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Unique S-100 computer (Z-80/Vector Graphic/Micropolis)

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    #31
    Originally posted by deramp5113 View Post
    First, make sure the Z80 reset line (pin 26) goes low and then high when you press and then release the reset button. It looks like your board has been strapped to use the standard PRESET line from the bus (pin 75) instead of the "special" short reset from the DRAM card (pin 55). This means you should be able to see the RESET pin on the Z80 go low when you press the reset button on the back.

    Second, verify the Z80 is getting the 4mhz clock on pin 6.

    Mike
    20200225_130629.jpg20200225_131206.jpg20200225_131216.jpg

    Mike, Thanks for your help with a few months ago. Didn't make much progress and this took the back burner. I've had a little progress whereas sometimes I can get into the Vector Graphic monitor.. but something is still very 'off'. Please see the screenshots of when I am able to get into the monitor, things print incorrectly. Pressing 'ESC' gives me a new command prompt, but it seems to print randomly on the screen - sometimes it just starts dumping hex on the screen.

    Since we last spoke I've gone through and used contact cleaner (DeoxIT) on all the pins/sockets.

    Using the scope, I am able to verify I am getting the 4Mhz clock signal on CLK pin 6.

    Using the reset button on the back of the computer DOES reset the monitor (pin 26 goes low then high again upon release of reset button) - but only if that first power on got me into the monitor. About half the time at powerup I just get that uninitialized garbled text screen you see in my first set of screenshots. In that case the reset button does nothing and I do NOT get the reset signal at pin 26.

    Any thoughts on why I'm only sometimes able to get into the monitor, and even then getting very random behavior?
    Last edited by nullvalue; February 25, 2020, 11:44 AM.

    Comment


      #32
      The reset button should always cause reset to be asserted. There are several change notices towards the end of the Z-80 CPU manual linked below that are related to the reset circuit and power-on problems related to the crystal. As a first step, I'd go through ECRs and see where your board stands in regard to these updates.

      https://deramp.com/downloads/vector_.../Z80%20CPU.pdf

      When the computer does come up and run, I'd suspect the CPU is reading bad data from the monitor EPROM on occasion or the 1K RAM on that board has some problems and that causes the erratic display results you're seeing.

      Mike

      Comment


        #33
        Originally posted by deramp5113 View Post
        The reset button should always cause reset to be asserted. There are several change notices towards the end of the Z-80 CPU manual linked below that are related to the reset circuit and power-on problems related to the crystal. As a first step, I'd go through ECRs and see where your board stands in regard to these updates.

        https://deramp.com/downloads/vector_.../Z80%20CPU.pdf

        When the computer does come up and run, I'd suspect the CPU is reading bad data from the monitor EPROM on occasion or the 1K RAM on that board has some problems and that causes the erratic display results you're seeing.

        Mike
        Thanks, I'll go through the ECRs. Do you think I should reprogram the EPROMs? (And/or replace them)

        Comment


          #34
          I'd suspect the RAM before the EPROM, but let's just change one variable at a time for now and verify and/or implement the ECRs.

          Mike

          Comment


            #35
            Originally posted by deramp5113 View Post
            I'd suspect the RAM before the EPROM, but let's just change one variable at a time for now and verify and/or implement the ECRs.

            Mike
            Originally posted by deramp5113 View Post
            I'd suspect the RAM before the EPROM, but let's just change one variable at a time for now and verify and/or implement the ECRs.

            Mike
            I have a Rev 3 board, so am exempt from the first two (#44 & 35).
            I had previously installed the 22uf cap (#49) and it has not made a difference. I currently have it disconnected.
            Jumper B is already installed (#22)
            Resistor values are already correct (#17) though I'm a little unclear on the wording. There is a 470 resistor between U9-11 and U9-2. Not sure what it means about between U9 and U4. There is no resistor. And it doesn't mention pin #s.
            Resistor values are already correct (#16)
            #13 talks about adding a 390pf cap on the Z80 to fix MWRITE issues. This is not installed. Think it's worth bothering with?
            MZ-0018 describes my problem exactly and talks about replacing the crystal. However it looks like I have one of the unaffected model crystals. Should I try replacing it anyway? It also mentions installing that 22uF cap which didn't help.

            Comment


              #36
              You might as well bring the board current with all the recommended updates, but I doubt the MWRITE fix is related to the problems you’re seeing now.

              As far as the crystal goes, I can’t really make a recommendation without probing around on the board myself with a scope to see what’s going on with POC, reset, and the oscillator.

              To narrow things down, I’d put the PROM/RAM board into another system, read out the EPROMs to verify them, and run a test on the 1K of RAM.


              Mike

              Comment


                #37
                Originally posted by deramp5113 View Post
                You might as well bring the board current with all the recommended updates, but I doubt the MWRITE fix is related to the problems youíre seeing now.

                As far as the crystal goes, I canít really make a recommendation without probing around on the board myself with a scope to see whatís going on with POC, reset, and the oscillator.

                To narrow things down, Iíd put the PROM/RAM board into another system, read out the EPROMs to verify them, and run a test on the 1K of RAM.


                Mike
                Thanks but unfortunately I don't have another system to test in. Any way to test on the bench? I was looking at the TL866 programmers but unfortunately they don't do the 2708's. Any suggestions for a programmer/dumper? I did find an Arduino-based 2114 tester.

                Comment


                  #38
                  If you want, I can look at your Z80 CPU and PROM/RAM boards to figure out what’s going on.

                  Mike

                  Comment


                    #39
                    Originally posted by deramp5113 View Post
                    If you want, I can look at your Z80 CPU and PROM/RAM boards to figure out what’s going on.

                    Mike
                    That would be amazing! Please PM me where to send them.

                    Comment


                      #40
                      Originally posted by deramp5113 View Post
                      To narrow things down, I’d put the PROM/RAM board into another system, read out the EPROMs to verify them, and run a test on the 1K of RAM.
                      So I hooked up an Arduino with this 2114 Tester sketch . There are (2) 2114's in the PROM/RAM board and sure enough! One gets successful passes, and the other one fails consistently. I'm going to order some replacement 2114's and see if that helps. Will report back..

                      EDIT: This 2114 installed is a 300ns model. Jameco stocks a 200ns and a 450ns model. I'd assume I want to err on the side of the faster chip, correct? Or does it need to be replaced with an exact 300ns match?
                      Last edited by nullvalue; February 27, 2020, 07:22 AM.

                      Comment


                        #41
                        Cool! That's good progress.

                        Mike

                        Comment


                          #42
                          Originally posted by deramp5113 View Post
                          Cool! That's good progress.
                          Agreed! Thanks - Any thoughts on the 2114 speed ratings?

                          Originally posted by nullvalue View Post
                          EDIT: This 2114 installed is a 300ns model. Jameco stocks a 200ns and a 450ns model. I'd assume I want to err on the side of the faster chip, correct? Or does it need to be replaced with an exact 300ns match?

                          Comment


                            #43
                            Yeah, I'd go with the 200ns part as well.

                            Mike

                            Comment


                              #44
                              Yes, go with the faster 200ns chips. Order at least two just in case.....
                              Crazy old guy with a basement full of Pentium 1 laptops and parts

                              Comment


                                #45
                                Originally posted by DeltaDon View Post
                                Yes, go with the faster 200ns chips. Order at least two just in case.....
                                Yep I did, may as well replace both.

                                Comment

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