Programmable Logic Controllers Flashcards

Which PLC addressing mode uses a fixed memory location that does not change during program execution?

Answer: Direct addressing

Direct addressing specifies an exact, fixed memory location in the PLC program that remains constant during execution.

In a PLC ladder diagram, what does a normally closed (NC) contact represent in terms of the physical input signal?

Answer: The contact passes current when the input coil is de-energized

A normally closed contact in ladder logic passes current (is true) when its associated coil or input is de-energized or off.

What is the primary purpose of a PLC's watchdog timer?

Answer: To detect if the scan cycle exceeds a maximum allowed time and trigger a fault

The watchdog timer monitors the PLC scan cycle duration and triggers a fault or system reset if the cycle time exceeds a preset maximum, preventing runaway programs.

Which IEC 61131-3 programming language uses graphical blocks connected by signal lines to represent program logic?

Answer: Function Block Diagram (FBD)

Function Block Diagram (FBD) uses interconnected graphical function blocks and signal lines to represent control logic.

A PLC analog input module receives a 4–20 mA signal from a pressure transmitter. What does a 4 mA signal typically represent?

Answer: 0% of the process range (minimum span)

In a 4–20 mA loop, 4 mA corresponds to 0% (minimum) of the process variable range, and 20 mA corresponds to 100%.

What type of PLC instruction is used to move data from one memory register to another?

Answer: MOV (Move)

The MOV (Move) instruction copies the value from a source register to a destination register in PLC memory.

In PLC network communications, what does the term 'master-slave' topology refer to?

Answer: One device initiates all communications and others respond only when polled

In a master-slave topology, the master device controls the network by polling slave devices, which only respond when addressed.