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Troubleshooting 1 Ultimate Exam
Question 1. Which PLC component is responsible for executing the user-written ladder logic? A) Power supply B) I/O module C) CPU/Processor D) Communication module Answer: C Explanation: The CPU (central processing unit) fetches, decodes, and executes the program instructions during each scan cycle. Question 2. In a typical PLC scan cycle, which step occurs first? A) Output scan B) Program execution C) Input scan D) Housekeeping tasks Answer: C Explanation: The PLC first reads all input states (input scan) before executing the user program. Question 3. Which of the following best describes a fixed-type PLC? A) Modules can be added or removed without changing the chassis. B) All I/O and processor functions are built into a single, non-expandable unit. C) It uses a modular backplane for easy expansion. D) It requires a separate rack for communication modules. Answer: B
Troubleshooting 1 Ultimate Exam
Explanation: Fixed PLCs have a predetermined set of I/O and cannot be expanded, unlike modular systems. Question 4. Converting the binary number 101101₂ to decimal yields: A) 45 B) 53 C) 59 D) 61 Answer: B Explanation: 1·2⁵+0·2⁴+1·2³+1·2²+0·2¹+1·2⁰ = 32+0+8+4+0+1 = 45 (Oops – correct calculation gives 45). Actually the correct answer is A. Explanation: 101101₂ = 32+8+4+1 = 45₁₀. Question 5. Which PLC data type can store a value ranging from –32, to +32,767? A) Boolean B) 16-bit Integer (INT) C) 32-bit Floating Point (REAL) D) Counter (CTU) Answer: B Explanation: A signed 16-bit integer (INT) uses two’s complement representation covering that range. Question 6. A sensor that sources current to the PLC input (current flows out of the sensor) uses which wiring convention? A) NPN (sinking) B) PNP (sourcing)
Troubleshooting 1 Ultimate Exam
Question 9. Which software function allows you to verify that the programming PC is communicating with the PLC processor? A) Online/Offline toggle B) Force I/O window C) Tag browser D) Program dump utility Answer: A Explanation: Switching the software to “Online” confirms a live connection with the PLC. Question 10. In RSLogix, the LED on the processor’s front panel that blinks rapidly indicates: A) RUN mode B) PROGRAM mode C) COMMUNICATION fault D) POWER fault Answer: B Explanation: A blinking “PROG” LED shows the CPU is in programming (download) mode. Question 11. Forcing a PLC output while the machine is operating is considered safe when: A) The output controls a safety-critical device. B) The forced state matches the normal operating condition. C) The PLC is in RUN mode. D) The output is a high-current actuator. Answer: B
Troubleshooting 1 Ultimate Exam
Explanation: Forcing should only duplicate a normal state to avoid unintended motion or hazards. Question 12. Which ladder-logic instruction moves a value from one data file to another without alteration? A) ADD B) MOV C) SUB D) CMP Answer: B Explanation: The MOV (move) instruction copies source data to destination unchanged. Question 13. The purpose of rung comments in a PLC program is to: A) Increase execution speed. B) Provide human-readable documentation for troubleshooting. C) Reserve memory for future use. D) Enable cross-reference searching. Answer: B Explanation: Comments help technicians understand the intent of each rung during diagnosis. Question 14. A flashing red LED on the CPU’s “FAULT” indicator most likely means: A) Power supply is off. B) A minor fault such as a communication error. C) A major fault like memory corruption.
Troubleshooting 1 Ultimate Exam
B) Broken wiring or a blown fuse. C) Incorrect module address. D) Analog scaling error. Answer: B Explanation: Physical LED status reflects actual input voltage; a break or fuse loss prevents the module from sensing the device. Question 18. When the module LED is ON but the corresponding PLC bit does not change, the likely issue is: A) Power supply failure. B) Incorrect I/O address in the ladder logic. C) Sensor polarity reversal. D) Ground loop. Answer: B Explanation: The module sees the input, but the program references a different address, so the bit stays unchanged. Question 19. Which multimeter measurement is most appropriate to verify a 24 VDC discrete input signal? A) Resistance across the input terminals. B) AC voltage at the input. C) DC voltage between input and common. D) Continuity test with the input energized. Answer: C Explanation: Measuring DC voltage confirms the presence of the expected signal level.
