SACA - Smart Automation Certification Alliance PLC Programming and Troubleshooting Questions and Answers

Question 1

A technician is troubleshooting a PLC-controlled conveyor system.
A specific output module indicator light for the main drive motor contactor is ON, but the motor contactor itself is not engaging.
The external wiring from the module to the contactor has been verified as correct and the contactor coil is functional.
What is the most likely cause of this issue?

Accepted Answer

A defective output module where the internal switching device has failed.

Answer

A fault in the ladder logic program.

Answer

An incorrect IP address setting on the PLC processor.

Answer

The PLC is in PROGRAM or REMOTE mode.

Question 2

During the commissioning of a new machine, an operator reports that a pneumatic cylinder extends but never retracts. The ladder logic shows a standard seal-in circuit for the extend solenoid and a separate rung to energize the retract solenoid when an 'extended' limit switch is made. Monitoring the logic online, the technician sees the 'extended' limit switch input is active, but the retract solenoid output rung is not energized. Which of the following is the most probable cause?

Accepted Answer

A conflicting instruction is de-energizing the retract output elsewhere in the program.

Answer

The pneumatic tubing to the cylinder is disconnected.

Answer

The power supply to the PLC is underrated.

Answer

The 'extended' limit switch has failed mechanically.

Question 3

A maintenance technician arrives at a machine that has completely stopped and the PLC's 'FAULT' LED is solid red. What is the first and most critical step in the troubleshooting process?

Accepted Answer

Go online with the PLC using programming software to diagnose the fault code.

Answer

Immediately cycle power to the PLC to try and clear the fault.

Answer

Replace the CPU module with a known good spare.

Answer

Start checking all field wiring and sensors for shorts.

Question 4

Which of the following is a PLC programming best practice that significantly aids in future troubleshooting efforts?

Accepted Answer

Adding detailed comments and descriptions to rungs, tags, and routines.

Answer

Using as many nested branches as possible to shorten the code.

Answer

Assigning all I/O addresses directly in the logic instead of using symbolic tags.

Answer

Storing all program logic within a single, large main routine.

Question 5

An operator reports intermittent, random machine stoppages that occur a few times a day without any specific fault code being logged by the PLC. When troubleshooting, which of the following potential causes should be investigated first for such an issue?

Accepted Answer

Loose wiring connections or electrical noise/interference.

Answer

A corrupted PLC firmware.

Answer

An error in a math instruction within the program.

Answer

A faulty HMI screen.

Question 6

In a ladder logic program, a timer instruction (TON) is used to delay a motor from starting for 10 seconds after a start button is pressed. The motor never starts. Online monitoring shows the start button input is energized and the timer's enable (EN) bit is true, but the timer's done (DN) bit never becomes true, even after waiting 30 seconds. What is the most likely programming error?

Accepted Answer

The timer's accumulator value is continuously being reset to zero by another instruction on a different rung.

Answer

The timer's preset value is set to 0.

Answer

The rung containing the timer is being conditionally jumped over.

Answer

The wrong type of timer (e.g., TOF instead of TON) was used.