Logic output to open-collector output?

Discussion in 'General Electronics Chat' started by autorelease, Dec 17, 2010.

  1. autorelease

    Thread Starter Active Member

    Jan 26, 2009
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    I have a chip I'm trying to connect to the IRQ line of a Motorola 6800-style bus. Typical 68xx-family devices have open-collector interrupt outputs, so multiple chips can connected to the IRQ input in a wired-or configuration.

    However, the interrupt output of the chip I have (a TMS9918A Video Display Processor :)) is an active-low logic output; it's driven both high and low

    I figure I could connect the interrupt output to an inverter and then to the base of an NPN transistor, and then connect the collector to the IRQ line.

    I have two questions though:
    - What value should I use for the base resistor? I can never remember...
    - Can I use a PNP transistor and save an inverter? What would the schematic for this look like?

    Thanks,
    -matt
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think you can use a common base open collector stage, which is noninverting. Use a 2N3904. Connect the emitter directly to the INT output of your TMS9918A. connect a 10k resistor from the base to Vcc. If your IRQ line has very light loading you might need to add a 10pF capacitor from the base to GND. If the IRQ bus has one or more additional open collector drivers, you probably don't need the 10pF cap.
    Like I said, this should work. In any case, it can't hurt to try. I ran a simulation, and it worked well.

    A PNP still inverts, and it wouldn't work anyway. You need a bus pulldown. The PNP pulls up. The bus already has a pullup somewhere.
    Actually a PNP emitter follower might work, but it will have a 0.7V drop, so your logic 0 will be about 1V, which will give you very little noise immunity.
     
  3. autorelease

    Thread Starter Active Member

    Jan 26, 2009
    39
    0
    Thanks Ron! I just simulated it as well and it seems like it would work. (and I've got plenty of 2N3904s lying around.)

    I had forgotten about the common-base configuration. It's been too long since my engineering classes...
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    Common base is seldom used in switching circuits. I have used them for noninverting level shifters, but never as an open collector bus driver.
     
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