My FP Mini is working fine, with just the MWRT jumper installed. All the other jumpers for grounding those pins are out.
Subject: RE: [N8VEM-S100:6822] MiniFP users - check this
Date: Sat, 25 Apr 2015 08:38:59 -0700
Yes that was a much discussed issue at the time the IEEE specs came out. It meant for example people had to pull up their Godbout and the likes motherboards, and disconnect the lines from active termination. That is also why also the S100 boards we now do have P1,P2 and P3 jumpers to ground on them. It would have been great if the original Altair board had a pin to ground in the middle of the bus. It would make route tracing easier. However since the option are jumpers only, it effectively is un used in board tracing.
Not sure if this may be related to the problems we’re seeing with this board, but I was looking at my CompuPro backplane and I noticed that pins 20 and 70 are grounded per the IEEE696 specification, while on the IMSAI, only pin 70 is grounded. Pin 20, I believe, is the UNPROT signal.
Maybe it’s not the problem, but I think everyone should try isolating pin 20 from the bus by leaving both J3 and J4 open or otherwise confirming that pin 20 is not connected to ground at the bus. The only issue with this is if you use an early memory card that requires a high on pin 20 to enable writing it won’t work. I remember having to isolate 20/70 on certain CompuPro cards in order to get them to work with the IMSAI.
Just a thought…
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