Sink & Source
(As seen in AutomationFYI, 1 st Quarter 2000)
Everyone knows that a sink is where you wash your dishes and a source is where you get (hopefully) good tips. But did you know that "sink" and "source" also have meaning in the world of PLCs? Whether an I/O module is sinking (NPN) or sourcing (PNP) can make a big difference in whether the module will interface with the outside world.
Sinking and sourcing are particularly important in DC circuits. Consider diagram A, which represents a sourcing application. Here, positive voltage is applied to the module. In turn, each output (one shown) supplies current to the outside world.
Diagram B shows how a sinking-type module is hooked up. The positive voltage is supplied to one side of the load and module output "sinks" the current from the load. Typically, sinking modules are involved in interfacing electronics equipment (TTLor CMOS compatible) and sourcing modules to power solenoids, etc.
Input modules are also described as sinking or sourcing, but the terms have been defined two different ways. In one case (TTL compatible) the module is hooked up as in diagram C and called sinking. However, notice that the module actually sources current to the external device. So, is this a sinking module or a sourcing application?
Our advice? Ignore the words "sink" and "source" and choose the module that will work in your application, using the manufacturer's typical hookup diagrams as guides.