What is PLC I/O ?

PLC I/O stands for Programmable Logic Controller Input and Output.
These are the signals that a PLC receives (inputs) and sends (outputs) to control industrial equipment and processes.

These signals allow the PLC to monitor (through inputs) and control (through outputs) the operation of a process or Instruments.

In a PLC system, there are two main types of signals:

1. Digital (Discrete) Signals:
   These are ON/OFF signals, such as switches, sensors, or relays.

   • Example: Start/Stop push buttons, limit switches, contactors.

2. Analog Signals:
   These are variable signals, representing a range of values (for example 0–10V or 4–20mA).

   • Example: Temperature transmitters, pressure transmitters, speed controllers.

Input Signals are received from field devices (like sensors, switches).
Output Signals are sent from the PLC to control devices (like motors, valves, or lights).

Types of I/O Signals

PLC I/O signals are divided into two main categories based on their nature:

1. Digital (Discrete) I/O

2. Analog I/O

1. Digital (Discrete) I/O

Digital signals represent two states only — ON or OFF, TRUE or FALSE, 1 or 0.
These are used for devices that operate in binary conditions.

Digital Inputs:

These are signals received by the PLC from field devices.

• Examples:

  • Start/Stop Push Button

  • Limit Switch

  • Proximity Sensor

  • Pressure Switch

Digital Outputs:

These are signals sent by the PLC to control field devices.

• Examples:

  • Relay or Contactor Coil

  • Solenoid Valve

  • Indicator Lamp

  • Alarm Buzzer

Digital inputs and outputs are usually 24V DC or 230V AC, depending on system design.

2. Analog I/O

Analog signals represent a continuous range of values, not just ON or OFF.
They are used when a process variable (like temperature, pressure, or speed) needs to be measured or controlled with precision.

Analog Inputs:

These are continuous signals received by the PLC, typically from transmitters or sensors.

• Examples:

  • 4–20 mA or 0–10 V temperature signal

  • Pressure Transmitter

  • Level Transmitter

  • Flow Transmitter

Analog Outputs:

These are continuous signals sent by the PLC to control actuators or variable devices.

• Examples:

• Control signal to Variable Frequency Drive (VFD)

• Control Valve position signal

• Analog Meter output

How PLC I/O Works

1. Input Stage:
   Field devices send signals to the PLC input module. The input module converts these signals into a form that the PLC CPU can process.

2. Processing Stage:
   The PLC CPU executes the control logic (program) based on the input status.

3. Output Stage:
   After processing, the PLC sends signals to the output module to control the connected field devices.

Example 

Motor Start and stop Using VFD.

1. Start Button:- Digital Input (Sends ON signal to PLC)
2. Stop Button:- Digital Input (Sends OFF signal to PLC)
3. EMG button:- Digital Input (Sends EMG signal to PLC)
4. Local remote button:- Digital Input (Sends Local remote signal to PLC)
5. VFD start coil/CMD:- Digital Output (Energized by PLC to start motor)
6. Overload Relay:- Digital Input (Sends trip signal to PLC)
7. Speed Transmitter:- Analog Input (Sends actual speed signal)
8. VFD Control Signal :- Analog Output (PLC sends speed reference to VFD)