Top 30 PLC Basics Interview Questions and Answers
I. Fundamental Concepts
1. What is a PLC?
A PLC, or Programmable Logic Controller, is a ruggedized industrial computer used to automate specific processes, machine functions, or even entire production lines. It continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.
2. How does a PLC differ from a relay-based control system?
- Flexibility: PLCs are programmable. To change the logic in a relay system, you must physically rewire the relays, which is time-consuming and expensive. In a PLC, you simply change the code.
- Reliability: PLCs are solid-state devices with no moving parts, making them far more reliable and long-lasting than mechanical relays.
- Complexity: PLCs can handle complex logic, timers, counters, and mathematical operations that would be impractical with relays.
- Size: A single PLC can replace hundreds of relays, saving significant cabinet space.
3. What is the PLC scan cycle?
The PLC scan cycle is the repetitive, sequential process the PLC's Central Processing Unit (CPU) follows to execute its program. It consists of three main steps:
- Read Inputs: The PLC checks the status of all connected input devices (switches, sensors) and stores their states in memory.
- Execute Program: The PLC executes the user-written logic one instruction at a time, using the stored input values to solve the logic and determine the required output states.
- Write Outputs: The PLC updates the status of all connected output devices (motors, lights, valves) based on the results of the program execution.
4. What are the main components of a PLC?
- Central Processing Unit (CPU): The "brain" of the PLC that executes the program and performs all logic operations.
- Memory: Stores the operating system, user program, and data. This includes RAM (volatile) and ROM/EEPROM (non-volatile).
- Input/Output (I/O) Modules: Interface the PLC with the real world, connecting it to sensors, switches (inputs) and motors, lights (outputs).
- Power Supply: Converts the incoming AC voltage to the low-voltage DC required by the PLC components.
- Programming Device: A computer with specialized software used to write, download, and monitor the PLC program.
5. How does a PLC differ from a standard PC?
| Feature | PLC | PC |
|---|---|---|
| Environment | Designed for harsh industrial environments (vibration, temperature, electrical noise). | Designed for office environments. |
| Operating System | Real-time, proprietary OS focused on speed and reliability. | General-purpose OS (Windows, macOS) not designed for real-time control. |
| Programming | Uses specialized languages like Ladder Logic, focused on control tasks. | Uses general-purpose languages (C++, Java, Python). |
| I/O | Has built-in, robust interfaces for industrial inputs and outputs. | Requires special cards for I/O, which are not as rugged. |
II. PLC Hardware and I/O
6. Differentiate between Digital (Discrete) and Analog I/O.
- Digital I/O: Deals with signals that have only two states: ON or OFF (1 or 0). Examples include push buttons, limit switches, and motor starters.
- Analog I/O: Deals with signals that can have a continuous range of values. Examples include temperature sensors, pressure transmitters, and variable speed drives. These signals are typically represented as a voltage (0-10V) or current (4-20mA).
7. What is the difference between sourcing and sinking I/O modules?
This describes the direction of current flow between the I/O module and the field device.
- A sourcing module provides (sources) the power. Current flows from the module, through the field device, to ground. It is associated with PNP transistors.
- A sinking module receives (sinks) the power. Current flows from an external power supply, through the field device, into the module to ground. It is associated with NPN transistors.
8. What is the purpose of an optical isolator in an I/O module?
An optical isolator (or optocoupler) provides **electrical isolation** between the high-voltage field side and the low-voltage PLC logic side. It uses an LED and a phototransistor to transmit the signal via light, creating a physical air gap. This protects the sensitive CPU from damaging voltage spikes and electrical noise from the industrial environment.
9. What are the different types of memory in a PLC?
- ROM (Read-Only Memory): Stores the PLC's operating system firmware. It is non-volatile.
- RAM (Random-Access Memory): Volatile memory used to store the user's program, timer/counter values, and I/O status. It requires a battery backup to retain data during a power loss.
- EEPROM (Electrically Erasable Programmable ROM): Non-volatile memory often used to store a backup of the user program, which can be reloaded automatically after a prolonged power outage.
