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MANUAL PROGRAMMABLE LOGIC CONTROLLER
5TH EDITION PETRUZELLA (CHAPTER 1-15)
QUESTIONS AND CORRECT ANSWERS (VERIFIED
ANSWERS) Q&A 2026 |INSTANT DOWNLOAD PDF
1. What is the primary function of a Programmable Logic Controller (PLC)? A. To perform mechanical machining operations B. To control industrial processes using programmed logic C. To generate electrical power D. To replace sensors in systems Rationale: A PLC is designed to automate and control industrial machinery and processes through programmed logic instructions. Correct Answer: B 2. Which of the following best describes a PLC scan cycle? A. Output → Input → Program → Storage B. Input → Program → Output → Repeat C. Program → Input → Output → Stop D. Input → Output → Memory → Program
Rationale: The PLC continuously reads inputs, executes the program, updates outputs, and repeats the cycle. Correct Answer: B
3. Which component is considered the “brain” of the PLC? A. Input module B. Output module C. CPU D. Power supply Rationale: The CPU executes the control program and manages data processing. Correct Answer: C 4. What type of memory is commonly used in PLCs for user programs? A. RAM only B. ROM only C. EEPROM or Flash memory D. Optical memory Rationale: Non-volatile memory like EEPROM or Flash retains programs even when power is lost. Correct Answer: C 5. What is the purpose of the input module in a PLC system?
Rationale: Output modules send control signals to actuators like motors and solenoids. Correct Answer: B
8. Which programming language is most commonly associated with PLCs? A. Python B. Ladder Logic C. C++ D. JavaScript Rationale: Ladder logic is widely used in industrial PLC programming due to its simplicity. Correct Answer: B 9. In ladder logic, what does a normally open contact represent? A. A closed circuit by default B. A switch that is always ON C. A condition that allows current when activated D. A timer function Rationale: A normally open contact allows current flow only when activated. Correct Answer: C
10. What is the main advantage of using PLCs over relay systems? A. Higher cost B. More wiring required C. Easier modification of control logic D. Slower operation Rationale: PLC programs can be modified easily without rewiring hardware. Correct Answer: C 11. Which scan step occurs first in a PLC cycle? A. Program execution B. Output update C. Input scan D. Memory refresh Rationale: PLCs begin by scanning all input status. Correct Answer: C 12. What is a timer in PLC programming used for? A. Counting events B. Delaying or timing operations C. Storing data permanently D. Converting analog signals
A. Limit switch B. Push button C. Temperature sensor D. Relay contact Rationale: Temperature sensors provide continuous (analog) signals. Correct Answer: C
16. What does “ladder rung” represent in PLC programming? A. A memory block B. A single logic operation path C. A hardware module D. A wiring diagram only Rationale: Each rung represents a logical control statement in ladder logic. Correct Answer: B 17. What is the role of the power supply in a PLC system? A. Executes logic B. Converts field signals C. Provides electrical power to PLC components D. Stores program data
Rationale: The power supply energizes all PLC internal components. Correct Answer: C
18. Which device converts physical signals into electrical signals for PLC input? A. Actuator B. Sensor C. Relay D. Contact block Rationale: Sensors detect physical conditions and convert them into electrical signals. Correct Answer: B 19. What happens during the output scan phase of a PLC? A. Inputs are read B. Program is written C. Output devices are updated D. Memory is cleared Rationale: The PLC updates outputs based on program execution results. Correct Answer: C 20. What is the purpose of interlocking in PLC systems?
