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Thread: DEC8251 or N8251 wanted for reference

  1. #1

    Question DEC8251 or N8251 wanted for reference

    Hello everyone,

    I have a failing M8330 timing generator in my PDP8/F. To test the DEC8251 with my
    HP10529A comparator I bought a few N8251N from Ebay.
    My comparator gave an error on one of the outputs while testing. So I swapped the chip with an N8251N.
    The result is that the D0BB test programs halts 2000(oct) lines earlier.
    Funny enough, my tester with also that new N8251N in it as reference doesn't spot any difference anymore.
    So both Ebay chips behave the same in the board and tester. But the replacement chip messes everything up.
    I replaced the old chip, and at least I have my old error back.

    I'm really curious about what is happening here. I fear that the Ebay chip might be not compatible
    or maybe even Fake? I bought chips before from the same supplier and those worked great...

    So, does anyone have a good DEC8251 replacement chip for me?

    Thanks in advance!
    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #2


    Sorry I don't have this chip, but I read about SN7442 could sometimes work as replacement. The logic table is equal for input 0-9, but different for 10-15. And it is not pin compatible.
    But I could send you this.

  3. #3


    Hi Volker,

    Yes that is right, I used the 7442 also in several of my redesigned Omnibus boards.
    But as you say the truth table is different when you decode higher number outputs.
    So you should check that before using this replacement...

    I've drawn and ordered this converter PCB tonight. But I can't stand it that the
    original chips are so hard to find and that replacements seem to be fake...

    7442 - DEC8251.jpg

    After testing the board will be on my GitHub as well with a note of the differences.

    But for now it would be a great help to have at least one good N8251.

    Regards, Roland
    Last edited by Roland Huisman; May 25th, 2020 at 12:21 PM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  4. #4


    So for those who are interested, there is a DEC8251 replacement (SN7442) pcb on my GitHub.

    Picture 1.jpg Picture 2.jpg Picture 3.jpg Picture 4.jpg Picture 5.jpg

    I got the timing generator back to life again. A DEC8251 and a 74H74 were broken.

    The weird thing was that after replacing the DEC8251 the maindec halted earlier.
    So in the first place I thought that this was a faulty 8251 replacement. But, after finding an original
    DEC8251 in the basement, I got the same result. So that DEC8251 on the M8330 was really bad.

    All interrupt handling went wrong. But when enabling the interrupts the ION LED goes on
    and when setting the flag on the TTY controller INT BUS LED goes on.
    But the CPU didn't receive an interrupt...

    The cause was a defective 74H74 causing the INT STROBE signal to fail. The strange situation
    was that both errors caused by different defects were interfering with each other. So that was
    a bit confusing to say the least...

    But hey, now I've got a good DEC8251 chip for my tester as reference and a suitable replacement
    for future repairs. (Just watch the truth table for the board you are servicing.) But it seems that there
    are no problems to be expected for the way the chips are used by DEC. In all the clone boards which I
    designed I used t 7442 to replace the DEC8251 as well)

    I thought that this way of making a drop in replacement might be interesting for the
    SP380 and SP384 as well... I didn't want to use small SMD chips. I used a normal chip
    to keep the looks a bit the same... So if anyone thinks that is useful I can make a drawing for that too...

    Regards, Roland
    Last edited by Roland Huisman; June 13th, 2020 at 07:33 AM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  5. #5


    If I recall correctly, the DEC8640 is the preferred replacement for the SP380 (lower leakage spec).


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