S5 Timer Calculator

Why This Calculator?

If you have an old Siemens S5 program in front of you and need to migrate it to S7, the timers are almost always the first stumbling block. S5 uses the proprietary KT format with BCD encoding and four different time bases. S7 instead uses IEC timers (TON, TOF, TP) with the TIME data type (T#30s). There is no automatic 1:1 conversion between these two worlds — every single timer must be translated individually.

This calculator handles exactly that conversion. Enter a KT value and instantly receive: the time value in seconds, the IEC equivalent in T# format, the matching SCL call for TIA Portal, and the S7 STL function call. The calculator also works in reverse: enter a time in seconds and receive all possible S5 KT variants with the best available resolution.

How Is the S5 KT Format Structured?

An S5 timer value consists of two parts: a time value (BCD-encoded) and a time base. The notation is always KT [value].[time base]. In memory, it is stored as a 16-bit word — shown here for KT 030.2 (= 30 seconds):

The upper two bits (15–14) are unused. Bits 13–12 contain the time base as a 2-bit value (here 10 binary = 2, corresponding to 1 second resolution). The remaining 12 bits encode the numeric value as three BCD nibbles: 0000 (hundreds) / 0011 (tens = 3) / 0000 (ones = 0).

S5 was developed in 1979, when displays and inputs were often directly BCD-encoded. For S7 this is irrelevant — IEC timers use the binary TIME data type in milliseconds. This architectural decision from the 70s is the only reason why the conversion is so error-prone today.

Which Timer Type to Use?

The S5 → IEC mapping is not trivial because two S5 types are each mapped to one IEC type. As a rule of thumb for migration:

• SI → TPSimple pulse, resettable via reset input.
• SE → TPIdentical to SI in IEC semantics. The S5 behavior of extending on a new edge is built into TP.
• SD → TONThe standard on-delay. 90 % of all S5 timers in practice are SD.
• SS → TON with flag logicTON resets when the input drops. SS retained the elapsed time in S5. Solution: TON with an additional flag that stores the state — or the IEC block TONR in S7-1500.
• SA → TOFOff-delay, direct 1:1 conversion.

Real-World Conversion Examples

These values are taken from actual S5 programs and show the typical patterns found in legacy plants:

      U     E 1.0
      L     KT 030.2
      SD    T 1
      U     T 1
      =     A 4.0

"Timer_Conveyor"(
    IN := "Start_Button",
    PT := T#30s
);
"Motor_Conveyor" := "Timer_Conveyor".Q;

      U     E 0.5
      L     KT 020.1
      SD    T 5

"Timer_Debounce"(
    IN := "Input_Switch",
    PT := T#2s
);

      U     E 2.0
      L     KT 050.0
      SE    T 10

"Timer_Pulse"(
    IN := "Start_Condition",
    PT := T#500ms
);

      UN    E 3.0
      L     KT 600.3
      SA    T 20

"Timer_Cooldown"(
    IN := NOT "Stop_Signal",
    PT := T#1h40m
);

      L     KT 999.3
      SD    T 100

"Timer_Long"(
    IN := "Start_Condition",
    PT := T#2h46m30s
);

Common Mistakes in Timer Migration

From analyzing hundreds of S5 programs, these are the five mistakes that keep appearing during migration: