Glossary · TON
What is On-delay timer (TON)?
A standard function block whose output Q becomes true after IN has stayed true for PT without interruption, and resets as soon as IN drops.
Its most common industrial use is de-chattering alarms: when a level hovers around its trip point, a two-second delay stops dozens of alarms, which is what ISA-18.2 recommends.
The ET output gives the elapsed time, used for a progress bar or to detect a step timeout in a state machine.
Related terms
- Off-delay timer (TOF)
- A block whose output Q stays true for PT after IN goes false, then drops.
- Function block (FB)
- A unit with inputs, outputs and internal memory that persists between scans, used by declaring an instance of it.
- Alarm deadband
- A gap between the value that raises an alarm and the value that clears it, so the alarm does not chatter around the trip point.