PLC Operations Flashcards

A TOFF (Timer Off Delay) timer's output bit remains ON for the preset time after:

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.

Which PLC I/O module type would you use to control a variable-speed drive (VFD) with a 0-10 VDC speed reference signal?

Answer: Analog output module

An analog output module produces a variable voltage or current signal (like 0-10 VDC) to command VFD speed.

In ladder logic, what is the function of an Output Latch (OTL) instruction?

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.

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:

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.

In PROFIBUS DP, the 'DP' stands for:

Answer: Decentralized Periphery

PROFIBUS DP (Decentralized Periphery) is designed for fast, cyclic communication between a master PLC and distributed field I/O devices.

Which instruction in ladder logic is used to move a constant value or the content of one register into another register?

Answer: MOV (Move)

The MOV instruction copies a source value (constant or register) into a destination register without modifying the source.

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?

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.