PLC Operations Flashcards

Which PLC addressing mode directly specifies a memory location by its physical address without any calculation?

Answer: Direct addressing

Direct addressing specifies the exact memory location (e.g., I0.0, Q0.1) without any pointer calculation.

In a PLC program, a TON timer's accumulated value (ACC) resets to zero when:

Answer: The enable (EN) bit turns OFF

A TON timer's ACC resets to zero whenever the rung condition (enable input) goes FALSE.

What does the term 'scan time' refer to in PLC operation?

Answer: Time required to complete one full execution cycle of the ladder logic

Scan time is the total time for the PLC to read inputs, execute the program, and update outputs in one cycle.

A CTU (Count Up) counter's output bit (CU) activates when:

Answer: The accumulated value equals the preset value

The CTU done bit turns ON when the accumulated count reaches or exceeds the preset value.

Which communication protocol is most commonly used for high-speed deterministic communication between PLCs and I/O modules on a local network?

Answer: EtherNet/IP or PROFINET

EtherNet/IP and PROFINET are industrial Ethernet protocols designed for deterministic, high-speed PLC-to-I/O communication.

In ladder logic, an Examine If Closed (XIC) instruction represents:

Answer: A normally open contact that triggers when the referenced bit is ON

XIC (–| |–) passes power when the referenced bit is logic 1 (ON), mimicking a normally open contact that closes when energized.

When a PLC's CPU enters FAULT mode, what typically happens to the outputs?

Answer: Outputs are de-energized and go to a safe/off state

Most PLCs de-energize all outputs when a CPU fault occurs to prevent uncontrolled machine operation.