PLC Operations Flashcards

A PLC analog input module reads a 4-20 mA signal. A reading of 4 mA typically represents:

Answer: 0% (minimum) of the process range

In a 4-20 mA loop, 4 mA represents the zero/minimum process value and 20 mA represents full scale.

Which PLC memory type retains its data when power is removed WITHOUT a battery backup?

Answer: ROM or Flash EEPROM (non-volatile memory)

ROM and Flash EEPROM are non-volatile memory types that retain data without power or battery backup.

In a PLC network using Modbus RTU, the device that initiates all communication transactions is called the:

Answer: Master

In Modbus RTU, the master device initiates all requests; slave devices only respond when polled by the master.

What is the purpose of a 'watchdog timer' built into a PLC's CPU?

Answer: To reset the CPU if the scan cycle takes longer than a preset maximum time

The watchdog timer detects program hangs by resetting or faulting the CPU if a scan cycle exceeds the allowed maximum time.

During PLC troubleshooting, forcing an output ON while in RUN mode will:

Answer: Energize that output regardless of the rung logic condition

Forcing overrides the program logic and holds the output at the forced state until the force is removed.

What does PID stand for in the context of PLC process control?

Answer: Proportional-Integral-Derivative

PID (Proportional-Integral-Derivative) is a closed-loop control algorithm that minimizes the error between a setpoint and measured process variable.

In Allen-Bradley ControlLogix PLCs, tags are stored in which type of memory organization?

Answer: Tag-based (symbolic) memory structure

ControlLogix uses a tag-based memory structure where engineers assign symbolic names rather than fixed numeric addresses.