Unusual PLC Input voltage

I came across an OMRON PLC where the input signal voltage was provided by an external 24 VDC source wired in an unusual manner.
The +24 V was connected to the Input common and the 0 V was fed to the external switches. (All input signals come from external mechanical relay contacts).
I wonder what the reason is for this reverse wiring?

possibly the difference between source and sink?

I came across an AB PLC where the input signal voltage was provided by an external 24 VDC source wired in an unusual manner.
The 0V was connected to the Input common and the +24V was fed to the external switches. (All input signals come from external mechanical relay contacts).
I wonder what the reason is for this reverse wiring?


Actually the Omron PLC could have been wired either direction. Their inputs are bipolar.

Christopher, if you search this site on "source" and "sink" or "PNP" and "NPN" you will get more info on the topic than you dreamed existed! Look particularly for posts by Terry Woods on the subject.

Not all transistors are created equal. Some PLCs can have I/O wired as source or sink, some restrict your choice to one or the other. Some solid state field devices, like some proximity switches, can only be wired as a source or a sink. Some can go either way.

The guy that wired your system may have been accommodating specific I/O or field device requirements, he may have been wiring by habit and it didn't really matter, or he may have been guessing and got lucky. How would one know?

He must have been an Auto-mobile technician, not an Auto-mation one!

REVERSE WIRING?

One of the previous posts was correct, it is Sourcing / Sinking. A choice in wiring. Read up on it, it will all become clear.

Good luck.