Limit Test

Tests if value is within or outside specified range defined by low and high limits. This term is essential for understanding ladder logic in industrial automation and PLC programming.

Ladder logic relies on Limit Test as part of the small set of primitives that combine into every conceivable PLC program. Tests if value is within or outside specified range defined by low and high limits.

Limit Test is evaluated as part of the PLC's deterministic scan: inputs are sampled, ladder rungs run in sequence, outputs are written, and the process repeats. Limit Test sits inside that program-execution phase, where its specific logical or temporal behaviour shapes how the rung resolves.

For ladder programmers, Limit Test is one of the components you reach for instinctively after enough rungs. The visual nature of ladder makes it readable to electricians and operators in a way that text-based languages aren't, which matters in environments where maintenance happens at 2 AM by whoever is on call.