Q1. Which gate produces HIGH only when all inputs are HIGH?

A. OR B. AND C. NOT D. XOR

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: AND is HIGH only when every input is HIGH.

Q2. Which gate produces HIGH when at least one input is HIGH?

A. AND B. NOR C. OR D. XNOR

Check Answer

Correct Answer: C   |   Marks: 1

Explanation: OR is HIGH when one or more inputs are HIGH.

Q3. Which gate is known as an inverter?

A. NOT B. AND C. OR D. XOR

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A NOT gate complements its input.

Q4. What is 1 AND 0?

A. 1 B. 0 C. 2 D. Undefined

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: AND requires both inputs to be 1, so the result is 0.

Q5. What is 0 OR 0?

A. 1 B. 0 C. 2 D. Undefined

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: OR is 0 when all inputs are 0.

Q6. What is NOT 1?

A. 1 B. 0 C. Both D. High impedance

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: NOT reverses the input, so NOT 1 is 0.

Q7. Which gate is HIGH when two inputs are different?

A. XNOR B. AND C. XOR D. NOR

Check Answer

Correct Answer: C   |   Marks: 1

Explanation: XOR is HIGH for unequal input states.

Q8. Which gate is HIGH when two inputs are equal?

A. XOR B. XNOR C. OR D. NAND

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: XNOR is HIGH for equal input states.

Q9. Which gate is the inverse of AND?

A. NOR B. NAND C. XOR D. XNOR

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: NAND is the complement of the AND operation.

Q10. Which gate is the inverse of OR?

A. NAND B. XOR C. NOR D. NOT

Check Answer

Correct Answer: C   |   Marks: 1

Explanation: NOR is the complement of the OR operation.

Q11. What does PLC stand for?

A. Programmable Logic Controller B. Process Line Computer C. Power Logic Circuit D. Programmable Load Counter

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: PLC means Programmable Logic Controller.

Q12. Which PLC unit executes the program?

A. Power supply B. CPU C. Terminal block D. Fuse

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The CPU executes the user program and manages the scan.

Q13. Which PLC module receives sensor signals?

A. Output module B. Input module C. Power module D. Cable

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The input module receives signals from field sensors and switches.

Q14. Which PLC module controls actuators?

A. Input module B. Output module C. Memory card D. Programming port

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The output module sends signals to actuators.

Q15. What is the purpose of the PLC power supply?

A. Store programs B. Provide electrical power C. Measure temperature D. Increase speed

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The power supply provides the voltage required by the PLC.

Q16. A normally open ladder contact is true when its bit is:

A. 0 B. 1 C. Always true D. Disconnected

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: A normally open instruction is true when its bit is 1.

Q17. A normally closed ladder contact is true when its bit is:

A. 1 B. 0 C. Always false D. Analog

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: A normally closed instruction is true when its bit is 0.

Q18. What is the usual PLC scan sequence?

A. Output-input-stop B. Read inputs, execute program, update outputs C. Erase-program-power off D. Input-recharge-output

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The PLC reads inputs, executes logic, and updates outputs.

Q19. Which device commonly starts a motor?

A. Push button B. Printer C. Speaker D. Keyboard only

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A push button provides a discrete start command.

Q20. What is a PLC timer used for?

A. Create a time delay B. Increase voltage C. Convert AC to DC D. Measure weight only

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A timer creates or measures a programmed time interval.

Q21. What is a PLC counter used for?

A. Count events B. Change wire colour C. Reduce pressure D. Generate air

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A counter records events or pulses.

Q22. Which number system is used for digital logic?

A. Decimal only B. Binary C. Roman D. Octal only

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: Binary represents the two logic states 0 and 1.

Q23. A digital PLC input normally represents:

A. A continuous range B. An ON/OFF state C. Torque only D. Viscosity

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: Digital inputs represent discrete states such as ON and OFF.

Q24. What is the purpose of an emergency stop?

A. Increase speed B. Stop hazardous operation quickly C. Tune a timer D. Display a logo

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: An emergency stop is intended to bring hazardous operation to a safe state.

Q25. Which gate is universal?

A. NAND B. XOR only C. Buffer only D. XNOR only

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: NAND can be combined to implement other logic functions.

Q26. The Boolean expression for NAND is:

A. A+B B. A·B C. (A·B)' D. A'+B'

Check Answer

Correct Answer: C   |   Marks: 1

Explanation: NAND is the complement of the AND product.

Q27. The Boolean expression for NOR is:

A. A+B B. (A+B)' C. A·B D. A'·B'

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: NOR is the complement of the OR sum.

Q28. According to De Morgan's theorem, (A·B)' equals:

A. A'·B' B. A'+B' C. A+B D. A·B

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The complement of a product becomes the sum of the complements.

Q29. According to De Morgan's theorem, (A+B)' equals:

A. A'+B' B. A·B C. A'·B' D. A+B

Check Answer

Correct Answer: C   |   Marks: 1

Explanation: The complement of a sum becomes the product of the complements.

Q30. Simplify A + A·B.

A. A B. B C. A·B D. A+B

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: The absorption law gives A + A·B = A.

Q31. Simplify A·(A+B).

A. B B. A C. A+B D. 0

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: The absorption law gives A·(A+B) = A.

Q32. An OR gate produces 0 for which input?

A. 00 B. 01 C. 10 D. 11

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: OR is 0 only when both inputs are 0.

Q33. An AND gate produces 1 for which input?

A. 00 B. 01 C. 10 D. 11

Check Answer

Correct Answer: D   |   Marks: 1

Explanation: AND is 1 only when both inputs are 1.

Q34. PLC scan time is the time required to:

A. Replace the CPU B. Complete one scan cycle C. Charge the battery D. Change wiring

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: A scan includes input reading, program execution, and output updating.

Q35. Why are PLC input filters used?

A. Remove noise or short transients B. Increase torque C. Change frequency D. Erase memory

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Filtering prevents unwanted brief signals from being accepted.

Q36. What is the purpose of a seal-in circuit?

A. Maintain an output after momentary start B. Reduce voltage C. Reverse inputs D. Measure pressure

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A seal-in branch maintains the run command after the start button is released.

Q37. Why is a stop contact placed in series?

A. Interrupt the run logic B. Always energize the motor C. Count revolutions D. Measure current only

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: The series stop condition breaks the run logic when stopping is requested.

Q38. A normally closed instruction with bit 1 is:

A. True B. False C. Analog D. High impedance

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: Normally closed logic is false when its associated bit is 1.

Q39. Which instruction retains a bit until reset?

A. Set/latch B. Compare only C. Move only D. Input filter

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A set or latch instruction holds a bit until reset.

Q40. Digital and analog signals differ because:

A. Digital is discrete; analog varies over a range B. Digital is always AC C. Digital measures pressure only D. There is no difference

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Digital states are discrete, while analog values vary over a range.

Q41. A 4–20 mA signal is commonly used for:

A. Industrial analog measurement B. Only binary switching C. Lubrication only D. Network addressing

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: The 4–20 mA standard transmits analog process values.

Q42. What is the purpose of an HMI?

A. Operator monitoring and control B. Replace every sensor C. Supply compressed air D. Act as a gearbox

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: An HMI displays process information and allows operator commands.

Q43. What is interlocking?

A. Prevent incompatible operations together B. Increase memory C. Change PLC brand D. Remove sensors

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Interlocks prevent conflicting or unsafe actions simultaneously.

Q44. A rising-edge instruction detects:

A. A 0-to-1 transition B. A permanent 0 C. Resistance D. Power-supply change

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Rising-edge logic detects the transition from OFF to ON.

Q45. What is the purpose of a watchdog timer?

A. Detect abnormal execution or excessive scan time B. Count products C. Filter analog signals D. Control brightness

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A watchdog detects abnormal execution or excessive scan duration.

Q46. Which area stores sampled input states?

A. Input image table B. Output relay only C. Printer memory D. HMI screen

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: The input image table stores sampled input states used by the program.

Q47. A retentive timer is designed to:

A. Retain accumulated time B. Always reset each scan C. Convert AC to DC D. Operate without CPU

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A retentive timer preserves accumulated time until reset.

Q48. Why must output ratings not be exceeded?

A. The output module may be damaged B. The program gets shorter C. The CPU becomes faster D. The HMI disappears

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Exceeding voltage or current ratings can damage the output module.

Q49. A truth table is used to:

A. List outputs for all input combinations B. Show wire colours C. Calculate motor weight D. Store drawings

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A truth table lists the output for every possible input combination.

Q50. Y = A·B + C represents:

A. AND A and B, then OR with C B. Only NOT C. Only XOR D. NOR followed by NOT

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: The expression first performs AND and then ORs the result with C.

Q51. Simplify F = A·B + A·B'.

A. A B. B C. A+B D. 0

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Factor A: F=A(B+B')=A.

Q52. Simplify F = (A+B)(A+B').

A. A B. B C. A+B D. 0

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Using the Boolean identity, F=A+B·B'=A.

Q53. Simplify F = A + A'·B.

A. A+B B. A·B C. A'+B D. A

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: The identity X+X'Y=X+Y gives A+B.

Q54. Which expression represents XOR?

A. A·B + A'·B' B. A'·B + A·B' C. A+B D. A·B

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: XOR is true for unequal inputs and is A'B+AB'.

Q55. Which expression represents XNOR?

