Programmable Logic Controllers (PLCs) are the backbone of modern industrial automation.
They are used everywhere, from controlling machines on factory floors to managing complex processes in power plants and manufacturing industries. For students, technicians, and engineers, a solid understanding of PLCs is not just useful but essential for career growth and problem-solving in the field of automation.
In this blog, we have compiled 100 objective questions with answers on PLC and automation concepts. These questions cover the basics as well as advanced topics, making them a great resource for interview preparation, competitive exams, and refreshing practical knowledge.
So, Lets start !!
PLC and Automation Q&A - Part 1
1. A PLC is primarily designed to replace:
a) Relays and timers in control circuits
b) Analog meters
c) Microprocessors in computers
d) Digital voltmeters
Answer: a
2. The main advantage of PLCs over relay logic is:
a) Higher power rating
b) Flexibility and easy reprogramming
c) No need for electricity
d) Cheaper wiring only
Answer: b
3. The CPU of a PLC consists of:
a) Memory and I/O modules
b) Input/output cards only
c) Processor, memory, and power supply
d) Keyboard and display
Answer: c
4. The PLC scan cycle includes:
a) Program execution only
b) Input scan → Program execution → Output update
c) Output update → Input scan
d) Interrupt routines only
Answer: b
5. In PLC terminology, “ladder diagram” is:
a) A physical wiring layout
b) A graphical programming language resembling relay logic
c) A mechanical drawing
d) An input device
Answer: b
6. The memory that stores the user program in a PLC is usually:
a) ROM
b) EEPROM or Flash memory
c) RAM only
d) Cache
Answer: b
7. Inputs to a PLC can include:
a) Switches and sensors
b) Encoders
c) Analog transmitters
d) All of the above
Answer: d
8. Which of the following is NOT an output device for PLCs?
a) Motor starter
b) Solenoid valve
c) Thermocouple
d) Alarm buzzer
Answer: c
9. A PLC timer instruction is used to:
a) Count pulses
b) Delay operations for a set time
c) Measure temperature
d) Store data in memory
Answer: b
10. The difference between an on-delay and off-delay timer in PLC is:
a) No difference
b) On-delay waits before turning ON; off-delay waits before turning OFF
c) On-delay waits before turning OFF
d) Off-delay resets immediately
Answer: b
11. Which communication protocol is widely used for PLC networking in industries?
a) Wi-Fi
b) Modbus / Profibus / Ethernet/IP
c) RS-232 only
d) HTTP
Answer: b
12. A PLC counter instruction is mainly used for:
a) Time delay
b) Counting events, pulses, or objects
c) Arithmetic operations
d) Generating alarms only
Answer: b
13. The smallest unit of PLC memory is a:
a) Word
b) Bit
c) Byte
d) Register
Answer: b
14. Which type of PLC I/O module converts 4–20 mA signals to digital values?
a) Digital output
b) Relay output
c) Analog input module (A/D conversion)
d) Digital input module
Answer: c
15. PLC ladder logic contacts that represent field inputs are called:
a) Coils
b) Outputs
c) Examine ON/Examine OFF instructions
d) Registers
Answer: c
16. Which PLC programming language is standardized by IEC 61131-3?
a) Ladder diagram (LD)
b) Function block diagram (FBD)
c) Structured text (ST)
d) All of the above
Answer: d
17. The watchdog timer in a PLC ensures:
a) Time delays are correct
b) The CPU scan cycle is within limits (prevents system hang)
