Updated: Nov 11, 2020

In this article, we will learn about types of the PLC input and output used in the industries. There are many types of industries which require inputs and outputs to monitor and control the field devices.

Industries like automobile use digital input and output but in industries like petrochemical, chemical, food and beverages, etc require not only digital but analog input and output.

In every PLC you will find two types of module:

  1. Analog

  2. Digital

Now, in the analog and digital, they each have types in which we have to connect field devices.

In the digital, it has two categories, out of them one is digital input and the second one is a digital output.

Same as above we have two category name for the analog type. Analog inputs and analog output.


  1. Digital input: Pushbuttons, Proximity sensors, selector switches, all types of switches, Limit switches.

  2. Digital output: Solenoid valve, all type of ON/OFF valve, motor, pump, etc.


  1. Analog input: Signal coming from all the types of transmitter, VFD's speed control, strain gauge, etc.

  2. Analog output: Control valve, motor speed control command using VFD.

-Digital signal can be only binary like ON/OFF or zero(0) and one(1) or true or false.

-In the digital signal, you will have two different DC voltages. For zero it can be 0 volts and for one value it can be 24 volts.

- Analog signal can be voltage, current, millivolt of the thermocouple and strain gauge.

- In analog card, the signal coming from the field is analog so, the signal needs to be converted from analog to digital. Conversion happens inside each analog card which contains internal circuitry.

- mA signal which is coming from the field needs to convert in the voltage. For that resistor inside the analog module is used.

In the next article, I will discuss internal circuitry which converts the signal. If you have doubts or questions the please put it into the comment section.

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