I wouldn’t worry about connecting a computer until you have a solid P5. When you’re taking your 24v readings what are you referencing for ground? Use the G24 test point on the IOA. If you see a flicker when reading P24 with the black lead there but not when reading from controller ground there is probably is a bad G24 connection. You may already be reading from the test point though. Pulling FB3 and FB4 fuses would isolate the P24 that makes P5 a little bit more. What do you actually read on P5 from G24? You can read get P5 on Con35 pins 27-30. I would want to install a new IOA with just Con28 plugged in and no CPU. Is it possible you replaced the IOA first with the old CPU so that a bad CPU could’ve taken out the new IOA? That’s all I can think of except putting a scope on P24 to see if there’s something you aren’t seeing with a meter. Or using a different 24v power supply plugged into the outlet in the machine room to rule power out.