c) Counting pulses
d) Fast I/O scanning
Answer: b
18. Retentive timers in PLCs are used to:
a) Reset after power loss
b) Keep elapsed time even after power cycle
c) Work faster than normal timers
d) Ignore power supply issues
Answer: b
19. Which of the following is an advantage of PLC over DCS (Distributed Control System)?
a) Better suited for discrete and machine automation
b) Handles large process plants
c) Direct fieldbus integration only
d) Cheaper sensors
Answer: a
20. Which addressing system is commonly used in PLC programming for I/O?
a) Hexadecimal only
b) Octal/decimal with slot, word, and bit identifiers
c) Binary only
d) ASCII addressing
Answer: b
PLC and Automation Q&A - Part 2
21. Which type of PLC I/O module provides electrical isolation between field devices and the CPU?
a) Opto-isolated modules
b) Analog input modules
c) Relay output modules
d) Encoder modules
Answer: a
22. Which PLC instruction is used to hold a coil output active until reset?
a) Latch (SET)
b) Reset (RST)
c) Toggle
d) Pulse
Answer: a
23. The resolution of an analog input module depends on:
a) Memory size
b) Number of I/O slots
c) A/D converter bit size
d) Power supply voltage
Answer: c
24. Which PLC function is used to compare two values?
a) Timer
b) Comparator instruction (e.g., LES, GRT, EQU)
c) Counter
d) Scan watchdog
Answer: b
25. A PLC retentive memory location stores data:
a) Only during power OFF
b) Only during program scan
c) Even after power is switched OFF
d) Until cleared by CPU
Answer: c
26. The I/O table in a PLC contains:
a) Program instructions
b) Status of inputs and outputs
c) Scan cycle data
d) Communication settings
Answer: b
27. Which PLC instruction is used for mathematical operations?
a) MOV
b) ADD, SUB, MUL, DIV
c) CMP
d) OUT
Answer: b
28. The primary function of an analog output module is to:
a) Read analog signals
b) Convert digital data from CPU to 4–20 mA or 0–10 V signal
c) Store analog setpoints
d) Amplify digital signals
Answer: b
29. A PLC subroutine is used to:
a) Replace I/O modules
b) Save program memory
c) Divide program into logical sections
d) Scan inputs faster
Answer: c
30. Which type of PLC memory is cleared when power is switched off?
a) EEPROM
b) Flash
c) Volatile RAM
d) ROM
Answer: c
31. In ladder logic, a coil instruction represents:
a) An input contact
b) An output device
c) A timer
d) A memory register
Answer: b
32. Which PLC function is used to shift data bits?
a) MOVE
b) SHIFT REGISTER (SHL/SHR)
c) XOR
d) RET
Answer: b
33. The main purpose of PLC scan time is to determine:
a) Program length
b) Memory usage
c) Speed of program execution per cycle
d) Accuracy of outputs
Answer: c
34. Which PLC communication port is most commonly used for programming?
a) USB or RS-232/RS-485
b) HDMI
c) Ethernet only
d) CAN bus
Answer: a
35. Which function allows PLCs to continue operation in case of minor power dips?
a) Redundant I/O
b) Capacitor backup
c) UPS support
d) Power fail routine
Answer: d
36. Which PLC instruction is used to move a value from one register to another?
a) ADD
b) MOV
c) CMP
d) RET
Answer: b
37. The cyclic redundancy check (CRC) in PLC communication is used for:
a) Data encryption
b) Memory saving
c) Error detection
d) Speed control
Answer: c
38. Which of the following is NOT a PLC programming language as per IEC 61131-3?
a) Ladder Diagram (LD)
b) Function Block Diagram (FBD)
c) Assembly Language (ASM)
d) Structured Text (ST)
Answer: c
39. The HOLD function in a PLC is used to:
a) Pause the program permanently
b) Suspend scan cycle temporarily
c) Retain a timer or counter value
d) Block outputs
Answer: b
40. In PLC terms, “forcing an I/O” means:
a) Physically disconnecting the I/O
b) Forcing the operator to reset system
c) Manually overriding an input or output in software for testing
d) Locking the PLC in run mode
Answer: c
PLC and Automation Q&A - Part 3
41. Which PLC addressing mode refers to data by memory word and bit position?
a) Direct addressing
b) Indexed addressing
c) Absolute addressing
d) Indirect addressing
Answer: a
42. A shift register instruction in PLCs is useful for:
a) Counting speed
b) Tracking items on a conveyor
c) Generating pulses
d) Resetting outputs
Answer: b
43. The RET instruction in a PLC program is used to:
a) Restart CPU scan
b) Return from subroutine
c) Reset outputs
d) Re-enable timers
Answer: b
44. Which PLC instruction allows branching of program execution?
a) JMP (jump)
b) CNT (counter)
c) TON (on-delay timer)
d) MOV (move)
Answer: a
45. In ladder logic, normally closed contacts are represented by:
a) Parallel lines
b) Broken line contacts
c) “Examine OFF” instruction
d) Memory registers
Answer: c
46. The retentive counter instruction is used when:
a) The count must reset at power OFF
b) The count must reset every scan
c) The count must remain stored after power loss
d) The count must not exceed 10
Answer: c
47. Which PLC memory type stores I/O status during each scan?
a) Data registers
b) Image register (I/O image table)
