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

 

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.

 

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...

 

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...

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