For Next Loop

Iterative control structure executing code block specified number of times. This term is essential for understanding ladder logic in industrial automation and PLC programming.

Ladder logic relies on For Next Loop as part of the small set of primitives that combine into every conceivable PLC program. Iterative control structure executing code block specified number of times.

From the PLC's perspective, For Next Loop executes during the program-execution phase of the scan cycle, where each rung is evaluated left-to-right and the corresponding outputs are updated before the controller writes physical I/O. Understanding when For Next Loop executes — and how it interacts with the scan cycle — is the difference between code that works in simulation and code that works on real hardware.

Mastering For Next Loop is part of becoming productive in ladder logic — the language in which roughly 80% of industrial automation is still written. Visual readability is the reason ladder hasn't been displaced; engineers continue choosing it for binary logic and machine sequencing.