View Single Post
Old 24th Feb 2021, 12:14 am   #641
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: Non-working Commodore PET 3016

Assuming that goes OK, let's go on to a NOP test.

This is basically the procedure described in the document in your post #359, but in that version the CPU data pins are connected directly to 0V and 5V respectively. We don't like that because those pins are outputs as well as inputs and if they somehow go into output mode, they won't appreciate being welded to the supply lines.

So our recommendation (as per Mark) is that you make those connections from the data pins to 0V and 5V via resistors of perhaps 4K7 value rather than wires.

Physically this may involve stacking two or more sockets in order to allow you to make the necessary connections. Maybe you can do it with a conventional socket with all of its pins fitted but with the pins of the data pin receptacles bent out sideways, flat, so you can solder the resistors to those. If you do adopt this method bear in mind that the bent-out-flat pins with the resistors soldered to them must not make contact with the contacts in the socket below.

However you decide to do it, do not solder to the CPU pins because if you get any solder on the thin lower portion of the IC pins they may never fit into a turned-pin socket again.

Just to recap, the reason for doing this is to feed the CPU with a known harmless, predictable instruction to execute over and over again. Each time it reads a NOP instruction, it will advance the address by one step and then read the next instruction (which will also be a NOP) and so on. The end result is that the CPU runs smoothly through all the addresses trying to read from each one as it goes and selecting all the system devices one after the other in a nice predictable way, all of which can be observed on a scope. This is especially useful for spotting address line problems and address decoder problems but it should make it easier to observe data line activity as well.

Last edited by SiriusHardware; 24th Feb 2021 at 12:29 am.
SiriusHardware is online now