open-collectoropen-drain
I have a comparator with open collector output (LM311) and a gate with open drain output (SN74LVC1G06). I want join these two outputs to make a wired-or, with a pull-up resistor tied to +3.3V.
In a general way, is it appropriate to join an open-drain output with an open-collector output?
Yes. Open drain outputs act like a switch to ground. The resistor acts as a pullup for when none of the outputs are active.
If you replace the open drain outputs in your circuit with buttons its easier to visualise:
simulate this circuit – Schematic created using CircuitLab
If you want to use a PSMNOR9-25, which is an 100A N-Channel MOSFET, then the following is a suitable circuit. You could replace Q1 with a small N-Channel MOSFET such as a 2N7000.
simulate this circuit – Schematic created using CircuitLab
Best Answer
There really isn't an issue with that. Open drain and open-collector are essentially the same in this application.
Two things to watch out for:
Make sure you get your polarities right and that you need an OR gate and not a NOR gate because the open-collector/open-drain style often invert your signal. If the output of either opamp is high, then the output of that IC will be pulled low, and vice versa.
Just make sure each IC can sink (Vcc/R1)+(Vcc/R2). This will be the worst case current if one output is low, and the other is high.
Picture for reference.