Troubleshooting 1 Ultimate Exam
Question 20. A 4- 20 mA analog input is reading 0 mA on the PLC. Which is the most common cause? A) The sensor is out of range. B) The input module is set to voltage mode. C) The sensor wiring is reversed. D) The PLC program scales incorrectly. Answer: B Explanation: If the module expects voltage but receives current, it will read 0 mA. Question 21. In scaling an analog input, the raw count of 32768 corresponds to which engineering value if the range is 0- 100 °C? A) 25 °C B) 50 °C C) 75 °C D) 100 °C Answer: B Explanation: 32768 is half of the 16-bit range (65535), representing the midpoint of the engineering range: 50 °C. Question 22. An analog output shows a fluctuating value despite a constant setpoint. The most likely fault is: A) A broken power cable to the PLC. B) A noisy sensor feeding the same channel. C) A faulty output module or external wiring short. D) Incorrect timer instruction. Answer: C
Troubleshooting 1 Ultimate Exam
C) Input pulses rapidly. D) No change in input state. Answer: A Explanation: When the actuator presses the NC switch, the contacts open, removing the signal (logic 0). Question 26. When troubleshooting a pushbutton that never registers a press, the first test should be: A) Verify the PLC program logic. B) Measure voltage at the button terminals while pressed. C) Replace the button with a known good one. D) Check the CPU’s fault register. Answer: B Explanation: Confirming the physical signal verifies whether the problem is wiring/device or program related. Question 27. A solenoid valve controlled by a PLC output is not actuating. Which is the most systematic first step? A) Force the output ON from the programming software. B) Replace the solenoid coil. C) Check the output module LED status. D) Increase the scan time. Answer: C Explanation: The output LED indicates whether the module is attempting to drive the load; if it’s off, the fault is likely upstream. Question 28. Interposing relays are used in PLC output circuits primarily to:
Troubleshooting 1 Ultimate Exam
A) Provide isolation between the PLC and high-current loads. B) Reduce scan time. C) Convert analog signals to digital. D) Store program data. Answer: A Explanation: Relays protect the low-power PLC output from high current and voltage spikes. Question 29. A ground loop in a control panel can cause which symptom? A) PLC processor overheating. B) Erratic analog input readings. C] Immediate processor fault. D) Loss of Ethernet connectivity. Answer: B Explanation: Ground loops introduce noise that especially affects analog signals, causing unstable readings. Question 30. The TON (Timer ON Delay) instruction will reset its accumulated time when: A) The timer’s preset value changes. B) The timer’s enable rung condition becomes false. C) The PLC enters PROGRAM mode. D) The timer reaches 0 ms. Answer: B Explanation: TON resets to 0 whenever its enable condition is de-energized.
Troubleshooting 1 Ultimate Exam
Explanation: If the coil address is wrong, the program never energizes the output for step 3. Question 34. Safety interlock logic often uses which PLC instruction to prevent machine start? A) XIC (Examine If Closed) contact linked to a guard-door sensor. B) ADD instruction. C) TON timer. D) MOV data file. Answer: A Explanation: An XIC (normally open) contact tied to a “door closed” sensor ensures the start rung only energizes when the door is secured. Question 35. When a program uses the DIV instruction and the divisor can become zero, the PLC will: A) Automatically reset the CPU. B) Set the result to maximum integer value. C) Generate a divide-by-zero fault and stop the program. D) Ignore the instruction. Answer: C Explanation: Most PLCs flag a runtime error for division by zero, halting execution of that rung. Question 36. Overflow in a 16-bit integer operation will cause which result? A) The result wraps around to the negative range. B] The PLC to shut down. C) The result to be clipped at 32,767.
Troubleshooting 1 Ultimate Exam
D) No effect; the PLC ignores overflow. Answer: A Explanation: Two’s-complement arithmetic wraps around, producing an incorrect signed value. Question 37. The first step in a systematic troubleshooting methodology is to: A) Implement a solution. B) Define the problem by observing symptoms. C) Document the fault. D) Update the program. Answer: B Explanation: Observation and problem definition guide all subsequent diagnostic actions. Question 38. When documenting a fault, which piece of information is least essential? A) Exact time the fault occurred. B) The color of the wiring insulation. C) The PLC scan time setting. D) The fault code displayed. Answer: B Explanation: While useful for future reference, wiring color is not critical for diagnosing the immediate fault. Question 39. A PLC’s backup battery is low. Which symptom is most likely to appear first? A) Loss of Ethernet connectivity.
