Wiring PLCs - PLC How To | Simple PLC How-To Guides Internal ConnectionsCommonsRelaysSource & SinkThe inputs and outputs of PLCs are connected internally in 1 of 3 ways: Relays, Source or Sink. PLC manufacturers often produce all 3 versions, so you can select the version that best fits your application. Below are descriptions of each variety and what their advantages/disadvantages are go over a couple symbols that will be used. DC PowerSwitch. It represents an input. Could be as simple as a light switch that you’re monitoring to see if it’s on or off. It represents something you..See more on plchowto Wiring Diagrams - FACTORY I/O Wiring Diagrams¶ Generic Wiring Diagram¶ Generic diagrams explaining how to wire the Advantech USB-4750 to a PLC.