S5 Timer Types to S7 Equivalents: SI/SE/SD/SS/SA → TON/TOF/TP

Complete Comparison Table#

Behavior Summary#

SI (Pulse) → TP (approximate)#

• SI: Output TRUE while input TRUE, max for set time. Goes FALSE if input goes FALSE early.
• TP: Output TRUE for set time regardless of input. Not cut short.
• Migration: Use S_PULSE for exact behavior. Use TP only if the "cut short" behavior is not needed.

SE (Extended Pulse) → TP (approximate)#

• SE: Output TRUE for full set time. Retriggerable — new edge restarts timer.
• TP: Output TRUE for full set time. NOT retriggerable.
• Migration: Use S_PEXT for exact behavior. If retrigger is not used in the program, TP works.

SD (On-Delay) → TON ✅#

• Exact 1:1 match. Both start timing when input goes TRUE, output goes TRUE after delay, both reset when input goes FALSE.
• Migration: Direct replacement. SD → TON.

SS (Retentive On-Delay) → No IEC equivalent ❌#

• SS: Starts timing on rising edge. Continues even if input goes FALSE. Output latches TRUE. Must be reset with R.
• TONR (common misconception): TONR accumulates time across multiple input pulses. SS does NOT accumulate — it starts once and runs to completion.
• Migration: Use S_ODTS for exact behavior. Or implement with TON + SR flip-flop in SCL.

SA (Off-Delay) → TOF ✅#

• Exact 1:1 match. Output goes TRUE immediately when input goes TRUE. When input goes FALSE, output stays TRUE for set time.
• Migration: Direct replacement. SA → TOF.

Migration Decision Tree#

Which S5 timer? ──→ SD? ──→ Use TON (exact match)
                 ──→ SA? ──→ Use TOF (exact match)
                 ──→ SI? ──→ Does input go FALSE before timer expires?
                              ──→ Yes, and that matters ──→ Use S_PULSE
                              ──→ No / doesn't matter ──→ Use TP
                 ──→ SE? ──→ Is retrigger used?
                              ──→ Yes ──→ Use S_PEXT
                              ──→ No ──→ Use TP
                 ──→ SS? ──→ Use S_ODTS (or TON + SR in SCL)

S7 Code Examples#

SD → TON (SCL):

TON_Instance(IN := StartCondition, PT := T#5s);
Output := TON_Instance.Q;

SA → TOF (SCL):

TOF_Instance(IN := RunSignal, PT := T#10s);
FanOutput := TOF_Instance.Q;  // Fan runs 10s after motor stops

For individual timer command details, see: SI | SE | SD | SS | SA

Part of the S5→S7 Timer Reference. Maintained by PLCcheck.ai.