Interview Questions

Top 15 PLC interview Questions

1. What is PLC?

PLC stands for Programmable Logic Controllers, PLC is an industrial computer used to monitor inputs and to regulate (turn on / off) their outputs to automate a device or process, depending on their state making choices based on their program or logic.

PLC is a software and hardware mixture. It functions as the machine’s brain or system for control mechanisms for automation.

2. What is the role of PLC in automation?

PLC plays most important role in automation. All the monitoring as well as the control actions are taken by PLCs.

PLC Senses input via I / P modules, processes logic via CPU and memory, and provides output via output module.

Within a very short moment, it conducts accurate activities. That’s one of the reasons why PLC is commonly used in the industrial sector today.

3. What are the general functions of a PLC?

The functions of PLC are as follows:

  • Sequential Control: PLC processes binary signal input into output that is used for the purposes of sequential technical processing, here PLC keeps all steps in the sequential process in the right order.
  • Plant Monitoring: PLC continuously monitors the status of a system and takes the necessary actions in connection with the process being controlled or displaying the message to the operator.
  • Shutdown System: The working principle of a PLC is to receive a process input signal that is controlled and then process the input signal in accordance with the program stored in memory and then produce an output signal to control the actuator or other equipment

4. What are the types of PLC programming languages?

Based on IEC-61131-3 International Standards, there are 5 types of PLC programming languages, namely:

  1. Ladder Diagram (LD)
  2. Function Block Diagram (FBD)
  3. Sequential Function Chart (SFC)
  4. Structure Text (ST)
  5. Instruction List (IL)

5. Describe about basic elements in Ladder logic programming:

There are three basic organizational elements associated with ladder logic:

  • RUNG: Ladder logic instructions are written on a function. During the scan program, the processor scans from left to right, one by one ladder from top to bottom.
  • INSTRUCTION: Ladder logic Instruction is divided into 2 parts;
    Input instructions:
    Displayed on the left side of the ladder Check, comparison, or specific conditions
    Output instructions:
    Shown on the right side Do something action.
  • BRANCHES: Branches are used in ladder logic to create different paths that directly read the state of inputs and outputs.

6. What are the different PLC brands known?

  1. ABB
  2. Allen-Bradley
  3. Siemens
  4. Honeywell
  5. Mitsubishi
  6. Yokogawa
  7. Toshiba
  8. General Electric
  9. Omron
  10. Schneider Automation
  11. Reliance
  12. Hitachi
  13. Modicon/Gould
  14. AutomationDirect/PLC Direct/Koyo
  15. Cutler Hammer

7. Power supply in PLC system:

Power supply provides processor, I / O and communication modules with system energy requirements. Typically the input voltage of the energy supply is 120 V – 230 V AC or 24 V DC and the output voltage of the back plane is 2 A to 5 A at 5 V DC.

8. What is Sinking & Sourcing?

The most brief definition of these two concepts would be:
SINKING = Common GND line (‐)
SOURCING = Common VCC line (+)

  • Sinking I/O circuits on the I/O modules receive (sink) current from sourcing field devices. Sinking output modules used for interfacing with electronic equipment.
  • Sourcing I/O are the sourcing output modules used for interfacing with solenoids.

9. Types of PLC:

There are two fundamental kinds of PLC bases based on PLC building and operation:

  • Compact PLC
  • Modular PLC

10. Difference Between Fixed and Modular PLCs ?

The processor will have integrated power supply and I / Os in one unit in non-modular PLCs. The modular PLC, for parts such as power supply, I / O modules, will have distinct slots. The I / Os or energy supply can be selected as required.

11. Timer instruction for PLC & Types of Timers:

Timer is a instruction that waits for a certain amount of time before doing anything (control time). Timers use the internal CPU clock to count fractions of seconds or seconds. The preset is called the time duration for which a timer was set and is set in multiples of the time base used.

Timers Types:
On‐Delay timer‐ simply “delays turning on”. It is called TON, TIM or TMR.
Off‐Delay timer‐ simply “delays turning off”. It is called TOF and is less common than the on‐delay type.

12. What are counters?

A counter is set to some preset value and will function its contacts when this value of input pulses is obtained.

The normal counters are typically “software” counters – they don’t physically exist in the PLC but rather they are simulated in software

13. Types of counters

  • Up‐counters counts from zero up to the preset value. These are called CTU, CNT, C, or CTR.
  • Down‐counters count down from the preset value to zero. These are called CTD.
  • Up‐down counters count up and/or down. These are called CTUD.

14. Types of PLC inputs & Outputs:


  • DC Inputs: DC input modules allow to connect either PNP (sourcing) or NPN (sinking) transistor type devices to them.
  • AC Inputs: There is no polarized AC voltage. The AC voltage is most frequently shifted via a limit switch or other form of switch.


  • Relay Outputs: One of the most common types of outputs available is the relay output. Existence of relays as outputs makes it easier to connect with external devices.
  • Transistor Outputs: Transistor type outputs can only switch a DC current. The PLC applies a small current to the transistor base and the transistor output “closes”.
  • Triac Output: Triac output can be used to control AC loads only. Triac output is faster in operation and has longer life than relay output.

15. How to test and commission a PLC system?

  • Ensure that all cable connections between the PLC and the plant are full, secure and in accordance with the specifications needed and meet local requirements.
  • Checking that all the incoming power supply matches the voltage setting for which the PLC is set.
  • Check that all protective devices are set to suitable environments for the journey.
  • Checking that emergency stop button work.
  • Checking that all input/output devices are connected to the correct input/output points and giving the correct signals.
  • Loading and testing the software.


Instrumentation Engineer

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