Which statement accurately describes how a PLC determines the status of each rung in its ladder logic program?

Which statement accurately describes how a PLC determines the status of each rung in its ladder logic program?

• A. The controller uses the status of input and output tags to determine rung status.
• B. The controller uses the statuses of internal timers to determine rung status.
• C. The controller uses the operator's manual to determine rung status.
• D. The controller uses the processor's clock to determine rung status.

Answer: (A)

In ladder logic, the PLC determines the status of a rung during each program scan by evaluating the current on/off states of the tags involved in that rung. These tags come from physical inputs and from internal memory bits that may be used as conditions in the rung. If the combined conditions are true, the rung energizes its associated coil, setting the coil’s tag to true for use by other rungs in the next scan. The important point is that the rung’s truth value depends on the real-time status of input points and any internal or feedback tags used in the rung, and the coil’s tag is then updated to reflect that result. Timers, counters, or clocks influence rung results only through their own internal statuses and timing logic—not as a universal signal that determines rung status by itself. The operator’s manual isn’t involved in automatic evaluation. Outputs reflect the coil state after a rung is evaluated, and in some designs an output tag can be referenced by other rungs as a condition, which is why input and output tag statuses together describe how rung status is determined.

In ladder logic, the PLC determines the status of a rung during each program scan by evaluating the current on/off states of the tags involved in that rung. These tags come from physical inputs and from internal memory bits that may be used as conditions in the rung. If the combined conditions are true, the rung energizes its associated coil, setting the coil’s tag to true for use by other rungs in the next scan. The important point is that the rung’s truth value depends on the real-time status of input points and any internal or feedback tags used in the rung, and the coil’s tag is then updated to reflect that result.

Timers, counters, or clocks influence rung results only through their own internal statuses and timing logic—not as a universal signal that determines rung status by itself. The operator’s manual isn’t involved in automatic evaluation. Outputs reflect the coil state after a rung is evaluated, and in some designs an output tag can be referenced by other rungs as a condition, which is why input and output tag statuses together describe how rung status is determined.