Troubleshooting 1 Ultimate Exam
Question 42. When a discrete output module’s LED is ON but the connected pilot light remains dark, the most probable cause is: A) The PLC program has a fault. B) The pilot light is burnt out or wiring is open. C) The output module is in fault mode. D) The CPU is in PROGRAM mode. Answer: B Explanation: The module is delivering power (LED on); a dark light indicates a downstream fault. Question 43. A 0- 10 VDC analog input reads 12 V on the PLC. Which is the most plausible explanation? A) The input module is set to 4- 20 mA mode. B) The sensor is out of range and the module is saturating. C) The PLC is reading a digital input instead. D) The wiring is reversed. Answer: B Explanation: When the input voltage exceeds the module’s maximum, the reading often saturates at the top of the scale. Question 44. Which Ethernet cable defect most commonly causes intermittent communication loss to a remote I/O? A) Incorrect pin-out (crossover instead of straight). B) A broken pair within the shield. C) Use of Cat-3 cable. D) Excessive cable length beyond 100 m. Answer: B
Troubleshooting 1 Ultimate Exam
Explanation: A broken wire in one of the data pairs creates intermittent errors while the rest of the cable may appear intact. Question 45. In a PLC that uses a modular backplane, a missing power supply module will most likely result in: A) All I/O modules showing “offline” status. B) Only analog modules failing. C) The CPU still running but with reduced scan speed. D) No effect; the CPU draws power from the rack. Answer: A Explanation: Without the power supply, the backplane cannot deliver power to any installed modules. Question 46. When converting a hexadecimal address “0x1A3” to decimal for an I/O reference, the result is: A) 419 B) 451 C) 483 D) 511 Answer: C Explanation: 1·16² + 10·16¹ + 3·16⁰ = 256 + 160 + 3 = 419 (Oops). Actually 0x1A = 419 decimal. Answer: A Explanation: 0x1A3 = 419₁₀. Question 47. Which instruction can be used to compare two values and set a bit if they are equal?
Troubleshooting 1 Ultimate Exam
Question 50. When a PLC’s watchdog timer trips, the processor typically: A) Restarts the scan cycle automatically. B) Enters a fault state and halts program execution. C] Switches to PROGRAM mode. D) Increases the scan time to compensate. Answer: B Explanation: A watchdog fault indicates the CPU missed its timing window, causing a fault condition. Question 51. The primary advantage of using a modular PLC over a fixed PLC in troubleshooting is: A) Faster scan times. B) Ability to replace individual faulty modules without replacing the entire controller. C) Lower initial cost. D) Built-in Ethernet port. Answer: B Explanation: Modular designs allow hot-swap or replacement of specific I/O or power modules. Question 52. Which of the following statements about analog scaling is true? A) Scaling is performed only in hardware. B) Changing the engineering units does not affect the raw count. C) The scaling equation is always linear. D) Scaling can be performed in the PLC program using a multiply-and-divide instruction.
Troubleshooting 1 Ultimate Exam
Answer: D Explanation: PLC programs often implement scaling with arithmetic instructions. Question 53. A PLC input module is configured for “sink” (NPN) devices, but a PNP sensor is connected. The expected result is: A) The input reads HIGH when the sensor is active. B) The input reads LOW regardless of sensor state. C) The module will be damaged. D) The PLC will generate a fault code. Answer: B Explanation: A sinking input expects current to flow into the module; a sourcing sensor will not provide the correct polarity, leaving the input at 0 V. Question 54. Which diagnostic tool allows you to view the real-time status of all PLC I/O points without stopping the processor? A) Offline program editor. B) Online I/O table (or data file viewer). C) Fault register dump. D) Program upload utility. Answer: B Explanation: The online I/O table displays live input and output states while the PLC runs. Question 55. A PLC program uses a CTD (Count Down) instruction that never reaches zero. The most probable cause is: A) The preset value is set to zero. B) The count enable condition is never true.
