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No pulsing on an OR gate?

Discussion in 'Electronic Design' started by Anthony Girasoli, Nov 8, 2004.

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  1. Hello all,

    I'm currently trying to troubleshoot a faulty arcade game board from the
    80s (Star Castle, remember that??) :) Anyway, there's a problem in the
    intensity circuit, so I'm tracing those particular outputs back with a
    logic probe. When I get to an OR gate (actually, the schematics says
    it's an AND gate with the two inputs and output all inverted), I found
    something weird. The output of the OR gate has no pulsing. But both
    inputs to the OR gate have pulsing. (The inputs are stuck high and low,
    as well.)

    What is going on here? Shouldn't I have pulsing on the output of an OR
    gate, when both inputs have pulsing? Is there something that would
    cause no pulsing on the output?

    Thanks for any help!

  2. Tim Wescott

    Tim Wescott Guest

    An OR gate will have a high output if one or the other inputs is high.
    If input A is always high when input B is low and visa-versa, the output
    will stay high.

    The schematic notation is to indicate that the signals are active low,
    i.e. they are "false" when the signal is high. This was a common thing
    when logic was TTL and could pull lines down faster than it could pull
    them up, and is still common for "wired-or" logic where you have a line
    with one pullup resistor and a bunch of open-drain outputs.
  3. Same thing. Just a different way of drawing it. All to do with logic and
    negative logic. Usually the type of drawing used depends upon what one would
    expect as being an "active" input. The end result is the same, it just
    sometimes makes it easier to understand the schematic.
    Make your mind up!. They're either pulsing or they're stuck!
    Not if they are pulsing in perfect antiphase. The only simple way to check that
    is with a scope on each input.
    A faulty chip.
    You're welcome.

  4. Gibbo and Tim,

    Thanks for the insight and explanations.

    Let me clarify a bit... when I say one input is stuck high to the OR
    gate and the other input is stuck low, I mean that using my logic probe,
    I have a steady high LED and steady low LED, respectively. And, my
    logic probe's pulse LED is flashing. Which (I am delving back to my CSE
    logic design lab days) means the probe reads a clock pulse, correct?

    So, one input is a steady high with pulsing, and the other input is a
    steady low with pulsing, and yet the output is a steady high, with no
    pulsing. If the clock pulses going to the two inputs are 180 degrees
    out of phase with each other, I understand that they would be cancelled

    I looked at the schematics, and the two inputs of the OR gate are being
    fed with two different circuits that have different clocks. One circuit
    gets it's timing from "CLK1" and the other is from "Inverse CLK2"... so
    that kind of makes sense. When I put my probe to CLK1, I don't see a
    pulse, but when I put my probe to Inverse CLK2 I do see a pulse.

    OK, I went back to my notes to see what that particular OR gate should
    be doing, from a working boardset that I used to have. It says that I
    should have pulsing on the output of this particular OR gate, with the
    high and low LEDs of the probe flashing rapidly, suggesting activity.
    Am I too quick to conclude that I have a bad IC where this OR gate

    Thanks again for all your help!

  5. If one input is almost permanently low, with high pulses on it, and the other
    is almost permanently high, with low pulses on it, and they are the compliment
    of each other then your output will be permanently high. Which seems to be what
    you are saying you have.

    Borrow a scope, and look at both inputs at the same time.

    You'll probably find something like this....

    (view in fixed width font)


    ____ ____ ____ ____ ____
    | | | | |

    Which will give you a permanently high output as, quite simply, one OR the
    other is always high.
    Yes. Though if you don't have a scope it may actually be quicker (and cheaper)
    to simply change the chip. From the suggested era they are probably 74LS series
    TTL or 4000 series CMOS which cost peanuts each.

    If you're not experienced at removing them, instead of trying to be clever and
    remove the chip carefully (which can be real difficult without a lot of
    practice) just cut all the legs off, then remove each leg individually. Dead
    easy, and almost no risk to the PCB (unless you're a complete knob!)

    You will then be left with a machine that does either exactly the same (my
    guess) OR (no pun intended) nothing (in which case you probably fucked up the
    PCB removing the IC) OR works perfectly (doubtful as, from your [admittedly
    limited] readings I don't think that IC is faulty)

  6. Tim Wescott

    Tim Wescott Guest

    With just the logic probe, unless you have a beeter idea of what's going
    into the gate, you don't know. You should either buy an O-scope
    (they're not _that_ spendy any more!), see if you can compare the inputs
    to a known-good board, or just change the gate.

    I would get a scope, because I can't imagine effectively doing this
    stuff without one.
  7. When I repaired video games, when we got to this point, we'd just
    replace the chip, because any more screwing around costs more.

    If one took more than 10 min. to diagnose, we'd just replace
    every chip on the board at $0.15 each, rather than continue
    to troubleshoot at $30.00/hr.

  8. Thanks again guys for enlightening me on the logic stuff. You've been
    very helpful as I've been researching my problem in my old books and the
    Internet, and had to go to usenet as a last resort.

    I agree... If I'm going to work on these games as a hobby, I should just
    front the bucks (or quid) and get a 'scope!

    At any rate, I changed that OR gate IC, and of course, same problem
    (ahhh, hopes were up for a short while!)

    On to further troubleshooting this board. I am going deeper into the
    schematics. Thanks again to all for your replies!!

  9. Rich Grise

    Rich Grise Guest

    Yeah, like I said, at that point we'd shotgun them, but we had a
    customer paying $30.00/hr. and losing who-knows-how-much with a
    down game. Well, that's kind of an exaggeration. Usually we'd
    send them a loaner. Not a whole game, just a board. :) But we
    still had to fix _his_ board, and get the loaner back.
    Yeah, if you're doing it for fun and learning (there's a difference?)
    then go for it! Keep probing and keep asking questions. And save your
    pence for a proper scope. :)

    Have Fun, and welcome to the zoo!

  10. Bob Stephens

    Bob Stephens Guest

    You really should do yourself a favor and get your hands on a dual trace
    oscilloscope. Stick the logic probe in a dart board or something.

  11. markp

    markp Guest

    I agree, the logic probe probably has very limited bandwidth anyway and I
    wouldn't trust it to indicate a pulse. Both inputs need to be shown on a
    'scope to be sure.

  12. The output of an NOR is only low if both inputs are low. If either input
    is high then the output is high.

    If the pulses are arraigned so that while one signal is low the other is
    high the output will always be high.

    Try using pencil and paper, draw some truth tables and timing diagrams.
  13. If it were me, the last thing I would suspect is a defective logic IC.

    #0 Anything burned, smell bad, blown fuse?

    #1 Connectors. If plug in cards are used check for bent pins, clean
    gold fingers with a pink eraser.

    #2 Wire connections - are the wires crimped tightly?

    #3 Power supply outputs, all within 5%. Check for ripple by setting
    the multimeter to AC. Check for spikes by placing a 1N4148 diode
    in series with a digital multimeter (cathode to + input), if the
    reading goes up then you have a high frequency noise problem.
    You can also use an old Heathkit Demodulator probe.

    #4 Check all electrolytic capacitors. And old VOM with a needle (the cheaper
    the VOM the better, cheap ones use more current) is the thing to use.

    #5 Check power semiconductors

    If it appears the problem is in the itty-bitty stuff then look at the schematic,
    divide the circuit into two logical parts and check the signals in the middle.
    If you can determine which half looks bad then cut that section into two and
    check the signals in the middle ... You can usually get to the defect in a
    half-dozen probes.
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