data lines??

No no, didn't cause the swap. The data cable _was_
backwards. Bit 7 (the high bit) was "sort of" (about
5K ohms) grounded to a neighboring pin at U15 which was
often low. This resulted in that bit almost always being
0. When the cable was backwards, you would get an 0xC2
(JNZ) instead of an 0xC3 (JMP) which would work, with the
exception of the address being reversed by the byte and
the "low" (really high) address bit always zero.
If you put the data cable in the right way, you got an
0x43 (invalid instruction) which did not put an address
on the address bus and gave the impression that random
lights were flashing on each press of the EXAMINE switch.

The system is still flakey as heck. With my limited time,
resources and knowledge it might take me a couple of years
to get it working and stable. I'm considering asking Jim
Willing what he would charge to get it put back together.

Bill
Received on Wed Jul 07 1999 - 18:56:52 BST