PLC Programming and Troubleshooting Flashcards

A PLC program uses a CTU (Count Up) counter with a preset of 100. After the counter reaches 100, the done bit turns ON. What happens to the accumulated count if additional pulses continue to arrive?

Answer: The accumulated count continues to increment beyond the preset value

Most PLC CTU counters continue to accumulate beyond the preset value; the done bit remains ON but the count keeps incrementing until a reset instruction clears it.

Which communication protocol is commonly used for deterministic, real-time control communication between a PLC and servo drives on a motion control network?

Answer: EtherCAT or PROFINET IRT

EtherCAT and PROFINET IRT provide isochronous, deterministic communication with sub-millisecond cycle times required for precise motion control.

In Allen-Bradley ControlLogix terminology, what is a 'task' and how does it affect program execution?

Answer: A task is a scheduling mechanism that determines when and how often programs within it are executed

In ControlLogix, a task (continuous, periodic, or event) defines the execution schedule for the programs assigned to it, controlling scan rates and priorities.

A technician notices that a PLC analog input is reading 0 mA on a 4-20 mA loop when the transmitter is powered and the field device is operational. What is the MOST likely cause?

Answer: An open circuit in the field wiring loop

A 4-20 mA loop reading 0 mA indicates an open circuit, since even a failed transmitter in a live loop would typically output at least 3.6 mA as a fault signal.

What does the IEC 61131-3 standard define?

Answer: Five standard programming languages for programmable controllers

IEC 61131-3 is the international standard that defines five PLC programming languages: Ladder Diagram, Structured Text, Function Block Diagram, Instruction List, and Sequential Function Chart.

When using Sequential Function Chart (SFC) programming, what must be true before a transition fires and the next step becomes active?

Answer: The current step must have been active for at least one scan cycle and the transition condition must be TRUE

An SFC transition fires when the preceding step is active (for at least one scan) and the transition condition evaluates to TRUE, then deactivates the current step and activates the next.

A PLC program experiences a major fault with error code indicating 'stack overflow.' What programming condition MOST likely caused this?

Answer: Excessive nested subroutine calls or a recursive call chain that exceeded the call stack depth

A stack overflow fault occurs when subroutine calls are nested too deeply or a recursive loop is created, exhausting the CPU's call stack memory.