PLC Input Units

Example of input lines can be connection of external input device. Sensor outputs can be different depending on a sensor itself and also on a particular application.

In practice we use a system of connecting several inputs (or outputs) to one return line. These common lines are usually marked “COMM” on the PLC controller housing.

DC Inputs
DC input modules allow to connect either PNP (sourcing) or NPN (sinking)
transistor type devices to them. When we are using a sensor have to worry about its output configuration. If we are using a regular switch (toggle or pushbutton) we typically don’t have to worry about whether we wire it as NPN or PNP.

AC Inputs
An ac voltage is non-polarized. Most commonly, the AC voltage is being
switched through a limit switch or other switch type. AC input modules are less common than DC input modules, because today’s sensors typically have transistor outputs. If application is using a sensor it probably is operating on a DC voltage.

Typical connection of an AC device
to PLC input module

Typically an AC input takes longer than a DC input for the PLC to see.
In most cases it doesn’t matter to the programmer because an AC input device is typically a mechanical switch and mechanical devices are slow.

It’s quite common for a plc to require that the input be on for 25 ms (or more) before it’s seen. This delay is required because of the filtering which is needed by the PLC internal circuit.