Calculating Min, Max, Average of Two Sensors on S7-1500 in SCL

1. Problem Definition and Engineering Scope

A typical industrial measurement task requires a single statistical summary (minimum, maximum, arithmetic mean) from two redundant process sensors over a defined observation window. The window is fixed at 30 minutes in this reference design, and the controller is a Siemens SIMATIC S7-1500 CPU programmed in Structured Control Language (SCL) under TIA Portal. The two inputs are REAL (32-bit IEEE-754) temperature values in °C, typically delivered by PT100/RTD modules (e.g., SM 1231 on S7-1200 or SM 531 on S7-1500) or by analog input cards with 4–20 mA transmitters.

The required outputs are:

• Q_MIN — minimum value observed in the rolling window
• Q_MAX — maximum value observed in the rolling window
• Q_AV — arithmetic mean of the samples in the rolling window

The design is built as a reusable Function Block (FB) with its own Instance Data Block (IDB), so it can be called from any cyclic OB (typically OB1) or from a time-driven OB such as OB30 (cyclic interrupt) if a deterministic sample period is required.

2. Prerequisites

Confirm the following before coding the FB:

1. Hardware — S7-1500 CPU (any firmware version that supports TIA Portal V15 or later; the SCL syntax used here is compatible with all current S7-1500 firmware). See the SIMATIC S7-1500 product page for the current CPU portfolio.
2. Software — TIA Portal with the SCL compiler installed. Open Project → Properties → Task cards → SCL to verify the package is active.
3. Sensor scaling — the two inputs I_TEMPERATURE_1 and I_TEMPERATURE_2 must already be scaled to engineering units (°C) as REAL. A typical scaling block is SCALE/NORM_X plus SCALE_X from the TIA Portal instruction palette.
4. Time base — a 1-second pulse I_PULSE generated by a clock bit (e.g., the system clock Clock_1Hz in the CPU properties → System and clock memory) or by a cyclic-interrupt OB.
5. Memory budget — a 30-minute window at 1-second sampling requires 1800 REAL samples (1800 × 4 bytes = 7200 bytes) plus a 4-byte pointer. A 30-second window needs only 1800 / 60 = 30 samples for 30 seconds at 1 Hz, so verify your sample period before sizing the array.

3. Algorithm: Rolling Buffer with a Single-Pass Statistic

The implementation uses a circular buffer indexed by POINTER. Each pulse, the oldest sample is overwritten by the newest composite value. After every write, a single FOR loop walks the entire array once, computing min, max, and the running sum in the same pass. This is the same pattern recommended for aggregate functions in general-purpose languages: initialize the accumulators from the first element, then iterate.

The complexity is O(n) per output update and O(1) memory beyond the buffer itself. On an S7-1500 with a 1-second pulse and 1800 elements, the loop runs well inside the cycle-time budget; the OB1 scan is not measurably impacted.

4. Step-by-Step: Build the FB in TIA Portal

4.1 Declare the Block Interface

Create a new Function Block named FB_StatWindow (number range 1000–1999 is conventional). In the Block interface editor, declare the following variables exactly:

4.2 Network 1 — Average the two sensors

// Composite sample: arithmetic mean of the two sensors
Temperature := (I_TEMPERATURE_1 + I_TEMPERATURE_2) / 2.0;

4.3 Network 2 — First-scan initialization

On the very first call (driven from OB100 startup or a one-shot FirstScan tag in the IDB), the array is filled with the current composite so that min/max/mean are well-defined before the first full window elapses.

IF I_FIRST_SCAN THEN
    POINTER := 1;
    // FILL_BLK_INI would also work; a loop keeps the FB self-contained
    FOR i := 1 TO CONST_PERIOD DO
        BUFFER[i] := Temperature;
    END_FOR;
END_IF;

4.4 Network 3 — Gating on the sample pulse

Statistics are only updated on a rising edge of I_PULSE. This keeps the FB idempotent if it is called more often than the sample period.

IF NOT I_PULSE THEN
    RETURN;   // leave Q_MIN / Q_MAX / Q_AV unchanged this scan
END_IF;

4.5 Network 4 — Write the new sample into the ring buffer

POINTER := POINTER + 1;
IF POINTER > CONST_PERIOD OR POINTER < 1 THEN
    POINTER := 1;
END_IF;
BUFFER[POINTER] := Temperature;

4.6 Network 5 — Single-pass statistic

Initialize all three accumulators from BUFFER[1] and walk the rest of the array. This pattern (init from element 0 / 1, then iterate from element 1 / 2) is the standard idiom and is also documented for the Power Fx aggregate functions Average, Max, and Min.

Q_AV  := BUFFER[1];
Q_MIN := BUFFER[1];
Q_MAX := BUFFER[1];

FOR i := 2 TO CONST_PERIOD DO
    Temperature := BUFFER[i];
    Q_AV := Q_AV + Temperature;
    IF Q_MIN > Temperature THEN
        Q_MIN := Temperature;
    END_IF;
    IF Q_MAX < Temperature THEN
        Q_MAX := Temperature;
    END_IF;
END_FOR;

Q_AV := Q_AV / INT_TO_REAL(CONST_PERIOD);

5. Wiring the FB in OB1

Call the FB from the main cyclic OB. FirstScan is the standard first-cycle flag available on every S7-1500 CPU; Clock_1Hz must be enabled in CPU properties → System and clock memory.

