PLC timer and counter symbols: TON, TOF, TP, CTU, CTD
Timers and counters are function blocks: a box with named inputs on the left and outputs on the right, placed on the rung like a coil or between contacts.
SymbolIEC 61131-3What it is
TON on-delay timerAn on-delay timer: when IN goes true it starts timing, and Q goes true once the elapsed time ET reaches the preset PT.
TOF off-delay timerAn off-delay timer: Q goes true as soon as IN is true, and stays true for the preset PT after IN goes false.
TP pulse timerA pulse timer: on a rising edge of IN, Q goes true for exactly the preset PT, however long IN stays on.
CTU up counterAn up counter: each rising edge on CU adds one to the count CV, and Q goes true when CV reaches the preset PV.
CTD down counterA down counter: LD loads the preset PV into the count CV, each rising edge on CD takes one off, and Q goes true when CV reaches zero.
Timers and counters
IEC 61131-3 defines TON (on delay), TOF (off delay) and TP (pulse) timers, and CTU, CTD and CTUD counters. Allen-Bradley uses TON, TOF and RTO (retentive) timers and CTU and CTD counters, with a preset and an accumulated value shown inside the instruction box.
Frequently asked questions
Which timers and counters appear on a ladder diagram?
This legend covers TON on-delay timer, TOF off-delay timer, TP pulse timer, CTU up counter, CTD down counter. Project legends may add more.
Can I download these symbols?
Yes. Each symbol page has free SVG files, and the hub has printable PDF charts.
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