Connecting open collector outputs to inputs of another device

Thread Starter


Joined Dec 9, 2013
This is a great forum and has inspired me to take on some electronics projects of my own. Looking for some help interacting some devices together. I’ve got two devices (let’s call them A and B) that need to send a pulses to trigger actions in a third device (since this third device will receive the pulses, let’s call it R)
Device A has a dry relay output.
Device B is an open collector output with the following statement from the manufacturer: indicator output provided by open collector outputs, <40 V, 50 mA

Device R has many inputs, each in a configuration like the one shown in this diagram:

My question is about what I need to do to connect the outputs of device A and the outputs of device B individually to the inputs of device R?
It would seem, from my limited knowledge, that if I connect the output relay pins of device A directly to the input pins of device R, it should just work so that device A can signal device R.

With regards to device B, from my limited knowledge of how open collectors work, it would seem like I could also directly connect device B to device R, but I may be wrong about this. Friends have told me that I should put a logic controlled relay (PDF Link) between device B and device R, to basically turn the outputs of device B into relay outputs. But from looking at that device R input diagram, it seems to me everything is there to just drive this directly (like doesn’t that diagram show a voltage source and a pull-up resister?). Am I wrong?

I would love to hear the thoughts of the people on this board.


Joined Oct 2, 2009
Connect a relay contact between GPi#P an Gnd in your diagram, or connect the collector of an NPN transistor to GPi#P and connect its emitter to Gnd, either will pull GPii#P low...


Joined Nov 7, 2013
The open collector outputs can be paralleled if no special logic is needed.

Any one will activate input.


Joined Aug 7, 2008
If R-1 is sensitive to multiple neg pulses, then relay contacts need to be debounced. R-2 , direct connection to collector is OK.


Joined Oct 2, 2009
The beauty about open-collector outputs is that you can parallel multiple control signals to create a WIRED-OR function without the need for extra gates. This works for NEGATIVE LOGIC (i.e. ACTIVE-LOW signals). Any LOW signal asserts the function.