LIFO Unload

Pops most recent data from LIFO stack when triggered. This term is essential for understanding ladder logic in industrial automation and PLC programming.

Ladder logic relies on LIFO Unload as part of the small set of primitives that combine into every conceivable PLC program. Pops most recent data from LIFO stack when triggered.

From the PLC's perspective, LIFO Unload 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 LIFO Unload 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.

In a multi-vendor world, LIFO Unload appears with subtle syntactic differences across Siemens TIA Portal, Rockwell Studio 5000, Mitsubishi GX Works, and Codesys-based IDEs — the IEC 61131-3 standard governs the concept, but vendors layer their own conventions on top.