Top PLC Interview Questions and Answers

plc is programmable control system which reduced complex hardwired relay logic. Programmable logic controller (PLC) store instructions, like sequencing, timing, counting, arithmetic, data manipulation, communication etc. to control industrial machines, systems and processes. Controlling process is depends on inputs, outputs condition of system. plc have basic features. plc is used for the purpose of efficiency and economy. It reduced human error and provides continues result of process or system. plc save fuel, cost, processing material, labor and increased production. below are some common question for plc asked in interview.

---

1. What is PLC?

Programmable Logic Controller (PLC) is a microprocessor or microcontroller based industrial computer used for automate processes by receiving inputs from sensors, processing data based on a program, and controlling outputs.

---

2. What are the different types of PLC?

• 1. Fixed/Integrated/Compact/monolithic PLC
• 2. modular PLCs

---

3. difference between Compact PLC and & Modular PLC?

• Compact PLC typical of small PLCs that come in one package with no separate, removable units. The processor and I/O are packaged together, and the I/O terminals will have a fixed number of connections built in for inputs and outputs. The main advantage of this type of packaging is lower cost. PLCs have a fixed amount of Input output capability.
• modular PLC - The basic modular controller consists of a rack, power supply, processor module (CPU), input/output (I/O modules), and an operator interface for programming and monitoring. The modules plug into a rack. When a module is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack. The PLC processor is also connected to the backplane and can communicate with all the modules in the rack.

---

4. what is analog and digital signal

• an analog signals are continuous signals that can vary over a range (e.g., 0–10V, 4–20mA).
• digital signals are discrete signals which have only two state ON or OFF(HIGH or LOW, 0 or 1).

---

5. what is redundancy in PLC control system?

in plc redundancy Two PLC CPUs (primary and backup) operate in tandem. plc redundancy means providing an alternative control system in the fault condition. This will provide reliability to the process control system as well as reduce downtime during a failure.in redundancy we implement a backup plc to ensure continuous operation of an automated process in case of primary plc failure.

---

6. What are the Basic Components of PLC Systems?

following are basic component of plc

• Power supply unit
• Central processing unit(CPU) Unit
• input and output module unit
• communication unit
• memory unit

---

7. why industry use plc for controlling process?

• Faster Response Time.
• Programmability
• perform Real-time Operation
• Communications Capability
• Easier to Troubleshoot
• Increased Reliability
• High number of input, output
• higher scan times
• microprocessor based control system

---

8. what is sensor?

Sensors are devices that convert one form of energy into another. They measure physical properties, such as temperature, light, motion, pressure, humidity, or sound, and convert them into electrical signals or data that can be interpreted, analyzed, or used by other systems, devices, or applications.

---

9. which type of programming languages used in plc?

PLC programming languages are standardized by IEC 61131-3, which defines following five primary languages used for programming PLCs.

1. Ladder Diagram (LD)

2. functional block diagram(FBD)

3. structured text

4.Instruction List (IL)

5.Sequential Function Chart (SFC)

10. what is ladder diagram language?

ladder diagram is a graphical based programming language for plc. It visually resembles a ladder, with two vertical rails representing power supply and horizontal rungs representing control logic.

---

11. what structured text programming language?

Structured text programming language is a high level text based programming language for plc. it is a combination of statement and conditional statement where you define your function to run a process.

---

12. what is functional block diagram programming language?

functional block diagram is graphically blocks based plc programming language. these graphical blocks represent logic and functions. functional block diagram have graphical block like logical blocks, timer and counter block, arithmetic and comparison blocks.

---

13. what is cfc in plc?

CFC (Continuous Function Chart) is a graphical programming language used in industrial automation for programming PLC and DCS systems. It allows the user to create control logic by arranging and interconnecting functional blocks freely on a workspace. Unlike other structured languages, CFC does not impose a strict top-to-bottom or left-to-right flow, providing flexibility in logic design.

---

14. What is a Rack or Chassis in a PLC system?

rack or chassis is physical structure for mounting and connecting all necessary modules within the PLC, such as input, output, CPU, power supply, and communication modules etc.in plc rack power supply module supply necessary power to the different input-output modules through the backplane bus system. rack or chassis used in modular plc system.

---

15. What are active backplane and passive backplane in PLC?

• 1. active backplane: An active backplane is a more sophisticated system that not only provides physical connections between modules but also includes electronics that support communication, power distribution, and system management.
• 2. passive backplane: A passive backplane is a simpler, more basic system that primarily functions as a connector between modules, without the added electronics for communication or system management.

16. What is Program mode, Run mode, and Test mode in PLCs.

• Program Mode: this mode used to modify and download plc program into the plc. In this mode ladder I/O scan sequence is stopped.
• RUN MODE: in this mode PLC continuous monitored and controlled the process, the PLC executes the control program.
• test mode: partial execution of the PLC program while enabling the user to test or simulate parts of the program without fully executing the entire control logic.

---

17. how many types of communication protocol available for plc?

communication protocol used to communicate plc with other devices like hmi, scada, development software, programming station etc. some of the standard common protocol for plc

• RS-232/RS-485
• MODBUS RTU/
• Profinet
• Modbus TCP
• Ethernet/IP (Ethernet Industrial Protocol)
• OPC (OLE for Process Control)
• DeviceNet
• Profibus DP

---

18. what is use of 4-20 mA signal in plc?

4-20 mA signal represent analog signal standard in plc. 4-20 mA signal used for transmitting analog sensor data to plc or from the PLC to field devices by using analog input or analog output module.

---

19. how many types of module available for plc

• Digital module
• analog module
• Temperature module
• Pulse/Encoder module
• High-speed module

---

20. what is analog module

analog devices represent physical quantities that can have an infinite number of values. Analog input modules normally have multiple input channels that allow 4, 8, or 16 devices to be interface to the PLC. The two basic types of analog input modules are voltage sensing and current sensing.

---

21. what is digital module

PLC DIGITAL MODULE This type of interface connects field input devices of the ON/OFF nature such as selector switches, pushbuttons, and limit switches. Likewise, output control is limited to devices such as lights, relays, solenoids, and motor starters that require simple ON/OFF switching.

---

22. What is a Scan Cycle?

The scan cycle is the time taken by a PLC to continuously monitor its inputs, execute its logic, and control outputs in real-time. This cycle repeats and is continuously executed for the duration of the operation.

---

23. Explain sinking and sourcing in plc system?

• sinking : Sinking refers to a configuration where the PLC provides the negative current path, and the current flows from the load to the PLC’s input/output terminal.
• sourcing : Sourcing refers to a configuration where the PLC provides the positive current to output device, and the current flows from the output module to the load (e.g., a relay, lamp, or solenoid).

---

24. what is plc data type?

Data Types Represent Values Such As numbers, real numbers, strings, and Boolean values, Each data type has a specific range of values and operations that can be performed on it. All Data Type Have Some Reserved Memory For Storing Value In That Memory. following are data types used in different type of plc

---

25. How many types of plc company are there?

following are most common plc manufacturer

• Siemens (plc modal S7-300, S7-400, S-1200, S7-1500)
• Allen Bradley(AB) (MicroLogix, CompactLogix and ControlLogix)
• ABB (AC 500 and AC 800)
• Honeywell (HC900, Experion PKS )
• Mitsubishi Electric(MELSEC FX, MELSEC Q and MELSEC L )
• Schneider Electric (Modicon M221, M241, M258, M340 and M580)
• Omron (CP1H, CJ2, CS1, NX1P and NX7)
• Panasonic (FP-X, FP2)

---