One of the many advantages to using a PLC/PAC is the simplicity of the I/O wiring.
I/O devices are wired to I/O points on a fixed I/O unit and to I/O modules in a modular unit. Input devices such as switches, pushbuttons and sensors are wired to input module points and output devices such as indicator lights, solenoids and motor starter coils are wired to output module points.
Note: For this tutorial we will be referring to modular PLC/PAC's.
I/O modules can accept Acv, DCV or a combination of ACV and DCV. Most modern automation systems use DCV I/O, therefore we will focus on DCV.
DC voltage has a polarity. We already know that current flow is from negative to positive. This current flow is also referred to as electron flow and is also the current that is measured when making measurements with a meter.
There is also another current flow. This current flow is called 'conventional current' and is the current flow that solid state physis use. This current flows from positive to negative. When electrons flow from one atom to another they leave a hole behind on the atom that they leave. If you could watch the electrons moving from atom to atom, the electrons would be moving in one direction (negive to positive) and the holes would look like they are moving in the opposite direction (positive to negative).
DCV I/O module polarity is referenced using the terms sink and source. The terms are derived from conventional current flow, but we will explain sink and source in terms of electron flow.
Why should we care what direction current is flowing? Mechanical switches, indicator lamps and solenoids don't care what polarity the current is flowing. But sensors such as photoelectric, inductive and capacitive proximity sensors are also wired to PLC/PAC I/O modules. These sensors are solid state, using diodes and transistors in their internal workings. These devices require current to be flowing in the right direction.
Click the terms for details and wiring diagram examples).
Sink is designated NPN.
Source is designated PNP.