c) Flash ROM
d) EEPROM
Answer: b
48. Which PLC feature allows monitoring and editing programs in real time?
a) Watchdog timer
b) Online programming
c) Scan interrupt
d) Remote I/O
Answer: b
49. A rung in ladder logic is considered true when:
a) Only timers are active
b) Power supply is stable
c) All input conditions in that rung are satisfied
d) The CPU is in STOP mode
Answer: c
50. Which PLC instruction performs bitwise logical operations?
a) AND, OR, XOR
b) ADD, SUB, DIV
c) MOV
d) RET
Answer: a
51. A PLC with scan time of 10 ms will update outputs:
a) Once per second
b) Every 10 ms
c) Every 1 ms
d) Randomly
Answer: b
52. Which is an advantage of modular PLCs over compact PLCs?
a) Smaller size
b) Lower price
c) Expandability and flexibility
d) Built-in HMI
Answer: c
53. In PLCs, analog values are generally scaled between:
a) 0–255
b) 0–4095 or 0–32767 (depending on resolution)
c) 0–100 only
d) 0–999
Answer: b
54. Which PLC programming method is closest to high-level languages?
a) Structured Text (ST)
b) Ladder Diagram (LD)
c) Function Block Diagram (FBD)
d) Instruction List (IL)
Answer: a
55. Which of these is NOT a fieldbus protocol used with PLCs?
a) Profibus
b) DeviceNet
c) Foundation Fieldbus
d) SMTP
Answer: d
56. Which PLC feature allows handling of non-sequential events?
a) Subroutines
b) Interrupts
c) Scan cycle
d) Comparators
Answer: b
57. A PID instruction in PLCs is used for:
a) Timing events
b) Arithmetic calculations
c) Proportional-integral-derivative control of process variables
d) Shifting registers
Answer: c
58. Which is the most commonly used programming method for discrete control?
a) Structured Text
b) Ladder Logic
c) Instruction List
d) Function Block Diagram
Answer: b
59. Which PLC instruction is used to clear a data register?
a) CLR
b) MOV
c) ADD
d) OUT
Answer: a
60. In PLC communication, baud rate refers to:
a) Error correction speed
b) Data storage capacity
c) Number of signal changes per second (bits per second rate)
