IMSAI (moving along...)

From: Bill Sudbrink <>
Date: Thu Jul 8 15:41:00 1999

> I would follow up on the posts about "Mwrite" signals.
> Mwrite = S100 pin 68, active high,
> pWR (processor write) = S100 pin 77, active low.

OK. S-100 pins:

On the memory card:
Pin 68 goes to the N74LS37A pin 9 (seems to be an input pin).
Pin 77 has no connection, right at the card edge.
       It does not go to any jumper or option pad, etc.

On the front panel:
Pin 68 goes to U24 (8T97) pin 5 (an output pin). I
       have not verified the logic on the other side
       of that pin (which would be pin 4).
Pin 77 goes to U5 (8212) pin 2 which seems to control
       the latch for the user data lights. It also
       seems to go into the DEPOSIT logic.

On the MPU-A:
Pin 68 has no connection, right at the card edge.
Pin 77 goes to an 8T97 pin 9 (an output pin).

Now that I look at it this way, this is VERY WEIRD!
Maybe very bad? Does this mean that the CPU can not
write to this memory card? Does the front panel
arbitrate pin 68 for the CPU?

Received on Thu Jul 08 1999 - 15:41:00 BST

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