ITMC PLC Operations 4 — Questions and Answers
Question 1: A TOFF (Timer Off Delay) timer's output bit remains ON for the preset time after:
- The rung condition becomes TRUE
- The rung condition transitions from TRUE to FALSE (Correct answer)
- The accumulated value reaches zero
- Power is first applied to the PLC
Correct answer: The rung condition transitions from TRUE to FALSE
A TOFF timer starts timing when its enable input goes FALSE, keeping the output ON until the preset delay expires.
Question 2: Which PLC I/O module type would you use to control a variable-speed drive (VFD) with a 0-10 VDC speed reference signal?
- Digital output module (relay type)
- Analog output module (Correct answer)
- High-speed counter module
- Digital input module
Correct answer: Analog output module
An analog output module produces a variable voltage or current signal (like 0-10 VDC) to command VFD speed.
Question 3: In ladder logic, what is the function of an Output Latch (OTL) instruction?
- It energizes an output coil only while the rung condition is TRUE
- It sets a bit ON and holds it ON even after the rung goes FALSE (Correct answer)
- It compares two values and outputs the larger one
- It counts false-to-true transitions on the rung
Correct answer: It sets a bit ON and holds it ON even after the rung goes FALSE
OTL (latch) sets a bit to 1 and it stays 1 regardless of rung logic until an OTU (unlatch) instruction resets it.
Question 4: When troubleshooting a PLC digital input that shows ON in software but the field device is known to be OFF, the FIRST check should be:
- Replace the input module immediately
- Verify wiring polarity and check for voltage leakage or a stuck contact (Correct answer)
- Reload the PLC program from backup
- Upgrade the PLC firmware
Correct answer: Verify wiring polarity and check for voltage leakage or a stuck contact
Wiring issues, leakage current, or stuck field contacts can falsely energize a digital input—verify these before hardware replacement.
Question 5: In PROFIBUS DP, the 'DP' stands for:
- Digital Processing
- Decentralized Periphery (Correct answer)
- Dual Protocol
- Data Port
Correct answer: Decentralized Periphery
PROFIBUS DP (Decentralized Periphery) is designed for fast, cyclic communication between a master PLC and distributed field I/O devices.
Question 6: Which instruction in ladder logic is used to move a constant value or the content of one register into another register?
- ADD
- MOV (Move) (Correct answer)
- CMP (Compare)
- TON (Timer On Delay)
Correct answer: MOV (Move)
The MOV instruction copies a source value (constant or register) into a destination register without modifying the source.
Question 7: A PLC program uses a one-shot rising (OSR) instruction. How many times will the OSR output bit turn ON per false-to-true transition of the rung?
- Continuously, as long as the rung stays TRUE
- Exactly once per false-to-true transition (Correct answer)
- Twice—once on the rising edge and once on the falling edge
- Zero times—OSR only detects falling edges
Correct answer: Exactly once per false-to-true transition
An OSR produces exactly one scan's worth of ON time per rising edge, preventing repeated triggering while the rung stays TRUE.
ITMC PLC Operations 4
A TOFF (Timer Off Delay) timer's output bit remains ON for the preset time after: