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Question 1. Which component of a PLC is primarily responsible for executing the user program and managing the scan cycle? A) Power Supply B) Input Module C) CPU (Processor) D) Communication Port Answer: C Explanation: The CPU fetches, decodes, and executes the ladder logic, controlling the entire scan cycle. Question 2. In PLC terminology, what does the term “I/O Image Table” refer to? A) A graphical representation of the wiring diagram B) The memory area that holds the current status of all inputs and outputs C) The program editing screen on the PC D) The list of ladder rung comments Answer: B Explanation: The I/O Image Table mirrors the real-time state of physical I/O points for the PLC logic to use. Question 3. Which historical development most directly led to the replacement of electromechanical relay control with PLCs? A) Introduction of microprocessors in the 1970s B) Invention of the transistor in the 1940s C) Development of Ethernet networking D) Creation of SCADA systems Answer: A Explanation: Early microprocessors enabled flexible, programmable control, making PLCs feasible and superior to hard-wired relays. Question 4. A PLC input module that is “sinking” is best described as:
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A) Supplying voltage to the field device B) Pulling current from the field device to ground C) Providing isolation through an optocoupler D) Converting analog signals to digital Answer: B Explanation: Sinking inputs draw current from the external device to the PLC’s common (ground) side. Question 5. Which of the following is NOT an advantage of using a PLC over a traditional relay control system? A) Reduced wiring complexity B) Ability to execute complex arithmetic in ladder logic C) Unlimited scan time D) Easy reprogramming for process changes Answer: C Explanation: PLC scan time is finite and must be managed; unlimited scan time is not an advantage. Question 6. In a typical PLC scan cycle, which step occurs first? A) Update outputs B) Execute program logic C) Read inputs D) Perform housekeeping tasks Answer: C Explanation: The PLC begins each scan by sampling the physical inputs into the I/O image table. Question 7. Which type of analog input signal is most commonly used for transmitting sensor data over long distances? A) 0-10 V DC B) 4-20 mA current loop
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D) A circle with a cross inside Answer: A Explanation: The diagonal slash indicates the contact is closed when the associated bit is FALSE. Question 11. Which memory type in a PLC is typically used to store the user program so that it is retained after power is removed? A) RAM (volatile) B) EEPROM (non-volatile) C) Cache memory D) Register file Answer: B Explanation: EEPROM or flash memory retains program code without power. Question 12. The primary purpose of a “seal-in” (holding) circuit in motor control ladder logic is to: A) Provide overload protection B) Keep the motor starter coil energized after the start button is released C) Reverse motor direction automatically D) Measure motor speed Answer: B Explanation: The seal-in contact maintains coil energization until a stop condition occurs. Question 13. Which of the following best describes the function of a “one-shot” (OSR) instruction? A) It repeats an output every scan cycle B) It generates a single pulse that lasts only one scan when its input transitions from false to true C) It continuously toggles a coil while the input is true D) It resets all timers in the program
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Answer: B Explanation: One-shot creates a one-scan pulse on a rising edge, preventing repeated activation. Question 14. In a PLC, the “preset value” (PRE) of a timer represents: A) The current accumulated time B) The time required before the timer’s Done bit becomes true C) The maximum allowable scan time D) The number of inputs scanned per cycle Answer: B Explanation: PRE is the target duration; when ACC reaches PRE, DN is set. Question 15. Which addressing scheme is most commonly used in modern PLC programming environments? A) Fixed absolute addresses (e.g., I0.0) only B) Tag-based symbolic names (e.g., Motor_Start) C) Binary coded decimal addresses D) Physical memory locations only Answer: B Explanation: Tag-based addressing improves readability and portability of programs. Question 16. A “CTU” (Count Up) instruction increments its accumulator when: A) Its Count Up Enable (CU) bit is true B) Its Count Down Enable (CD) bit is true C) The preset value is zero D) The Done (DN) bit is set Answer: A Explanation: CU being true allows the counter to increase its ACC value.
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B) Scan time is constant for all PLC models regardless of program size C) Scan time includes input reading, program execution, and output updating D) Scan time can be measured using a timer instruction within the program Answer: B Explanation: Scan time varies with CPU speed, program complexity, and I/O count; it is not constant across all PLCs. Question 21. A “TOF” (Off-Delay) timer will set its Done bit (DN) after: A) The input becomes true and the preset time elapses B) The input becomes false and the preset time elapses C) The accumulator reaches zero D) The timer is reset manually Answer: B Explanation: TOF starts timing when the input goes false; DN becomes true after the delay. Question 22. Which instruction would you use to compare two integer values and activate a coil only if the first value is greater than the second? A) EQU (Equal) B) GRT (Greater Than) C) LES (Less Than) D) NEQ (Not Equal) Answer: B Explanation: GRT evaluates the condition “first > second” and can drive the coil. Question 23. In a PLC that supports modular expansion, a “fixed” PLC differs from a “modular” PLC in that: A) Fixed PLCs cannot be expanded with additional I/O slots B) Fixed PLCs have higher processing speed C) Fixed PLCs use analog inputs only D) Fixed PLCs require external programming devices
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Answer: A Explanation: Fixed PLCs have a set number of I/O points; modular PLCs allow adding modules. Question 24. Which of the following is a typical function of a “watchdog timer” in a PLC system? A) To monitor the scan cycle and restart the CPU if a scan exceeds a limit B) To provide analog scaling for temperature sensors C) To generate PWM signals for motor speed control D) To store user programs in non-volatile memory Answer: A Explanation: The watchdog timer ensures the PLC is operating correctly; it triggers a reset if scans take too long. Question 25. When an input module is configured for “sourcing” operation, it: A) Pulls current from the field device to ground B) Supplies voltage/current to the field device when activated C) Provides isolation via optocouplers only D) Converts AC inputs to DC internally Answer: B Explanation: Sourcing outputs provide positive voltage/current to the connected device. Question 26. Which of the following best describes a “burst mode” in PLC operation? A) Continuous execution of the program without input scanning B) Execution of a fixed number of scan cycles on demand C) Automatic restart after a power failure D) Parallel processing of multiple ladder logic sections Answer: B
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Question 30. Which of the following statements about “non-volatile” memory in a PLC is TRUE? A) It loses all data when power is removed B) It is used only for temporary variables during a scan C) It retains the user program and data after a power outage D) It can be accessed only during the Run mode Answer: C Explanation: Non-volatile memory (flash, EEPROM) preserves program and data when power is lost. Question 31. In ladder logic, a “parallel” arrangement of contacts implements which logical operation? A) AND B) NOT C) OR D) XOR Answer: C Explanation: Parallel contacts allow current flow if any one of the contacts is true, representing OR logic. Question 32. A PLC’s “communication port” is primarily used for: A) Supplying power to the CPU B) Directly driving high-current loads C) Exchanging data with HMIs, SCADA, or other PLCs D) Converting analog signals to digital Answer: C Explanation: Communication ports (Ethernet, serial) enable networking with external devices. Question 33. Which of the following is NOT a typical feature of a “PID control” module in a PLC?
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A) Proportional gain adjustment B) Integral time setting C) Derivative action configuration D) High-speed pulse counting Answer: D Explanation: PID modules focus on process control, not counting high-speed pulses. Question 34. When a PLC is placed in “Program Mode”, which of the following actions is allowed? A) Running the user program continuously B) Editing and downloading new ladder logic C) Executing the scan cycle without I/O updates D) Communicating with external devices in real time Answer: B Explanation: Program Mode permits editing, compiling, and transferring new code to the PLC. Question 35. The “EN” (Enable) bit of a timer instruction is set to TRUE when: A) The timer’s preset value is zero B) The timer’s input condition is satisfied C) The accumulator has reached the preset value D) The timer has been manually reset Answer: B Explanation: EN becomes true when the timer’s rung condition (input) is true, allowing the timer to count. Question 36. Which type of PLC I/O module would you select to directly control a 120 V AC motor starter coil? A) Relay output module B) Transistor (solid-state) output module
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Explanation: Scaling maps digital counts to real-world units for use in control logic. Question 40. When using a “TRIAC” output module, which type of load is it most suitable for? A) DC motor B) Resistive heating element C) AC lighting or fan D) Pneumatic valve Answer: C Explanation: TRIACs are designed to switch AC loads such as lights and fans. Question 41. The “CTU” counter’s “OV” (Overflow) bit indicates: A) The counter has reached its preset value and stopped counting B) The accumulator has exceeded the maximum countable value (e.g., 65535) C) The counter has been reset manually D) The counter is currently disabled Answer: B Explanation: OV signals that the accumulator wrapped around because it exceeded the maximum count. Question 42. Which of the following is a valid reason to use a “floating” analog input module? A) To reduce scan time B) To allow connection of either current-loop or voltage signals without wiring changes C) To provide built-in PID control D) To increase the number of discrete inputs Answer: B Explanation: Floating analog modules can accept multiple signal types, offering flexibility.
