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Build your own PDP 8I, Part 2..

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    #76
    Hi All;

    I have found a multitude of problems, Mostly wiring errors, about 5 or 6..
    But, the one that has caused my most head shaking, is that I bought 3 new 7489's and put them in, and one or more is Bad..
    I replaced all three of them, and it works better..
    I have not yet tried to put any programs in, or tried any of the switches, other than '0000..
    Which will be next.. and which may prove to find more mistakes, but for now I am Happy..

    THANK YOU Marty

    Comment


      #77
      Originally posted by Marty View Post
      Hi All;

      I have found a multitude of problems, Mostly wiring errors, about 5 or 6..
      But, the one that has caused my most head shaking, is that I bought 3 new 7489's and put them in, and one or more is Bad..
      I replaced all three of them, and it works better..
      I have not yet tried to put any programs in, or tried any of the switches, other than '0000..
      Which will be next.. and which may prove to find more mistakes, but for now I am Happy..

      THANK YOU Marty
      You deserve it :->! Did you get those 7489s from a reputable supplier?

      Comment


        #78
        Hi All;

        PBirkel, Yes, Which is what surprises me.. I bought them, at the same time I bought most of the IC's that I would need in 74LSxx , instead of 74(plain).. Thinking that was what was wrong, That I had some Bad Ic's in my 74xx's that I was using, and so new ones would eliminate that problem.. I also, rewired in the 74181 Sockets, from some old (think) trash sockets to new machine sockets..
        So, one of the wiring errors (problems) was a missing wire to one of the S(0-3) control signals.. Another couple was in the PC (Program Counter) circuitry that I had worked on months ago..
        So, now You are up to date on my progress..

        I have been working on it, on and off for the last few weeks..

        THANK YOU Marty

        Comment


          #79
          Hi All;

          I got some things done this afternoon and night, I need to check the Accumulator and Ma and Mb and the Write Circuits for correctness..
          The last three bits of the Accumulator have a problem when doing a bit rotate, which just might be a bad Ic, but, I might just as well check everything affecting it..

          THANK YOU Marty

          Comment


            #80
            Hi All;

            I think I have found a few more mistakes, I need to do some more checking for sure..
            On the Accumulator, I replaced one of the Ic's, which seemed to be Bad (74194)..
            The MA Register is Stepping by itself, some of the time, also, when the left hand of the Board is flexed it can make the computer work and or cause the MA register to continuously increment it's contents..

            THANK YOU Marty
            Last edited by Marty; September 15, 2015, 02:29 PM.

            Comment


              #81
              Hi All;

              I have been thinking about making some changes to this circuit, there is a fairly great amount of changes, and at present, the way I have wired up the Capacitors, that they might be shorting to the pins around them, so I am for all these reasons and a few more.. I am going to draw up the new circuit and replace all of the 14 pin sockets with 16 pin sockets and the bottom two pins will be for the Capacitor..
              So, it will be like starting all over again..

              Comment


                #82
                Originally posted by Marty View Post
                Hi All;

                I have been thinking about making some changes to this circuit, there is a fairly great amount of changes, and at present, the way I have wired up the Capacitors, that they might be shorting to the pins around them, so I am for all these reasons and a few more.. I am going to draw up the new circuit and replace all of the 14 pin sockets with 16 pin sockets and the bottom two pins will be for the Capacitor..
                So, it will be like starting all over again..
                :-<. But it might be a good point at which to take your "new circuit" and first verifying it by simulation before committing it to new-wires-and-sockets?

                Comment


                  #83
                  Originally posted by Marty View Post
                  Hi All;

                  I have been thinking about making some changes to this circuit, there is a fairly great amount of changes, and at present, the way I have wired up the Capacitors, that they might be shorting to the pins around them, so I am for all these reasons and a few more.. I am going to draw up the new circuit and replace all of the 14 pin sockets with 16 pin sockets and the bottom two pins will be for the Capacitor..
                  So, it will be like starting all over again..
                  Way back in my youth, while earning my Boy Scout Woodcarving Merit Badge, I was taught the most common mistake a beginning whittler makes is taking too big a chip. In later years in Computer Science school we learned to divide a program or OS into manageable sections and debug them as separate pieces before combining them into a whole. I believe they called this "Top Down Design - Bottom Up Coding".

                  I hesitated to say anything before, because I didn't want to rain on your parade as you "boldly went where no man has gone before". But now that you've decided to reboot your project you might also want to rethink how you're going about it.

                  What I would do is break the design into subsections, then breadboard, test and debug each one before trying to put it all together. That's how the "big boys" originally did it and they had good reason to do so. Build up a generic "register" then test your ability to gate data into and out of it. Then expand that and model each of the specific PDP8 registers in the design. Do the same with the ALU. Then model the connecting busses. Test how data flows from one part to another under single step. Then build the clock and test how it does or does not control the rest of the circuit.

                  Smaller "chips". Smaller headaches. Localized and therefore easier to find and fix errors.

                  Just my $0.02 suggestion.
                  "It's all bits on the bus, Cowboy! It's all bits on the bus!" -- Tom Beck, #1ESS Instructor, Southern Bell Opa Locka Training Center

                  Comment


                    #84
                    Originally posted by Marty View Post
                    Hi All;

                    I have been thinking about making some changes to this circuit, there is a fairly great amount of changes, and at present, the way I have wired up the Capacitors, that they might be shorting to the pins around them, so I am for all these reasons and a few more.. I am going to draw up the new circuit and replace all of the 14 pin sockets with 16 pin sockets and the bottom two pins will be for the Capacitor..
                    So, it will be like starting all over again..
                    Way back in my youth, while earning my Boy Scout Woodcarving Merit Badge, I was taught the most common mistake a beginning whittler makes is taking too big a chip. In later years in Computer Science school we learned to divide a program or OS into manageable sections and debug them as separate pieces before combining them into a whole. I believe they called this "Top Down Design - Bottom Up Coding".

                    I hesitated to say anything before, because I didn't want to rain on your parade as you "boldly went where no man has gone before". But now that you've decided to reboot your project you might also want to rethink how you're going about it.

                    What I would do is break the design into subsections, then breadboard, test and debug each one before trying to put it all together. That's how the "big boys" originally did it and they had good reason to do so. Build up a generic "register" then test your ability to gate data into and out of it. Then expand that and model each of the specific PDP8 registers in the design. Do the same with the ALU. Then model the connecting busses. Test how data flows from one part to another under single step. Then build the clock and test how it does or does not control the rest of the circuit.

                    Smaller "chips". Smaller headaches. Localized and therefore easier to find and fix errors.

                    Just my $0.02 suggestion.
                    "It's all bits on the bus, Cowboy! It's all bits on the bus!" -- Tom Beck, #1ESS Instructor, Southern Bell Opa Locka Training Center

                    Comment


                      #85
                      Hi All;

                      PBirkel, Thank You for You suggestion.. "" But it might be a good point at which to take your "new circuit" and first verifying it by simulation before committing it to new-wires-and-sockets? ""
                      I have for a long time tried to stay away from Simulation, because of the steep Learning Curve of getting it into my head and the computer..
                      As well as all of the typing and debugging of that just to get it right, for the Schematic.. I could be all Wrong, but it seemed easier to just wire it, than all that would be needed to get it done by getting it into simulation..
                      I have looked at what everyone has offered earlier, and I would need to figure out, exactly how to enter it in IC mode instead of gate mode, which I never got past..

                      DDS, Thank You for the reply and the suggestion, Actually, I have been kind of doing this already..
                      As the Schematic is divided into sections, that are parts of the whole thing, and I have been checking each section..
                      But, I can in some cases further divide the sections into smaller parts..
                      And since they are machine wire wrap sockets, I could only put in the Ic's that are needed for a particular circuit..
                      I can and did put a circuit or two on my Breadboard and made what didn't work, work.. Trying different things, until I proved it would work as the origional circuit was, or what was needed to make it work..
                      Mostly, the 'fixes' are having to do with Ic counts, in other words on the Alu there are five signals that control what the Alu does, and in each of these there are anywhere from three to maybe eight gate delays on any of the control signals, and so they all don't reach the Alu at the same time..
                      Most of these are things like having a 7402 followed by a 7404, which I can instead replace with a 7432, and eliminate the 7404.. Or a 7400 followed by a 7404, which I can replace by a 7408 and eliminate the 7404..
                      And so by doing this I can bring them all closer to the three gates of delay..
                      Also, I can replace the 74175 which is a 6 register part, so two are required, with a 74174, which has 4 registers per part, so it will require three of these for the 12 bits.. But by doing this I can in a number of places eliminate 12 (7404) inversion gates and have less loading on the 74174, than was on the 74175..

                      THANK YOU Marty
                      Last edited by Marty; September 16, 2015, 04:25 AM.

                      Comment


                        #86
                        "Mostly, the 'fixes' are having to do with Ic counts, in other words on the Alu there are five signals that control what the Alu does, and in each of these there are anywhere from three to maybe eight gate delays on any of the control signals, and so they all don't reach the Alu at the same time.."

                        Seems like you're looking at two main possibilities. In all of the systems I've worked with gate delays were mostly made irrelevant by either the processor clock's gating of data and signal flows or by delay lines. Here I should point out that I've never worked with a PDP8. Things were sometimes done differently back at the dawn of time. Machines spoke in inscrutable tongues like FIELDATA and EBCDIC. But IMHO your ALU should be inhibited by the clock from doing anything with its inputs until such a time as all the inputs should be ready. If that's not happening you likely have a problem with the clock control of the ALU. IF the ALU is waiting until the correct time and one or more of your inputs are not yet ready, then you likely have a problem with whatever is supposed to be propagating that signal.

                        The key to debugging this would be the processor timing diagrams for the design you're building. Hopefully you have them, can make them, or can get them from somebody on here who can. You might be able to use the diagrams for the PDP8 that your clone is trying to mimic if your clone is close enough to the original hardware wise. Then it will be a matter of dragging out the scope or logic analyzer of your choice and nailing it down.
                        "It's all bits on the bus, Cowboy! It's all bits on the bus!" -- Tom Beck, #1ESS Instructor, Southern Bell Opa Locka Training Center

                        Comment


                          #87
                          Hi All;

                          DDS, Thank You for Your answer, and suggestions..

                          ""
                          Seems like you're looking at two main possibilities. In all of the systems I've worked with gate delays were mostly made irrelevant by either the processor clock's gating of data and signal flows or by delay lines. ""
                          I think that this is mostly true, but, I can't say in this case..
                          "" Here I should point out that I've never worked with a PDP8. Things were sometimes done differently back at the dawn of time. Machines spoke in inscrutable tongues like FIELDATA and EBCDIC.
                          ""But IMHO your ALU should be inhibited by the clock from doing anything with its inputs until such a time as all the inputs should be ready. ""
                          I can't say that it has this..
                          "" If that's not happening you likely have a problem with the clock control of the ALU. IF the ALU is waiting until the correct time and one or more of your inputs are not yet ready, then you likely have a problem with whatever is supposed to be propagating that signal. ""
                          That is what I hope to fix, this time around..
                          Remember that this Design is not a Do all, have all, But just enough to get it working.. As this probably, took a student all school Year to get it wired and up and running.. It was a teaching tool.. With the Professor, there to Help with any inherit problems, which I don't have..
                          "" The key to debugging this would be the processor timing diagrams for the design you're building. ""
                          They DON'T exist.. Nor can I say that there were ever any..
                          ""Hopefully you have them, can make them, or can get them from somebody on here who can. ""
                          ""You might be able to use the diagrams for the PDP8 that your clone is trying to mimic if your clone is close enough to the original hardware wise. Then it will be a matter of dragging out the scope or logic analyzer of your choice and nailing it down. ""
                          This is Not even close enough to any PDP 8, for it to be of any help..
                          Also, Remember, Early on I gave the link to get the files, which had been used to build this.. I think only a few people actually got them and looked at them.. And it contained a Classroom Manual, a wire list for Wire-Wrapping, and a Partial Schematic, the Schematic does Not have everything, that is included in the wire List.. I have had to draw up my own..
                          And Last, but not Least, much of it I do Not understand what is happening, what it is doing and/or how it all fits together..
                          One of the Reasons for doing what I am doing now, is when I would look at what was happening at (say) at Clock pulse one, it would not be the same from time to time, So by trying to align things up, I hope to eliminate that problem.. (Hopefully)..

                          THANK YOU Marty

                          Comment


                            #88
                            Welcome back Marty.

                            I have also temporarily shelved my PDP8 for the time being (I had to do some work to pay the bills!)

                            I managed to get my simulation fully working (except for the single clock cycle switch). It runs the diagnostics perfectly - and it runs CHECKMO-II as well so I can play chess with it. Of course, in a software logic simulator it runs rather slooooooooooow... but it does prove that if I built it; it has a high probability of working.

                            I will post the latest version of my simulator again when I get a bit of time.

                            Can you remind me of the website where you obtained your base documentation from? If I get some (further) time whilst 'holed up' in a hotel I may have a look at what you have used to see if I can find some obvious fundamental problem with what I am seeing. It may help?

                            Dave

                            Comment


                              #89
                              Hi All;

                              Dave, here is a copy of the files from PBirkel..

                              I have set up a folder with all LD12-related files in my possession at:

                              http://n8vem-sbc.pbworks.com/w/brows...12%20PDP-8%2FI

                              I believe that at the moment you'll need to register in order to access that folder, but that's no worse than registering at VCF.
                              Yes, You need to register..
                              Thank You PBirkel..

                              THANK YOU Marty

                              Comment


                                #90
                                Marty,

                                Once you get it working you can use the front panel from this project:
                                http://obsolescence.wix.com/obsolescence#!pidp-8/cbie
                                Member of the Rhode Island Computer Museum
                                http://www.ricomputermuseum.org

                                Comment

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