23. What does a PLC comparator instruction do? A. Adds two values B. Compares two data values C. Converts analog to digital D. Stores memory values Rationale: Comparator instructions evaluate relationships like equal, greater than, or less than. Correct Answer: B 24. What is the purpose of a shift register in PLC programming? A. Stores program code B. Shifts data sequentially through memory bits C. Converts signals D. Controls motor speed Rationale: Shift registers move binary data across registers in sequence. Correct Answer: B 25. Which device is commonly used for analog output? A. Solenoid B. Motor starter
C. Variable frequency drive (VFD) D. Pushbutton Rationale: VFDs receive analog signals to control motor speed. Correct Answer: C
26. What is the main purpose of analog scaling in PLCs? A. Convert logic to mechanical motion B. Convert raw input into engineering units C. Increase CPU speed D. Store binary values Rationale: Scaling converts raw digital values into meaningful engineering measurements. Correct Answer: B 27. What is a latch in PLC programming? A. A memory that stores permanent data B. A circuit that maintains output ON after input is removed C. A timer function D. A sensor type Rationale: Latches maintain output state until explicitly reset. Correct Answer: B
Rationale: SCADA systems monitor and control industrial processes. Correct Answer: B
31. What is a PLC memory map used for? A. Wiring diagrams B. Organizing data storage locations C. Power distribution D. Mechanical layout Rationale: Memory maps define addresses for inputs, outputs, and internal bits. Correct Answer: B 32. Which PLC element stores temporary results during execution? A. Input module B. CPU registers C. Output module D. Power supply Rationale: CPU registers temporarily store computation data. Correct Answer: B 33. What is the main function of an HMI in PLC systems?
A. Replace sensors B. Provide user interface for monitoring and control C. Increase voltage D. Execute ladder logic Rationale: HMI allows operators to interact with the PLC system. Correct Answer: B
34. What is a one-shot instruction in PLC programming? A. Continuous output B. Output triggered once per input change C. Analog scaling function D. Timer reset function Rationale: One-shot generates a single pulse per event. Correct Answer: B 35. What is the function of a watchdog timer in PLCs? A. Measures temperature B. Detects program execution faults C. Controls outputs D. Stores program code Rationale: It monitors PLC operation and triggers fault if scan time exceeds limit. Correct Answer: B
Rationale: It allows PLCs to share data across systems. Correct Answer: B
39. What is a fault in a PLC system? A. Normal operation B. Program or hardware error condition C. Input signal D. Output instruction Rationale: Faults indicate system errors or malfunctions. Correct Answer: B 40. What happens when a PLC enters stop mode? A. Outputs remain active B. Program execution stops C. Inputs are ignored permanently D. Memory is cleared automatically Rationale: Stop mode halts program execution but retains memory. Correct Answer: B 41. What is the purpose of data handling instructions in PLCs? A. Control wiring B. Move, copy, and manipulate data C. Increase voltage D. Generate alarms
Correct Answer: B
42. Which instruction is used to move data in PLC programming? A. ADD B. MOV C. CMP D. RES Correct Answer: B 43. What is binary-coded decimal (BCD)? A. Analog signal format B. Digital representation of decimal numbers C. Motor control method D. Sensor type Correct Answer: B 44. What is the function of a PLC network node? A. Stores memory B. Connects devices in communication network C. Generates output signals D. Controls voltage Correct Answer: B
A. Voltage unit B. Binary digit (0 or 1) C. Sensor type D. Output device Correct Answer: B
49. What is a word in PLC memory? A. Single bit B. Group of bits (usually 16) C. Analog sensor D. Output module Correct Answer: B 50. What is the purpose of forcing in PLC programming? A. Increase CPU speed B. Override input/output states manually C. Reset program D. Delete memory Correct Answer: B 51. What is the purpose of a PLC register? A. Store physical devices B. Store data and values temporarily
C. Increase voltage D. Control wiring Correct Answer: B Rationale: Registers hold temporary data used during program execution.
52. What is the function of a data table in PLC memory? A. Stores ladder diagrams B. Organizes input/output and internal data C. Controls motors directly D. Powers sensors Correct Answer: B Rationale: Data tables organize PLC memory addresses for efficient processing. 53. Which instruction is used for arithmetic addition in PLCs? A. SUB B. ADD C. MUL D. DIV Correct Answer: B Rationale: ADD instruction performs addition of numeric values.
