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New PET 2001N-16 Project

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    New PET 2001N-16 Project

    Picked up my first PET a couple weeks ago. Have things cleaned up and am starting to troubleshoot it now. Currently it has garbled text at the top of the screen and just the @ symbol filing the remainder. You can see a few photos here

    So far this is what I have checked to troubleshoot the CPU using an oscilloscope:

    Pin 40 RESET is high
    Pin 2 RDY is high
    Pin 6 NMI is high
    Pin 4 IRQ is high
    Pin 37 & 39 both have a 1 MHz square wave

    Pin 7 SYNC signal seems like it may not be right. My understanding is that it should be low and pulse high. Mostly it looks like a low signal with some noise in it to me. Being kind of new to this I checked the same signal on one of my working Apple iie computers and it is very different. I also went as far as to swap the PET processor into the Apple and it booted up just fine. So the processor seems to be fine.

    Looking for ideas on where to go next on the SYNC signal? Also I forgot to mention I did start by checking all of the voltage rails and they were right on 5, -5 and 12 volts at the memory sockets.

    Thanks for any insight you can provide, Chris

    Sync stuck high indicates that the CPU is not fetching instructions. It could mean that there is bad ROM or that bad low RAM is causing the CPU to execute in RAM. Both these conditions can cause the CPU to fetch and execute an illegal instruction that is popularly called a KILL instruction. This will cause the CPU to hang up. Nothing further can happen until the machine is reset.

    It may be time to run a diagnostic EPROM called PETTESTE2V4 that daver2 wrote. It goes in the Edit ROM location which is on a socket. Do you or a friend have the ability to burn a 2716 EPROM?


      I have a TL866 2Plus that I use to burn BIOS chips with mainly, but I don't think that is up to the task of burning a 2716. At least not from the research I have done so far. Was contemplating the need for a more "Vintage" burner or more expensive modern unit to handle chips like the 2716.

      Is it possible to read the ROMs at least with the TL866? Again, my google searching hasn't turned up much on this topic. Comparing them to known good files would be a starting point.


        From information on the website I see the programmer should be able to program AMD 2716B devices (12.5V VPP).

        UK - Worcester.


          Thanks for pointing that out, I do see 2716B as an option in the software as a chip selection now. Just to clarify, the SYNC signal is not stuck high, it's low.


            The SYNC signal from the CPU would normally be LOW and should pulse HIGH briefly when the CPU fetches an instruction. If it is permanently LOW (or HIGH come to that) the CPU is misbehaving. As dave_m has pointed out - the most likely cause of this is a dead CPU, the CPU is held in a reset state, there is no clock to the CPU - or it executed an invalid instruction. The last option is the more likely...

            UK - Worcester.


              Originally posted by acenewman View Post
              Thanks for pointing that out, I do see 2716B as an option in the software as a chip selection now.
              Also check to see if your programmer can handle a 2816 EEPROM. That is also pin compatible as long as pin 21 (/Write enable) is high as it is on the PET.


                looks like I have a few 2816 and 2816A,B,C options. Those would all be 2K ROMS though, any thoughts on possible 4K substitutions? Aren't most of the ROMS 4K?

                By the way, I was playing with the o-scope some more and most the time the SYNC line stays low, but occasionally (1 in 5 times) it does seem to go high and hang there.


                  Yes, there is the 4K equivalent - but there are a few gotchas as well. You MUST use a 2532 NOT a 2732 (unless you use an adaptor).

                  The purpose of locating a 2716 was to program it with my PETTESTER code and swap it for the EDIT ROM. This is a 2K part - hence why we are looking for one.

                  If we can get my PETTESTER running, it will check the other BASIC ROMs out, and we can take it from there.

                  Last edited by daver2; April 27, 2021, 12:08 AM.
                  UK - Worcester.


                    Got it, will start looking through the parts bin. I have lots of chips, but not sure I have any quit that small (2K)


                      A 2816 should be OK (providing your programmer can burn it of course).

                      UK - Worcester.


                        Sadly not a single 2816 or 2816B on hand. Did manage to get a few on order though. Should have them by early next week.


                          Well my ROMulator showed up in the mail today. Happy to report that I now have a fully functional PET. Now to set about the task of troubleshooting if the issue was system ram, video ram or the ROMs.


                            Great news! I think the simplest thing would be to update the ROMulator configuration using a Raspberry Pi, and then you can do a few things which will be useful. You can write daver's PETTEST rom as one of the switch settings, and that will tell you plenty of information. I also have a separate RAM/ROM test you can try as well. I had meant to have these preloaded for you, but I must have programmed the ROMulator with the previous default settings, sorry about that. It is pretty simple to reprogram, I'll send you a message to help walk through it. Also will make sure the docs are easy to follow.
                            You can also configure the switches in such a way that one turns RAM replacement on/off, and another ROM replacement, so you can figure out which is bad.
                            We know at this point that the video RAM is good, since the ROMulator does not replace that.


                              I was going from memory and thinking that it replaced the video ram as well. Guess that means the new chips I put in are good. Guess it's down to system ram and ROMs. Would be very interested to know what the DIP settings are to disable the RAM and / or ROM replacement. I didn't see anything about that in the documentation. Will that work in the stock configuration or do I need to flash it with your RAM/ROM test for that? Hoping to get some time this weekend to do some flashing and more troubleshooting.