plc

They are both switching outputs which either can source current i.e. output current and not able to drain current ....

Or for the sink output it can input current but not supply current....

So I guess its probably easier to say the source output can drive a load by supplying current to it.

And the sink output can drive a load by taking current from it...

Does that make sense? A picture would definitely be worth many words here!!!

John.

Both types use a solid state switch (transistor). In a sourcing output, the switch is connected to the positive lead of the power supply and your load is connected from the switched output to the power supply negative. In a sinking output, the switch is connected to the power supply negative, and your load is connected from the power supply positive to the switch.

Can you have a sink output to a coil on a hydrualic valve?

You can.