PLC Input and Output —Types and Examples

In this article, we will specifically look at what a PLC input and output is. We will also take a look at the different types of PLC inputs and outputs, where they can be found, and the typical external components that can be used as inputs and outputs.

In every system, input and output parts play an important role. The system takes the input, processes it, and passes it as output to the other system.

It means that it processes the input data and provides information to another system. Similarly, input and output modules are the fundamental parts of the Programmable Logic Controller (PLC) systems.

In this tutorial, I am describing the PLC input and output modules in detail.

Table of Contents

• What is Input Module?
• Type of PLC input
• What components can be used as a PLC input?
• What is Output Module?
• Type of PLC output
• What components can be used as a PLC output?
• Frequently ask questions (FAQ)

What is Input Module?

The module which interacts with the input signal is called as Input Module. It is required to connect input devices like different types of switches.

Type of PLC input 

• Digital Input (DI)
• Analog Input (AI)
• High speed Counter Input (HSC)

Digital Input (DI)

A digital input is a type of input used in the world of PLC’s. They simply have two states – on or off. They are available in both AC and DC powered components and include components such as sensors, limit switches, push buttons, relay contacts, pressure switches, overload contacts and proximity switches.

LEDs are normally located somewhere on the PLC to display whether a digital input is active or not. This makes fault finding on the input side of a system extremely easy.

Analog Inputs (AI)

Analog inputs are a type of input that can give a reading or physical quantity and convert it to an electrical signal to a PLC. When the analog input goes into the PLC it will go through an analog to digital converter which basically converts the reading internally to a digital signal. The nominal power supplies for these types of signals are normally around 3-20ma and 1-5VDC. Some examples of analog inputs are temperature sensors, voltage sensors, current sensors, flow meters, distance sensors or level/float switches.

Analog inputs generally do not have a LED light that displays whether the input is active but some of the newer PLCs are now incorporating this into their systems.

High-Speed counter Inputs (HSC)

A high-speed counter input is a lot like a digital input, they simply turn ON/OFF. The main difference between digital inputs and high-speed counter inputs is the speed at which the PLC can process them. High-speed counters can process up to 10,000 signals per second! A typical application for this type of input could be from a sensor that looks at parts on a fast-moving process line.

What components can be used as a PLC input?

A number of different components can be used as inputs on a PLC however digital inputs and analog inputs use different external devices.

Digital input components

All of the components that can be used for digital inputs must only display two states (ON/OFF). they include are Push switch, Toggle switch, Rocker switch, Selector switch, Proximity switch, Limit switch and etc. are the example of the Digital Input Signal.

Analog input components

Analog input components are any external device that can convert a physical quantity into an electrical signal. they are including Temperature detection switch, Pressure detection switch, Flow detection switch, Level detection switch, Limit detection switch, Position detection switch, PH Level detection switch are the best example of the Analog Input Signal.

What is Output Module?

The module which interacts with the output signal is called as Output Module. The output module is required to connect output devices like electric applications.

Type of PLC input

• Digital Output (DO)
• Analog Output (AO)

Digital Outputs (DO)

A digital output is for your control side circuits as they give two states – either on or off. Digital outputs are the simpler of the two as they only have two possible states. Some examples of components that can be used as digital outputs are solenoids, lights, start signals, inverter start signals, machine safety door locks, buzzers and machine alarms.

Analog Outputs (AO)

A analog output is a variable output that can be read by other devices that are somewhere in between the off and on signals. Some good examples of where they can be used are to give a reading to set a speed on an inverter for a fan or motor, to send a signal to a valve position actuator and a variable power supply.

What components can be used as a PLC Output?

Digital and analog outputs can be used to activate and control a range of different components. Digital outputs are components that can be controlled simply by an ON/OFF signal whereas analog outputs can be used to set a variable level or ranged output.

Components that can be connected to digital outputs

Lamp, Coil, Buzzer, Relay, Motor, Fan, Heater, Actuator, Solenoid Valve and etc are the example of the Digital Output Signal.

Components that can be connected to analog outputs

Temperature Transmitter, Thermocouples, Pressure Transmitter, Flow Transmitter, Level Transmitter, etc., are the example of the Analog Output Signal.

Frequently ask questions (FAQ)

1. What is the input and output voltage of a PLC?

If the sensor is designed for 10-30 VDC or 120 VAC, then it's perfect for a PLC. Some sensors are made for 5 volts. To use these, you either need a special low-voltage input or a way to amplify the output up to 12-24 volts. The types of outputs, or loads, are also quite broad.

2. How many io in PLC?

The number of inputs and outputs (I/O) in a PLC varies by type. Nano PLCs typically have fewer than 32 I/O points. Micro PLCs usually have between 32 and 128 I/O points. Larger, modular PLCs can support hundreds or even thousands of I/O points, depending on their configuration and expansion modules.

3. What is PLC analog input and output examples?

Analog input examples include temperature sensors, oil pressure sensors, CO2 sensors, and weight scales. Outputs would be used to control the power, current, or voltage sent to these sensors.

4. What is a PLC output?

The purpose of a PLC Output module is to operate or control a physical device based on field device conditions connected to an input module and decisions made by the PLC program.

5. Is PLC AC or DC?

PLCs can operate using both AC and DC power, depending on the application and specific requirements. DC power is commonly preferred because it provides stability and is compatible with many industrial components and sensors, ensuring reliable and efficient operation.

6. What is PLC and type?

A Programmable Logic Controller (PLC) is a specialized hardware designed to manage control tasks in industrial settings. A PLC is a special kind of digital computer that plays a crucial role in industrial automation. It has a CPU that runs the control program and manages other components.

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