EETC PLC Programming 3 — Questions and Answers
Question 1: In IEC 61131-3 Structured Text, what is the correct syntax for an IF-THEN-ELSE conditional?
- IF condition THEN action ELSE action END_IF; (Correct answer)
- IF (condition) { action } ELSE { action }
- WHEN condition DO action OTHERWISE action END;
- IF condition: action ELSE: action ENDIF
Correct answer: IF condition THEN action ELSE action END_IF;
IEC 61131-3 Structured Text uses IF...THEN...ELSE...END_IF syntax with a semicolon to terminate the block.
Question 2: A sequencer output (SQO) instruction in a PLC steps through a predefined sequence of output patterns. What advances the sequencer to the next step?
- A rising edge on the control rung (Correct answer)
- The internal timer reaching its preset
- A counter reaching its limit value
- A communication message from the HMI
Correct answer: A rising edge on the control rung
Each false-to-true (rising edge) transition of the SQO instruction's rung condition advances the sequencer to the next output step.
Question 3: What is the purpose of subroutine (JSR/RET) instructions in a PLC program?
- To jump to a fault routine automatically on error
- To call a separate program file and return execution after completion (Correct answer)
- To reset all timers and counters to zero
- To switch between RUN and PROGRAM modes
Correct answer: To call a separate program file and return execution after completion
JSR (Jump to Subroutine) transfers program execution to another routine, and RET (Return) sends it back to the calling point.
Question 4: In Function Block Diagram (FBD) programming, how is data flow between function blocks represented?
- By numbered step transitions
- By horizontal rungs and vertical rails
- By connecting lines (wires) between block inputs and outputs (Correct answer)
- By shared global variable names
Correct answer: By connecting lines (wires) between block inputs and outputs
In FBD, signal lines (wires) graphically connect the output pins of one function block to the input pins of another, showing data flow.
Question 5: When using a MOV (Move) instruction in ladder logic, what happens to the source value after execution?
- It is deleted from the source address
- It is doubled in value
- It remains unchanged in the source address (Correct answer)
- It is inverted and stored at the source
Correct answer: It remains unchanged in the source address
The MOV instruction copies the source value to the destination address; the source value is read but not modified.
Question 6: What is a retentive timer (RTO) most commonly used for in industrial applications?
- Measuring time between two consecutive faults
- Accumulating total machine run-time across multiple power cycles (Correct answer)
- Replacing a standard TON timer in all applications
- Measuring the PLC scan time
Correct answer: Accumulating total machine run-time across multiple power cycles
An RTO timer retains its accumulated value when de-energized, making it ideal for tracking cumulative operating hours across interruptions.
Question 7: In Sequential Function Chart (SFC) programming, what is a 'transition'?
- A special step that runs indefinitely
- A condition that must be TRUE to move from one step to the next (Correct answer)
- An action associated with a parallel branch
- A forced output override command
Correct answer: A condition that must be TRUE to move from one step to the next
A transition in SFC is a Boolean condition; when it evaluates TRUE, execution advances from the preceding step to the following step.
EETC PLC Programming 3
In IEC 61131-3 Structured Text, what is the correct syntax for an IF-THEN-ELSE conditional?