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Build your own PDP 8I, Part 2..

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  • daver2
    replied
    Marty,

    I downloaded the documents this afternoon and had a quick scan through them.

    A couple of things came to mind:

    1. What did you do with the two wire lists?

    If it was me - I would have manually typed them into a spreadsheet and done some cross-checking between the two of them to ensure that what was typed was sensible. For example, if you couldn't read something from one printout - you may have been able to have read it from the other. I assume that both wiring lists were generated from the same source - but that is not guaranteed. I see someone has hand-edited one of them. I have checked one instance of a hand-edit (P1-60) and the two wiring lists seem to refer to the same 'chain of wires' so this hand-edit does not introduce a functional change. If you had the wiring list(s) as a spreadsheet - you could sort it according to 'signal' so that all the same signals should occur together. I suspect that some of the signal names may be mistyped in some places though.

    2. Checkout the registers / big mix and ALU separately.

    FIG LD1 on PDF page 7 of "LD8-LD23 Schematics".pdf shows the register layout, the large MUX gate and the ALU. This is the 'heart' of the machine. I would remove all of the peripheral chips - just leaving these in place. You should then be able to bring out the 'inputs' to the remaining chips and test out the various paths with a simple 1 Hz clock and some switches. Have you done this already?

    For example, you can set up the MUX gate to enable the SR switches (MUX CONTROL=5) and set the ALU for PASS (M, S0..S1 and CIN) and what you get out of the ALU should be whatever you enter on the switch register. By applying the relevant CLK and /CLR signals to the appropriate register - you should be able to latch the F outputs from the ALU into the selected register. Note that there are some extra inputs for AC to do with rotating - so you will need to set these to LATCH the input into the AC register for now. You should be able to exercise AC SHIFT LEFT and AC SHIFT RIGHT later. You should now be in a position to latch different values into the different registers (SR->MUX->ALU(PASS)->desired register); and then ensure that each register correctly passes through the 1 of 8 MUX gate. Then you can exercise the ALU by taking the SR->MUX->ALU(A) and AC->ALU(B) and using various combinations of the M, S0..S3 and CIN signals to ensure the correct thing happens in going through the ALU.

    As the memory is connected between MA/MB and MUX(6) you should also be able to exercise the memory as well.

    Only when this works in 'single-step' mode should you move on.

    Document the results from each step. In fact, identify what the expected response is first (before you try the test) and ensure you get what you expect. If you don't - go back to the IC data sheets and check that you have understood what should happen. When I tried this step on my simulation of the LD30 I realised I had misunderstood how the ALU works with CIN. I had the operation inverted in my mind!

    You should then be able to try something like "take AC, use the ALU to add 1 to it and store the result back in AC". You should then be able to increase the clock speed to the AC register and see what happens. It obviously should count in binary. If it starts to "stutter" as you increase the clock - you may have some noise-indices problems somewhere (or floating inputs to ICs). You may need to wire up some drivers and LEDs to see the pattern on the output from the MUX and ALU. You may have to video the LEDs if you don't have a scope or a logic analyser and play it back at slow speed to ensure that the counting is working properly. If the ALU will add 1 to the 'A' input (i.e. the output from the multiplexer) you should be able to try this test with all of the registers. I haven't got the ALU data sheet handy at the moment.

    I don't know how much of this you have already done - so I may be covering 'old ground' for you?

    Dave

    Leave a comment:


  • daver2
    replied
    Thanks Marty.

    The link seems to work OK for me.

    I will download the files and have a look when I get a few minutes.

    Dave

    Leave a comment:


  • smp
    replied
    Marty & DDS, thanks for the clarification. I'll go and rummage around.

    smp

    UPDATE:
    I finally got there... Yes, the URL is exactly as was stated. I have no idea why I had so much trouble, but I'm there now.
    Last edited by smp; September 18, 2015, 10:16 AM. Reason: added UPDATE:

    Leave a comment:


  • DDS
    replied
    Originally posted by smp View Post
    That's funny - I get "There are no items to display."

    smp
    To clarify:

    I used the link in Marty's post and was told I needed to register, just like he said we would. So I registered, retried the link and hit the dead end you hit. But I was able to rummage around and after some flailing about I found the documents. Later I tried the other link that pbirkel supplied and it took me directly to the documents.

    Leave a comment:


  • Marty
    replied
    Hi All;

    I did get to it.. BUT, I had to do some Backtracking..
    Yes, So, after getting "The page you were looking for was not found." ..
    I went to the top of the screen, where it displays, "" The N8VEM Home Brew Computer project "" I clicked on that rectangle..
    It then shows "Front Page" of the "" The N8VEM Home Brew Computer project "" And I type in the search Box, ""LD12" .. And it takes me to where all of the files are Located, Which is the exact same Link that You are looking for..
    n8vem-sbc.pbworks.com/w/browse/#view=ViewFolder&param=LD12%20PDP-8%2FI

    I don't understand Why it won't work directly from the link, but it doesn't seem to work..

    Here is the Reworked Board..

    002.jpg

    THANK YOU Marty
    Last edited by Marty; September 18, 2015, 07:21 AM.

    Leave a comment:


  • smp
    replied
    Originally posted by Marty View Post
    Hi All;

    SMP, "" That's funny - I get "There are no items to display." ""
    I think You get that because You have Not registered.. I think DDS got in because He had already registered.. DDS, Correct me If I am Wrong..
    So use the Link that I posted above, and Register then, You can use the other expanded link..

    THANK YOU Marty
    That's not it - I am a registered member.
    For whatever reason, I continue to get "The page you were looking for was not found."

    Sigh...

    smp

    Leave a comment:


  • Marty
    replied
    Hi All;

    SMP, "" That's funny - I get "There are no items to display." ""
    I think You get that because You have Not registered.. I think DDS got in because He had already registered.. DDS, Correct me If I am Wrong..
    So use the Link that I posted above, and Register then, You can use the other expanded link..

    THANK YOU Marty

    Leave a comment:


  • smp
    replied
    Originally posted by DDS View Post
    It was possible to find it without the link working (I did!) but this link takes you right to the good stuff. Thanks!
    That's funny - I get "There are no items to display."

    smp

    Leave a comment:


  • DDS
    replied
    Originally posted by pbirkel@gmail.com View Post
    That's: "http://n8vem-sbc.pbworks.com/w/browse/#view=ViewFolder&param=LD12%20PDP-8%2FI"
    It was possible to find it without the link working (I did!) but this link takes you right to the good stuff. Thanks!

    Leave a comment:


  • pbirkel@gmail.com
    replied
    Originally posted by Marty View Post
    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

    THANK YOU Marty
    That's: "http://n8vem-sbc.pbworks.com/w/browse/#view=ViewFolder&param=LD12%20PDP-8%2FI"

    Leave a comment:


  • m_thompson
    replied
    Marty,

    Once you get it working you can use the front panel from this project:
    http://obsolescence.wix.com/obsolescence#!pidp-8/cbie

    Leave a comment:


  • Marty
    replied
    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

    Leave a comment:


  • daver2
    replied
    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

    Leave a comment:


  • Marty
    replied
    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

    Leave a comment:


  • DDS
    replied
    "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.

    Leave a comment:

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