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Question 43. In ladder logic, a “latch” coil (SET) will remain energized until: A) The rung is scanned again B) A corresponding “reset” (RST) coil is energized C) The PLC is powered down D) The input contacts become false Answer: B Explanation: The SET coil stays true until an explicit reset instruction clears it. Question 44. Which PLC execution mode is specifically designed for testing individual rungs without affecting the rest of the program? A) Run Mode B) Program Mode C) Test Mode (or Simulation Mode) D) Maintenance Mode Answer: C Explanation: Test or Simulation mode allows isolated rung execution for debugging. Question 45. A PLC’s “Power Supply” typically converts which of the following? A) 24 V DC to 120 V AC B) 120/240 V AC to 24 V DC for internal circuitry C) 5 V DC to 12 V DC for I/O modules D) 4- 20 mA to 0- 10 V Answer: B Explanation: The internal power supply steps down mains AC to a low-voltage DC rail used by the CPU and modules. Question 46. When a PLC’s “CPU” encounters a “division by zero” error in a math instruction, the typical result is: A) The PLC continues normally, ignoring the error B) The offending rung is disabled and an alarm is generated
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Explanation: TT indicates the timer is actively counting; for TOF, this occurs after the input goes false. Question 50. Which of the following is the primary purpose of a “surge suppressor” on a PLC output module? A) To increase the output current capability B) To protect the module from voltage spikes caused by inductive loads C) To provide isolation between the PLC and the field device D) To convert AC output to DC Answer: B Explanation: Surge suppressors (e.g., snubber circuits) clamp voltage spikes generated by inductive loads. Question 51. A PLC programmed with ladder logic that includes a “GRT” (greater than) instruction comparing a temperature tag to a setpoint will: A) Energize the associated coil only when temperature > setpoint B) Energize the coil when temperature = setpoint C) Energize the coil when temperature < setpoint D) Never energize the coil because GRT is not a valid instruction Answer: A Explanation: GRT evaluates the “greater than” condition and triggers the coil accordingly. Question 52. Which of the following best describes a “fixed” PLC? A) A PLC that cannot be programmed after manufacture B) A PLC with a predetermined number of I/O points that cannot be expanded C fixed PLCs are only used in automotive applications D) A PLC that uses only analog I/O Answer: B Explanation: Fixed PLCs have a set I/O configuration, unlike modular systems that allow expansion.
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Question 53. When a PLC’s “scan time” exceeds the required response time for a safety interlock, the most appropriate corrective action is: A) Increase the number of inputs scanned per cycle B) Reduce the program complexity or upgrade to a faster CPU C) Add more relay contacts to the safety circuit D) Disable the watchdog timer Answer: B Explanation: Shortening scan time improves response; this can be achieved by simplifying the program or using a faster processor. Question 54. In PLC terminology, “de-bounce” is most commonly associated with which type of input? A) High-speed encoder pulses B) Mechanical pushbuttons or switches that may produce spurious transitions C) Analog temperature sensors D) Ethernet communication packets Answer: B Explanation: De-bouncing removes false multiple transitions caused by mechanical contacts. Question 55. Which instruction would you use to accumulate the total number of parts produced, assuming each part triggers a pulse on a digital input? A) TON (On-Delay Timer) B) CTU (Count Up) C) MOV (Move) D) GRT (Greater Than) Answer: B Explanation: CTU increments its accumulator Question 56. In a PLC, which of the following statements about the “ACC” (Accumulated) value of a timer is correct?
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B) Changes from true to false C) Remains true for more than one scan D) Is true continuously Answer: A Explanation: OSR detects a rising edge; OSF detects a falling edge. Question 60. Which PLC instruction would you use to convert a temperature reading from Fahrenheit to Celsius within ladder logic? A) SUB (Subtract) B) DIV (Divide) C) MUL (Multiply) D) A combination of MULTIPLY and SUBTRACT instructions Answer: D Explanation: Celsius = (Fahrenheit – 32) × 5/9, requiring subtraction followed by multiplication (or multiply by 5/9). Question 61. In a PLC, the “COM” (Common) address of a discrete input is typically used for: A) Providing power to the input device B) Grounding the input circuit in sinking configurations C) Storing the preset value of a timer D) Tagging the input for documentation Answer: B Explanation: In sinking inputs, the COM point is connected to the common (ground) side of the field device. Question 62. Which of the following is a primary benefit of using “tag-based” addressing over “fixed absolute” addressing? A) Faster scan times B) Reduced memory usage C) Improved program readability and maintainability
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D) Elimination of the need for a CPU Answer: C Explanation: Tags provide meaningful names, making the program easier to understand and modify. Question 63. A PLC’s “RTO” timer will retain its accumulated value after a power loss only if the CPU includes: A) A watchdog timer B) Battery-backed RAM or non-volatile memory for timers C) A high-speed Ethernet port D) An analog output module Answer: B Explanation: Retentive timers rely on non-volatile storage to preserve ACC across power cycles. Question 64. Which instruction is used to prevent a counter from incrementing when a certain condition is met? A) CTU with the CU (Count Up Enable) contact de-energized B) CTD with the CD (Count Down Enable) contact energized C) TON with its EN bit false D) MOV with a zero source value Answer: A Explanation: The CTU only increments when the CU input is true; de-energizing CU disables counting. Question 65. In ladder logic, a “normally open” (NO) contact placed in series with a coil will cause the coil to energize when the contact’s associated bit is: A) TRUE (1) B) FALSE (0) C) Either true or false D) Never, because NO contacts cannot energize coils