d) Memory refresh cycle
Answer: c
PLC and Automation Q&A - Part 4
61. Which PLC mode allows program editing while the process is running?
a) Stop mode
b) Run mode
c) Test mode
d) Online mode
Answer: d
62. In PLC terminology, an internal relay (flag bit) is used for:
a) Driving motors directly
b) Simulating outputs within logic
c) Powering external relays
d) Reading analog sensors
Answer: b
63. Which part of the PLC provides electrical power to the CPU and modules?
a) Communication port
b) Power supply module
c) Relay contacts
d) Analog input module
Answer: b
64. Which PLC register stores the accumulated value of counters and timers?
a) Status register
b) Accumulator register
c) Base register
d) Memory buffer
Answer: b
65. The I/O response time of a PLC depends mainly on:
a) Number of subroutines
b) Scan time and module speed
c) CPU frequency only
d) Communication baud rate
Answer: b
66. Which of the following is a continuous process control task for PLCs?
a) Motor start/stop
b) Bottle counting
c) Conveyor sequencing
d) Temperature loop control
Answer: d
67. A PLC is best described as:
a) Microprocessor-based computer used for automation
b) Digital voltmeter for monitoring
c) SCADA system replacement
d) Fieldbus controller only
Answer: a
68. Which of the following is NOT a PLC input device?
a) Limit switch
b) Proximity sensor
c) Push button
d) Contactor coil
Answer: d
69. The advantage of using structured programming in PLCs is:
a) Faster CPU
b) Easier debugging and maintenance
c) Reduced wiring
d) Lower scan time always
Answer: b
70. Which PLC programming instruction is used for dividing one number by another?
a) MUL
b) SUB
c) DIV
d) MOD
Answer: c
71. Which communication standard is widely used for short-distance PLC programming?
a) RS-232
b) RS-485
c) CAN bus
d) Modbus TCP
Answer: a
72. Which PLC function allows temporary storage of sensor data before processing?
a) Data buffer
b) Accumulator
c) Cache
d) Holding register
Answer: d
73. Which type of PLC module is required to control speed of a VFD (Variable Frequency Drive)?
a) Relay output
b) Digital input
c) Analog output
d) Encoder module
Answer: c
74. The HMI (Human-Machine Interface) is used in automation to:
a) Replace sensors
b) Allow operator interaction with PLC systems
c) Store programs permanently
d) Power input devices
Answer: b
75. Which programming instruction in PLC is used for decision making?
a) IF/THEN or equivalent compare instructions
b) ADD instruction
c) MOV instruction
d) RET instruction
Answer: a
76. The watchdog timer in PLCs resets the CPU when:
a) Program memory is full
b) Scan cycle exceeds defined limit
c) Communication port is overloaded
d) Outputs are short-circuited
Answer: b
77. The most common PLC scan sequence is:
a) Input → Output → Program
b) Program → Output → Input
c) Input → Program → Output
d) Output → Input → Program
Answer: c
78. Which PLC addressing mode refers to data indirectly via a pointer?
a) Direct addressing
b) Indexed addressing
c) Absolute addressing
d) Indirect addressing
Answer: d
79. Which PLC function block is used for arithmetic and logical operations combined?
a) PID
b) ALU block
c) Math block
d) Function block diagram (FBD)
Answer: d
80. Which of the following PLC instructions is used to compare two numbers and branch accordingly?
a) JMP
b) CMP
c) MOV
d) RET
Answer: b
81. Which PLC module interfaces with a thermocouple?
a) Digital input module
b) Analog input module
c) Special thermocouple module
d) Relay output
Answer: c
82. Which factor most affects PLC scan time?
a) Length of the user program
b) Size of I/O table
c) Instruction complexity
d) All of the above
Answer: d
83. Which industrial communication protocol is Ethernet-based?
a) Modbus RTU
b) DeviceNet
c) Profibus DP
d) Profinet
Answer: d
84. Which PLC output type is best for fast-switching applications?
a) Relay
b) Transistor
c) Triac
d) Pneumatic relay
Answer: b
85. Which PLC module converts pulses from an encoder into position/speed information?
a) Analog input
b) High-speed counter module
c) Relay output
d) Digital input
Answer: b
PLC and Automation Q&A - Part 5
86. Which PLC instruction is used to generate a single pulse output regardless of scan time?
a) Latch
b) One-shot (OSR)
c) Timer
d) Reset
Answer: b
87. Which PLC feature is critical for safety-related applications?
a) Online editing
b) Redundant CPU and power supply
c) Fast scan time
d) Ladder diagram
Answer: b
88. Which type of PLC output is most suitable for AC loads like lamps and heaters?
a) Relay output
b) Triac output
c) Transistor output
d) Pneumatic relay
Answer: b
89. Which instruction is typically used for analog signal scaling in PLCs?
a) PID
b) SCP (scale with parameters)
c) MOV
d) CMP
Answer: b
90. A non-retentive timer in PLC resets when:
a) CPU is reset
b) Input condition goes false
c) Power is switched off
d) All of the above
Answer: d
91. Which PLC register stores system fault information?
a) Status register
b) Accumulator
c) Watchdog register
d) Error log
Answer: a
92. Which PLC communication protocol is most common in automotive industry?
a) Profibus
b) CAN bus
c) Modbus
d) Foundation Fieldbus
Answer: b
93. The term “scan watchdog” refers to:
a) Output delay
b) Monitoring scan cycle completion
c) Power-on reset
d) Subroutine jump check
Answer: b
94. Which PLC function block is mainly used for batch processing?
a) PID block
b) Sequencer (SQO/SQC)
c) MOV block
d) Timer block
Answer: b
95. The instruction NOP in PLC programming means:
a) No operation
b) Normal output point
c) Null operating procedure
d) Node operation protocol
Answer: a
96. A high-speed counter (HSC) in PLCs is used to:
a) Count only slow pulses
b) Accurately count fast pulses beyond normal scan time
c) Replace timers
d) Generate interrupts
Answer: b
97. Which PLC memory area is used to store setpoints for process variables?
a) Status register
b) Data register
c) Output image
d) Accumulator
Answer: b
98. Which of the following is NOT a benefit of PLC-based automation?
a) Flexibility
b) Reliability
c) Easy troubleshooting
d) Higher manual labor requirement
Answer: d
99. A PLC instruction that stores logical results for later use is called:
a) Output latch
b) Internal coil (marker bit)
c) Reset instruction
d) Watchdog
Answer: b
100. Which PLC programming software feature allows simulating the logic without connecting hardware?
a) Data logging
b) Offline simulation
c) PID tuning
d) HMI design
Answer: b
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