Changing 'PD' (Presence Detect) on IBM 72-Pin SIMMs

Modifying 'PD' on four Toshiba 2Mb 70nS SIMMs to work in an IBM PS/2 Model 70 that only likes 2Mb 80nS modules. Originally the SIMMs have 'R1' and 'R2' closed, with 'R3' and 'R4' open. A modification is needed to open (remove the bridge) on 'R1', and bridge 'R3' closed. The needed change is 'R1' open, 'R2' and 'R3' closed, and 'R4' open.

Verify with a voltmeter when you are complete:

SIMM pin 72 is the 'common' - measure continuity from this pin for one side of your voltmeter. Pin 67 is 'R1', pin 68 is 'R2', pin 69 is 'R3', and pin 70 is 'R4'.

With four SIMMs modified correctly, the Model 70 counts to 8064Kb of RAM.

[hughnisbet]

Well done on the soldering, my old hands are arthritic now. It doesn't help that they are large to begin with, even my pinkies are a ring size Z. Lol.