DI/DO modules configuration

Dear,

with reference to the attached picture, please can you clarify If DI module is sink or source and if DO module is sink or source and which one do you prefer to be used, ( I mean which one will be safer to be used)

Thanks a lot!

Charbel

The Inputs are of Sink type and Outputs are of Source type. Because Inputs are draw power from 24V DC Bus and Output feeds power to the Load.

dear Basu,

thank you, is this configuration safe to be used?

thanks,

charbel

Sinking inputs are generally safer than sourcing. The issue is this:

Imagine you have a conduit between your panel and the input devices in question. Some idiot drives a lift truck into the conduit. One of the input wires gets crushed and grounds out. What happens?

If you have sinking inputs, the wire grounds out, causing a short in the circuit, and blowing a breaker. If you have sourcing inputs, the input wire is already grounded because with sourcing inputs the input devices are wired to the grounded side, so nothing blows. Instead, the PLC sees a path to ground through the input and happily updates the corresponding address to 1. Forklift driver backs up, sees nothing has really changed, and goes on his merry way, breathing a sigh of relief that he didn't damage anything.

Later, an operator approaches the machine and sees that some component of it is running when it shouldn't be. The plant spends days trying to troubleshoot the problem until one day a tech notices the crushed conduit by chance. And that's the best case scenario. Worst case the PLC turns something on when it shouldn't and shoots something across the room and kills someone. That is why, in a nutshell, inputs should always be sinking.

Outputs, on the other hand, can be sourcing because unlike inputs, outputs can be controlled.

dear,

thank you for your clarification, actually I made a small picture to try to understand, please check below. What I could depict is that if the switch in the field is normally open and a wire has been cut then it should not be a problem. what do you think?
thanks a lot.

Charbel said:

dear,

thank you for your clarification, actually I made a small picture to try to understand, please check below. What I could depict is that if the switch in the field is normally open and a wire has been cut then it should not be a problem. what do you think?
thanks a lot.

Click to expand...

I'm not sure what you're asking. It looks like you have an NPN (sinking) sensor and you're using an interposing relay to convert the signal in order to be compatible with the input, because a sinking input requires a sourcing sensor. Correct?

Anyway, I'm not sure what problems you're anticipating, but if the wire is cut where you indicated, the circuit will never be completed and thus the input will never be active.

What I was talking about was a situation where the field wire below where you indicated is grounded. Assuming your DC power supply is properly grounded, if that bottom field wire is ever grounded (which could happen if run in a metal conduit that gets crushed and the wire touches the inside wall), you would be completing the circuit to K1, because the negative of your supply and the field wire coming from K1 would connect through ground. This would make K1 fire and enable the input despite the fact that the sensor is not made. The ideal solution here would be to use a PNP (sourcing) sensor instead and not use K1 at all, if you can.