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PDP-8 logic levels

Discussion in 'Electronic Design' started by Tom Del Rosso, May 6, 2013.

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  1. From the wikipedia article on the PDP-8:

    "In the 8/S two different logic voltages were used, an inexpensive way to
    increase the fan-out of the inexpensive diode-transistor logic."

    What's that mean? Normally there are 2 logic levels. Do they mean 2 supply
    voltages?

    I know DTL modules in the Apollo guidance computer each had complementary
    outputs, so maybe that's what they're talking about.
     
  2. Bob Vines

    Bob Vines Guest

    Tom,

    The Wikipedia article appears to be incorrect as far as this sentence
    is concerned. The PDP-8/S is a negative logic machine using -3 and 0
    volts. The DC voltages required by the logic are +10V and -15V. The
    PDP-8/S Maintenance Manual at bitsavers.trailing-edge.com/pdf/dec/pdp8/
    pdp8s/PDP8S_MaintMan.pdf, page 1-3 states: "The dc voltages required
    by the logic are +10V and -15V. All logic is solid state; transistors
    and diodes operate on static logic levels of 0 and -3Vdc (tolerances
    are 0V to -0.3V and -3.2V to -3.9V)."

    A friend experienced with PDP-8 family machines stated, "I would
    hazard a guess this is just a misunderstanding of the fact that DEC
    predates integrated circuits so the newer ones [PDP-8s] are plus 3 and
    the power supply is +5 while the older ones are -15 power supply and
    the logic is -3 volts." He also pointed out that there are R-xxx
    [labeled] modules and there are S-xxx modules. The difference is
    there are different resistors. The more expensive "S" cards have less
    fanout but higher speed than the R-xxx modules.

    He also said, "Positive stuff has only +5. The old stuff has +10 and
    -15. That's not a logic level consideration. Just that some aspects
    of the module need the 10 to make the spec. Plus 10 volts is even
    "lower" than 0 volts when you are speaking to -3 or so as the
    reference point. In any case, a designer of R-series modules can
    count on a +10 voltage to help cleanup signals if need be."


    Bob
     
  3. THANKS for the expansive answer. That FTP site is quite a goldmine.
     
  4. Tauno Voipio

    Tauno Voipio Guest


    DTL is resistor pull-up open-collector logic. The supplies well
    out of the logic signal range help getting decent edge speeds
    from the pull-up. The opposite polarity supply is used to
    guarantee off-state bias on transistor switch bases.
     
  5. That PDP8 was a digital computer using two logic levels. Though to maintain
    that levels positive and negative voltages were used.

    petrus bitbyter
     
  6. gregz

    gregz Guest

    I never got the knack of using those DEC symbols. I remember them doing a
    voltage test changing power supply AC levels, to see if the computer passed
    limit testing. The power supplies just used constant voltage transformers.
    I got in there learning integrated circuits on PDP-15 and PDP-8I.

    Greg
     
  7. Guest

    That's not what "negative logic" means...
     
  8. Guest

    Do you remember the scene from Planet of the Apes when they were reading a passage from their "Bible" that turned out to be a GM repair manual or something? Well that's not too different from the kind of people writing articles for Wiki these days, complete shaved apes!
    You can look at the 8S series [gawd-awful] circuits and see they're using two different logic power supplies for internal pull-up/down biasing. This is what the /quote/ is referring to, and the Wiki author has no idea what itmeans so he conceals his ignorance by using quotation marks.
     
  9. Rod Serling wrote that script, and if a GM manual was his choice then I have
    faith in his judgement.

    I put the quotes on it. The words were composed by the wiki contributor. I
    guessed right at supply voltages, but it has no relevance to fanout
    obviously.
     
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