Hi there,

Very simply put:

PLC source - the PLC card will provide the 24Vdc

Sink - the PLC card is passive and the instrument will provide the 24Vdc

How to distinguish between the two. The best thing would be to read the instrument manual.

Usually (but not always) If the PLC is the source, you will only have two wires going to the instrument. The instrument would not have an external power supply.

If the Instrument is the source, it will have an external power supply cables as well as the signal cables.

These configurations in instrumentation are usually referred to as 2-wire (PLC source) or 4-wire (PLC Sink) loops.

Regards,

Craig

To be more precise, a sources always PROVIDES something whereas a sink always TAKES something.

In electronics (including PLCs), a signal SOURCE will have current coming OUT of it (that is, it provides current, which means that it provides 24 Vdc voltage). A signal SINK means that the currents ENTERS into the PLC and that the card provides the DC common (zero voltage).

Everything, including electricity, flows from a source to a sink.

To see whether a PLC card is sourcing or sinking, read the manual. Note that in many cases, a given card can have channels that are sources and others that are sinks ... Each channel could either be configurable by software (programming) or hardware (switches or jumpers on the electronic card), or they can be factory-set to be sources or sinks.

I hope this helps! :-)
DZ

Here is a schematic showing the "sinking" or "sourcing" circuits in the EZAutomation control modules. The circuits are the same in any controller.

Bigger pictures.....

The above comments are absolutely correct

http://www.automationdirect.com/static/specs/sinksource.pdf

The following link may give some more details..