A. A'·B + A·B' B. A·B + A'·B' C. A+B D. (A+B)'

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: XNOR is true for equal inputs and is AB+A'B'.

Q56. A three-input majority circuit outputs 1 when:

A. At least two inputs are 1 B. All inputs are 0 C. Exactly one input is 1 D. Inputs alternate

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Majority logic is true when two or more inputs are HIGH.

Q57. A three-input majority function is:

A. AB+AC+BC B. A+B+C C. ABC only D. A'B'C'

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: AB, AC, and BC represent all pairs of HIGH inputs.

Q58. Why can a normal PLC scan miss a short pulse?

A. It may occur between sampling events B. The CPU changes all 1s to 0s C. Outputs block inputs D. The HMI removes it

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A pulse can begin and end between two input sampling instants.

Q59. Which method is suitable for high-speed pulse counting?

A. High-speed counter B. Long software delay C. Ignore the input D. Use a lamp output

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: High-speed counter hardware captures pulses that ordinary scans may miss.

Q60. A motor requires Guard AND Start AND NOT Fault. Which expression is correct?

A. Guard+Start+Fault B. Guard·Start·Fault' C. Guard'·Start·Fault D. Guard·Start+Fault

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: All permissives must be true and the fault condition must be false.

Q61. Why should emergency-stop functions not rely only on ordinary PLC software?

A. Safety-rated architecture may be required B. Software is always faster C. It increases brightness D. It removes risk assessment

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Risk-based safety design may require independently monitored and validated safety functions.

Q62. A 5-second timer with 100-ms time base requires:

A. 5 B. 50 C. 500 D. 0.5

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: 5,000 ms divided by 100 ms equals 50 time-base units.

Q63. A counter target is 250 and current count is 187. Remaining count:

A. 53 B. 63 C. 73 D. 437

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: 250 minus 187 equals 63.

Q64. A 0–10 V input represents 0–100 °C. The value at 6 V is:

A. 6 °C B. 40 °C C. 60 °C D. 100 °C

Check Answer

Correct Answer: C   |   Marks: 1

Explanation: Linear scaling gives 6/10 × 100 = 60 °C.

Q65. A 4–20 mA transmitter represents 0–200 bar. The value at 12 mA is:

A. 50 bar B. 100 bar C. 150 bar D. 200 bar

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: 12 mA is halfway between 4 and 20 mA, so the value is 100 bar.

Q66. A major benefit of separating safety logic from standard logic is:

A. Improved independence and validation B. No sensors are needed C. Maintenance is eliminated D. All outputs become analog

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Separation supports independent validation and fault handling.

Q67. A seal-in bit has no reset path. What may happen?

A. The bit may remain latched B. Inputs cannot be read C. Scaling becomes zero D. Scan time doubles

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Without reset logic, the latched bit may remain ON indefinitely.

Q68. Which technique prevents repeated actions while a button is held?

A. One-shot or edge detection B. Permanent forcing C. Removing the input D. Increasing screen size

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Edge detection creates one event at the transition instead of repeating each scan.

Q69. Why are explicit states useful in a PLC state machine?

A. They organize stages and transitions B. They eliminate outputs C. They convert signals to analog D. They prevent every fault

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: States clarify the current sequence stage and valid transitions.

Q70. A race condition is:

A. An outcome dependent on timing or order B. A motor competition C. A timer preset D. An analog sensor

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: A race condition occurs when timing or execution order changes the result.

Q71. Why use descriptive PLC tag names?

A. Improve readability and troubleshooting B. Increase supply voltage C. Remove documentation D. Guarantee zero scan time

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Meaningful tags make programs easier to understand and maintain.

Q72. A conveyor stops if Sensor A OR Sensor B detects a jam. The logic is:

A. Stop=A·B B. Stop=A+B C. Stop=A'·B' D. Stop=A XOR B only

Check Answer

Correct Answer: B   |   Marks: 1

Explanation: OR logic activates the stop command when either sensor detects a jam.

Q73. Two-hand control requires both buttons within a time window. This is:

A. Simultaneity monitoring B. Simple OR logic C. Unconditional latch D. Analog scaling

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Simultaneity monitoring verifies that both inputs occur within the allowed interval.

Q74. Why can forcing PLC outputs be dangerous?

A. It can bypass normal logic and cause unexpected motion B. It repairs sensors C. It lowers motor current D. It guarantees safety

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Forcing overrides normal conditions and can energize equipment unexpectedly.

Q75. The purpose of a PLC permissive circuit is to:

A. Allow operation only when required conditions are satisfied B. Increase CPU memory C. Replace safety devices D. Convert digital to mechanical

Check Answer

Correct Answer: A   |   Marks: 1

Explanation: Permissives ensure required operating conditions are true before an action is allowed.