10. Differentiate between a fixed and a modular PLC.
- Fixed PLC: Comes in a single, compact unit with a fixed number and type of I/O points. It is less expensive and suitable for small, simple applications. It is not expandable.
- Modular PLC: Consists of a separate base or chassis, a power supply, a CPU, and multiple I/O modules that can be added or removed as needed. It is highly flexible, expandable, and suitable for large, complex applications.
III. PLC Programming
11. What is Ladder Logic (LAD)?
Ladder Logic is the most common PLC programming language. It is a graphical language that mimics the appearance of electrical relay wiring diagrams. This design makes it intuitive for electricians and technicians to understand and use without needing a formal programming background.
12. What are the rails and rungs in a ladder diagram?
- Rails: The two vertical lines on the far left and right of the diagram, representing the power supply (L1 and L2, or +V and 0V).
- Rungs: The horizontal lines that connect the rails. Each rung contains a set of input conditions (contacts) and at least one output action (coil). The logic on each rung is solved from left to right.
13. Explain the basic instructions: XIC, XIO, and OTE.
XIC(Examine If Closed): A normally open (NO) contact. This instruction is TRUE if the bit it is monitoring is a 1 (ON).XIO(Examine If Open): A normally closed (NC) contact. This instruction is TRUE if the bit it is monitoring is a 0 (OFF).OTE(Output Energize): An output coil. If the logic on the rung preceding it is TRUE, this instruction sets the specified output bit to 1 (ON). If the rung is FALSE, it sets the bit to 0 (OFF).
14. What is the difference between a Timer On-Delay (TON) and a Timer Off-Delay (TOF)?
- TON (Timer On-Delay): When the rung condition becomes TRUE, the timer starts timing. The timer's output (Done bit) becomes TRUE only after the accumulated time reaches the preset time. If the rung goes FALSE, the timer resets immediately.
- TOF (Timer Off-Delay): When the rung condition becomes TRUE, the timer's output becomes TRUE immediately. When the rung goes FALSE, the timer starts timing. The output remains TRUE until the accumulated time reaches the preset time, then it turns FALSE.
15. What are the primary counter instructions?
CTU(Count Up): Increments the accumulated value by one each time the rung condition goes from FALSE to TRUE. The output (Done bit) becomes TRUE when the accumulated value is equal to or greater than the preset value.CTD(Count Down): Decrements the accumulated value by one each time the rung condition goes from FALSE to TRUE.RES(Reset): An instruction used to reset the accumulated value of a timer or counter to zero.
16. What is a latching or seal-in circuit?
A latching circuit is a fundamental programming concept used to keep an output energized even after the initial input signal is gone. It typically uses a normally open contact associated with the output coil, placed in parallel with the start button contact. Once the output is energized, its own contact "seals in" the circuit, keeping it on until a normally closed stop button in series breaks the circuit.
17. What are the other PLC programming languages besides Ladder Logic?
The IEC 61131-3 standard defines five languages:
- Ladder Diagram (LD): Graphical, based on relay logic.
- Function Block Diagram (FBD): Graphical, shows signal flow through blocks of functions.
- Structured Text (ST): Text-based, high-level language similar to Pascal or C. Good for complex math and data manipulation.
- Instruction List (IL): Text-based, low-level language similar to assembly.
- Sequential Function Chart (SFC): Graphical, used for describing the sequential behavior of a control system.
18. What is a data file or tag in a PLC?
This is a designated area of PLC memory used to store different types of information. In older PLCs, these were fixed files (e.g., F8 for floats, N7 for integers). In modern PLCs (like Allen-Bradley's Logix platform), these are called "tags," which are user-defined names for memory locations, making programs much easier to read and understand (e.g., instead of
N7:0, you can use the tag Tank_Level).19. What is the difference between a bit, a byte, and a word?
- Bit: The smallest unit of data, can be a 1 or a 0.
- Byte: A group of 8 bits.
- Word: A group of 16 bits (2 bytes).
- Double Word (Dword): A group of 32 bits (4 bytes or 2 words).
20. What are internal relays or internal bits?
These are memory locations (bits) used within the PLC program for intermediate logic steps. They behave like output coils but are not physically connected to any output device. They are often called "memory coils" or "flags" and are essential for building complex logic without using up physical outputs.
IV. Practical and Application Questions
21. How would you troubleshoot a PLC system?
A systematic approach is best:
- Check Status Indicators: Look at the LEDs on the PLC (Power, Run, Fault) and on the I/O modules to quickly identify problems.
- Check Power: Verify that the PLC and I/O modules have the correct power.
- Go Online: Connect a programming device to the PLC to monitor the program logic in real-time.
- Check Inputs: Verify that the PLC is seeing the input signals correctly. Check the physical sensor and wiring if needed.
- Check Logic: Monitor the ladder logic to see why a specific output is not turning on. Is a permissive missing? Is a timer not done?
- Check Outputs: If the logic says an output should be on, but the device isn't working, check the output module, wiring, and the output device itself.
22. What is "forcing" in a PLC?
Forcing is a troubleshooting feature that allows a user to manually override the state of an I/O point or an internal bit, setting it to ON or OFF regardless of the program logic. It is a powerful but dangerous tool, as it can cause unexpected machine movement. It should only be used by trained personnel for testing and diagnostics, and all forces should be removed before returning the system to normal operation.
23. What is a PID loop and where is it used?
A PID (Proportional-Integral-Derivative) loop is a control loop feedback mechanism. PLCs use PID instructions to precisely control analog processes like temperature, pressure, or flow. The controller continuously calculates an error value as the difference between a desired setpoint and a measured process variable and applies a correction based on the P, I, and D terms to keep the process variable at the setpoint.
24. What is an HMI?
An HMI, or Human-Machine Interface, is a graphical user interface (often a touchscreen) that connects to a PLC. It allows an operator to monitor the process, view alarms, change setpoints, and manually control the machine. It provides a "window" into the automated system.
25. What is SCADA?
SCADA, or Supervisory Control and Data Acquisition, is a larger-scale system that allows for monitoring and control of an entire site or a process distributed over a large geographical area. It gathers data from multiple PLCs and other devices (RTUs) and presents it to a central control room. An HMI is the operator interface for a single machine or process, while SCADA is the interface for an entire plant or system.
26. What is the difference between a PLC and a DCS?
- PLC (Programmable Logic Controller): Best for high-speed machine control and discrete logic. Typically used for single-unit or OEM machine control.
- DCS (Distributed Control System): Best for continuous process control across an entire plant. It is designed for high reliability and integrates control, data acquisition, and operator interface into one native system. Typically used in industries like oil & gas, chemical, and power generation.
27. Name some major PLC manufacturers.
Major players in the PLC market include:
- Rockwell Automation (Allen-Bradley)
- Siemens
- Mitsubishi Electric
- Schneider Electric (Modicon)
- Omron
- ABB
28. What are some common applications of PLCs?
PLCs are used in almost every industry, including:
- Manufacturing: Assembly lines, packaging machines, robotics.
- Food & Beverage: Bottling, mixing, and batching processes.
- Automotive: Welding, painting, and assembly automation.
- Energy: Power plant control, pipeline monitoring.
- Building Automation: HVAC control, lighting systems, elevator control.
29. Why is documentation (commenting) important in a PLC program?
Documentation is critical for long-term maintainability. Well-commented code that explains the purpose of each rung, tag, and subroutine makes it significantly easier and faster for someone else (or yourself, months later) to troubleshoot, modify, or upgrade the system. It reduces downtime and prevents errors.
30. What is a "hot backup" or redundant PLC system?
This is a high-availability system used in critical processes where downtime is unacceptable. It consists of two identical PLCs (a primary and a backup) running the same program. The primary PLC controls the process while the backup runs in parallel, continuously monitored. If the primary PLC fails, the backup PLC automatically and seamlessly takes control of the process with no interruption.