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TTL logic problems (can't pull a signal low... how?)

Discussion in 'General Electronics Discussion' started by Rattanee, Aug 6, 2013.

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  1. Rattanee


    Aug 6, 2013
    Hey folks, I've been into circuit repair for a while, but I've just now gotten into designing my own circuits. I'm trying to build a custom interface between an arcade monitor (CGA) and a computer. I have the software part down, that's not an issue, I have the PC outputting 15khz fine. However I've added several layers of protection to the board so no signal can get to the monitor while it's not up and running in 15khz mode. To put things simple, my entire circuit is now working except for the sync signal blanking.

    I've built this sync combiner circuit:

    I have the enable line connected to a 74HC04's output that will go high while all conditions are met, and go low if not. In theory when low, it should pull the signal low as well, killing sync output.

    Now when the 7404 outputs high, all is fine and dandy, everythign works as it should. When it should go however, all that happens is that the sync signal appears on the output of the 7404 as well instead of getting properly pulled to ground. Could anyone point out the probably blatantly obvious problem I'm missing?

  2. Rattanee


    Aug 6, 2013
    Typical... as soon as you write the problem down you find the solution :D

    The schematics linked above are to blame. The enable line needs to go -before- the resistor so the input following it won't have a lower potential then the ground the signal is pulled to. Moving the diode before the resistor solved the issue immediately.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Glad to have helped ;)

    Seriously, I have found bugs after explaining code to the cat (It was the only thing that would listen).
  4. davenn

    davenn Moderator

    Sep 5, 2009
    speaking of cats and their uses....
    have a look in the comedy thread

  5. Rattanee


    Aug 6, 2013
    Well... based on the comedy thread... at least now I can say I have more space on my 'static workbench" ehehehe :)
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