PLC Programming Flashcards

In ladder logic, what does a normally closed (NC) contact do when its associated coil is energized?

Answer: Blocks power (opens)

A normally closed contact opens (blocks power flow) when its associated coil or bit is energized.

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

Answer: Direct addressing

Direct addressing specifies a fixed, absolute memory location in the PLC's data table that remains constant throughout execution.

What is the primary function of a PLC's accumulator register in arithmetic operations?

Answer: Holds intermediate results of math operations

The accumulator is a working register that holds intermediate values during arithmetic, comparison, and data manipulation operations.

A TOF (Timer Off-Delay) instruction begins timing when its rung condition:

Answer: Goes from true to false

A TOF timer starts its countdown when the rung condition transitions from true to false (de-energized).

In Allen-Bradley ControlLogix PLCs, which data type is used to store a 32-bit floating-point value?

Answer: REAL

The REAL data type in ControlLogix stores a 32-bit IEEE 754 single-precision floating-point number.

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

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

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

Which instruction is used in ladder logic to compare two values and energize an output only when the first value is greater than the second?

Answer: GRT

The GRT (Greater Than) compare instruction passes power when the value in Source A is strictly greater than the value in Source B.