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With the switch matrix test, the first portion at connector 2J2, you test at the actual .156 connector header. This gives you the status of the driven voltage on the columns.
But because of the differences in return side of the circuit the circuit, the other half cannot be tested at connector 2J3. You ground the pins on connector 2J3, but do the actual probing test at the legs on the PIA chip itself.
Switch matrix PIA is IC11, The PIA pins 2-9 are the input, and 10-17 are the drives.
-Hans
The resistor value really just tweaks the sensitivity of the switches. My basic understanding is that the software for system 3-6 had issues dealing with multiple switch closures in the same row, so I believe they de-sensitized them a bit to compensate. System 7 is better at dealing with multiple closures, so they went with the 0-ohms for maximum sensitivity.
Logic-wise, nothing changes in how the CPU chip reads the data on the switch PIA, the resistors just change the threshold at which the PIA senses switch closures.
-Hans
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