SACA PLC Programming and Troubleshooting 4 — Questions and Answers
Question 1: In a PLC ladder logic program, what is the function of a TON (Timer On-Delay) instruction?
- It immediately energizes the output when the rung goes true
- It energizes the output after the rung has been continuously true for a preset time (Correct answer)
- It energizes the output when the rung goes false for a preset time
- It counts rising edges on the input rung
Correct 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.
Question 2: Which PLC addressing mode uses a memory location that can be changed during program execution to reference different data table elements?
- Direct addressing
- Indirect addressing (Correct answer)
- Immediate addressing
- Symbolic addressing
Correct answer: Indirect addressing
Indirect addressing uses a pointer stored in memory to reference data table elements, allowing dynamic data access during runtime.
Question 3: A PLC output module is showing a fault LED. Which is the FIRST diagnostic step a technician should take?
- Replace the output module immediately
- Check the field device wiring and verify supply voltage at the module terminals (Correct answer)
- Reload the PLC program from backup
- Reset the CPU and cycle power
Correct 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.
Question 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);
- FALSE
- TRUE (Correct answer)
- 10
- 5
Correct answer: TRUE
Both conditions are true (10 > 5 is TRUE, and 5 ≠ 0 is TRUE), so the AND operation returns TRUE.
Question 5: What is the purpose of a watchdog timer in a PLC system?
- To schedule periodic I/O scans
- To detect if the scan cycle exceeds a maximum allowed time and fault the CPU (Correct answer)
- To synchronize multiple PLCs on a network
- To monitor analog input signal quality
Correct 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.
Question 6: When troubleshooting an intermittent false input signal on a PLC digital input card, which tool is MOST useful for capturing the transient condition?
- Multimeter set to DC voltage
- Data trend logging or a logic analyzer on the input channel (Correct answer)
- Megohmmeter for insulation testing
- Network packet sniffer
Correct 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.
Question 7: In a Function Block Diagram (FBD) program, how is execution order typically determined when multiple blocks are present?
- Alphabetically by block name
- By the data flow connections — outputs of one block feeding inputs of the next (Correct answer)
- By the order in which blocks were placed on the canvas
- Randomly by the runtime scheduler
Correct 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.
SACA PLC Programming and Troubleshooting 4
In a PLC ladder logic program, what is the function of a TON (Timer On-Delay) instruction?