Sink/Source wiring diagram help

A bit off topic, but I know this is the place to ask.

Long story short I had issues with a machine. an alarm and circuit not working.

Both were traced back to this Allen Bradley 1769-IQ16 Input Sink/Source Module.

We I jump 24VDC+ to point 4 and point everything works. My theory is that they both are source inputs and get the + voltage from that card. Card is bad not supplying voltage.

Maybe I’m just too tired today but I am having trouble testing/confirming this for a peace of mind. When I looked at the diagram I got really confused.

I would think some points are source and some are sink. The diagram looks like the points are interchangeable for source/sink. the points i jumped power to are IN4 and IN8. I traced IN4 eventually back to a negative. so the positive has to be from the card. But the diagram still has me confused.

Below is the diagram in the manual

I’d appreciate anyone’s help who can assist me in making sense of this.

Thank you in advance.

The card doesn’t supply voltage at all. Whatever you hook to the DC COM ports you will need the opposite to turn the input on.

So, if you tied 0 volts DC to DC COM 1 then you would need +24V DC to turn on inputs 0-7 (in Allen Bradley’s world this is called sinking, you would need a sourcing sensor or a PNP sensor). If you tied +24V DC to DC COM 1 then you would need 0 V DC to turn on inputs 0-7, this is called sourcing and you would need a sinking sensor or a NPN sensor.

This card apparently has two common terminals so you could have half of the card sinking and half of the card sourcing if you wanted to. Usually both commons are tied to the same point.


@Duffanator I guess my confusion came from assuming DC COM meant negative. Thank you!

No, in this case DC COM just means it’s the common for the inputs, and you can choose what that common voltage is.

You’re welcome.

@Duffanator ahh. Yeah in my short experience generally com is neg and something that has source provides the voltage. Which made me very confused. I think it was just weird verbiage.

again thank you

@Duffanator that also confirms if other inputs are working then the card must for sure be bad. as the common is obviously there, and jumping bypasses the internals .

Well, I would say that no, that is not the case based on your description of the problem.

If you jump 24 volts to input 4 and then the input turns on then the input card is good, right? If you aren’t getting 24 volts there then whatever device supplies the 24 volts is not supplying 24 volts. Do you have a print for the machine/system that shows what is connected to the inputs that are not turning on?

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@Duffanator I do. And it gets more confusing by the second. lol. I traced back IN8 to a contactor, cool. But there is no wire on the other side of the NO contact, and no loose wire in sight. and it happened mid production. I find it so weird that an alarm would trigger out of nowhere and the wire just be MIA mid production run.

Thank you for your input. I can’t really explain it well, but at least i know my wiring will be “correct” . its a rats nest of a panel that should be redone from scratch. I’m going to have to really dig into it, to sort out this mess.