CTUD (Up/Down Counter)

Bidirectional counter with separate count up and count down inputs. This term is essential for understanding ladder logic in industrial automation and PLC programming.

Ladder logic relies on CTUD (Up/Down Counter) as part of the small set of primitives that combine into every conceivable PLC program. Bidirectional counter with separate count up and count down inputs.

From the PLC's perspective, CTUD (Up/Down Counter) 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 CTUD (Up/Down Counter) 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, CTUD (Up/Down Counter) 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.