9401 current limiting resistor

[MJHanagan]

I have a couple of 9401 DIO modules for my cDAQmx and I want to use a few of these lines as inputs to detect (and trigger) relay contacts.  On page 9 of the 9401 manual it shows how to essentially convert an external +5 V signal and a set of switch contacts to a proper "TTL" input signal (see the "Dev 2" in the attachment). My question is: What is a good value for this current limiting resistor?  I will be sinking at least 4 channels worth of current on each module.  My initial thought is to keep the current fairly low (1-2 mA), but since I am sending these currents through a noisy environment of electromechanical relays should I boost the current level up to help prevent false signaling?

[lab_boy · Accepted solution]

Hey

It depends on how much current you want to drive, I found this document which might help you determine that also check is DF.

Good luck

lab

[MJHanagan]

I want to only drive as much current as needed to get the a good reliable input signal at the module pin itself and not sink too much current through the module's COM pin.  The manual does state that the the typical input current is ±250 uA, so the resistor should be less than ~2k to prevent the voltage at the module from dipping below 4.5V when the external switch is open (your link helped me think about this aspect).  When the switch is closed I initially assumed all of the current would sink through the module via the COM pin.  After rethinking this it is obvious that it does not (assuming the COM pin is properly wired to the ground terminal of the +5V supply).

So, I am going to try something in the 500 to 1kΩ range to produce a sinking current between 5 and 10 mA (something a whimpy 24 gauge wire can handle nicely!).

Thank you for the help!