PLC Timer Calculator
Calculate timer preset values and actual delay for PLC on-delay, off-delay, and pulse timers.
timer
PLC
on-delay
off-delay
pulse
preset
scan time
TON
TOF
TP
Input values
⚙
Enter values and click Calculate to see results here.
Formula
Actual delay = Preset × Time base
Preset = Desired delay / Time base
Actual delay ≈ Preset × Time base + Scan time
Worked Example
Given: You need a 2.5 second on-delay timer. The PLC scan time is 20 ms. Time base is 10 ms.
Step 1: Preset = 2500 / 10 = 250
Step 2: Actual delay = 250 × 10 + 20 = 2520 ms = 2.52 s
Error: 20 ms (0.8%). For critical timing, account for scan time. Reduce scan time or use interrupt-based timing.
Engineering Notes
- Time base: Common values: 1 ms, 10 ms, 100 ms. Smaller time base = higher resolution but more processing load.
- Scan time impact: PLC timers update once per scan. For a 20 ms scan, the actual delay is always at least 20 ms longer than requested.
- TON (on-delay): Output turns ON after input has been ON for the preset time. Used for startup delays and debouncing.
- TOF (off-delay): Output stays ON for the preset time after input turns OFF. Used for fan run-on and warning timers.
- TP (pulse): Output turns ON for a fixed duration regardless of input. Used for pump priming and solenoid pulses.
- Retentive timers (TONR): Accumulate time even when input goes false. Use for totalizing elapsed time.
References
- IEC 61131-3 — Timer functions (TON, TOF, TP)
- Allen-Bradley TON/TOF/TP instruction reference