Immediate Input

Reads physical input immediately instead of waiting for input scan phase. This term is essential for understanding ladder logic in industrial automation and PLC programming.

In ladder logic — the most widely used PLC programming language and the foundation that the IEC 61131-3 standard formalises — Immediate Input is one of the building blocks that turns electrical-engineering reasoning into executable PLC code. Reads physical input immediately instead of waiting for input scan phase.

From the PLC's perspective, Immediate Input 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 Immediate Input 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.

For ladder programmers, Immediate Input 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.