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PLC Programming and Troubleshooting Flashcards

7 cards from real SACA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 PLC Programming and Troubleshooting flashcards as text

  1. In a PLC ladder logic program, what is the function of a TON (Timer On-Delay) instruction?

    Answer: It energizes the output after the rung has been continuously true for a preset time

    A TON timer starts accumulating time when the rung goes true and energizes its done bit only after the accumulated time reaches the preset value.

  2. Which PLC addressing mode uses a memory location that can be changed during program execution to reference different data table elements?

    Answer: Indirect addressing

    Indirect addressing uses a pointer stored in memory to reference data table elements, allowing dynamic data access during runtime.

  3. A PLC output module is showing a fault LED. Which is the FIRST diagnostic step a technician should take?

    Answer: Check the field device wiring and verify supply voltage at the module terminals

    Verifying supply voltage and field wiring at the output module terminals rules out external causes before replacing internal components.

  4. In Structured Text (ST) programming, what does the following expression evaluate to if VarA = 10 and VarB = 5? Result := (VarA > VarB) AND (VarB <> 0);

    Answer: TRUE

    Both conditions are true (10 > 5 is TRUE, and 5 ≠ 0 is TRUE), so the AND operation returns TRUE.

  5. What is the purpose of a watchdog timer in a PLC system?

    Answer: To detect if the scan cycle exceeds a maximum allowed time and fault the CPU

    The watchdog timer faults the PLC CPU if the program scan does not complete within the configured maximum scan time, preventing runaway or stuck programs.

  6. When troubleshooting an intermittent false input signal on a PLC digital input card, which tool is MOST useful for capturing the transient condition?

    Answer: Data trend logging or a logic analyzer on the input channel

    Data trending or a logic analyzer captures time-stamped state changes, allowing you to detect and analyze brief intermittent faults that a multimeter would miss.

  7. In a Function Block Diagram (FBD) program, how is execution order typically determined when multiple blocks are present?

    Answer: By the data flow connections — outputs of one block feeding inputs of the next

    FBD execution follows data dependency — a block executes only after all blocks providing its input data have completed, establishing a data-flow-driven execution order.