// OB1 - Main
"iStatDB"(   // Instance DB auto-generated by TIA Portal
    I_TEMPERATURE_1 := "Raw_Temp_1",
    I_TEMPERATURE_2 := "Raw_Temp_2",
    I_PULSE         := "Clock_1Hz",
    I_FIRST_SCAN    := "FirstScan",
    Q_MIN           => "Stat_MIN",
    Q_MAX           => "Stat_MAX",
    Q_AV            => "Stat_AV"
);

6. Sizing the Window Correctly

The CONST_PERIOD parameter in the example above is set to 30. The unit depends entirely on the pulse rate driving I_PULSE:

For a 30-minute window at 1 Hz, change the constant to CONST_PERIOD : Int := 1800;. The arithmetic-mean divisor must be updated accordingly: Q_AV := Q_AV / INT_TO_REAL(CONST_PERIOD);.

7. LAD Implementation on S7-1200 and S7-1500

The same logic is implementable in LAD if SCL is not available on the target package. The recommended structure is:

1. Add instruction → Math functions → ADD + DIV by 2.0 to compute the per-scan composite (use REAL arithmetic, not INT).
2. Sample-and-hold on pulse — feed the composite into a MOVE_BLK / FILL_BLK chain that updates a DB array indexed by an INC-style counter that wraps at CONST_PERIOD.
3. Min — use the Compare instruction < inside a loop; on the S7-1500 the Comparison group includes a dedicated MIN/MAX instruction that returns the smaller / larger of two REALs.
4. Max — same as min, using the > compare and the MAX instruction.
5. Mean — accumulate into a REAL tag and divide once at the end.

For the loop on S7-1200/1500 LAD, the cleanest approach is to write a small SCL FB and call it from LAD, rather than building a long ladder chain with explicit compares per array element.

8. S7-300 Variant — Memory-Indirect Addressing in STL

On an S7-300 (or any classic S7-300/400 target), the S7-1500-style SCL array syntax is not always available; you typically use STL with memory-indirect addressing. The user requirement is the same: rolling min, max, mean from a 1 Hz sample. The implementation pattern is:

// STL sketch (S7-300, SCL not licensed)
// DBx.DBX0.0  = composite value (REAL, 4 bytes)
// DBx.DBD4    = pointer / index (DINT)
// DBx.DBD100  = sum accumulator (REAL)
// DBx.DBD104  = current min (REAL)
// DBx.DBD108  = current max (REAL)

      L     DBx.DBD4       // pointer
      L     1
      +I
      T     DBx.DBD4
      L     CONST_PERIOD   // e.g. 1800
      >I
      JC    RES
      L     0
      <I
      JC    RES
      JU    CONT
RES:  L     1
      T     DBx.DBD4
CONT: L     DBx.DBD4
      SLD   3              // index * 8 because stride is 8 here
      LAR1  P#DBx.DBX200.0 // base of array
      +AR1
      L     DBx.DBD0       // composite
      T     DBD [AR1,P#0.0]

The scan-loop that recomputes min / max / mean uses a separate counter from 0 to CONST_PERIOD–1 and the same indirect-addressing pattern. For a REAL array the index stride is 4 bytes (or 8 for a paired DINT); for a pure INT array the stride is 2 bytes. The source notes the stride difference explicitly: REAL/DINT addresses change by 4, INT addresses change by 2. Use a DINT accumulator when summing INT to prevent overflow.

9. Verification and Commissioning Steps

1. Static check — in TIA Portal, right-click the FB and select Compile → SCL. Fix any syntax errors. Open the IDB and confirm the BUFFER array size matches CONST_PERIOD.
2. Online watch — go online with the CPU, open the IDB in Monitoring mode, and force I_TEMPERATURE_1 = I_TEMPERATURE_2 = 25.0. Within one scan, Stat_MIN = Stat_MAX = Stat_AV = 25.0.
3. Min/max step test — toggle the inputs to known fixed values (e.g. 20.0 / 30.0 / 22.5 / 27.0) at every pulse, and verify that Q_MIN settles at the lowest value within the window and Q_MAX at the highest.
4. Ring-buffer wrap test — set CONST_PERIOD := 5 temporarily, set a 1 Hz pulse, and feed a saw-tooth pattern. The Q_MAX should equal the largest of the last five samples, and Q_MIN the smallest.
5. Mean sanity check — feed ten identical samples (e.g. all 50.0). Q_AV must equal 50.0 exactly. Then feed samples of 50 and 100 in alternation. After an even number of samples, Q_AV must be 75.0.
6. Cycle-time check — open the CPU diagnostics view and confirm the OB1 cycle time is well below the configured maximum (typically < 50 ms for a small S7-1511 with this block). If the cycle time approaches the limit, raise the constant or move the FB into a slower OB30 with a 100 ms cyclic interrupt.

10. Common Pitfalls and Field-Notes

11. Extensions and Production-Grade Variants

• Median — copy the buffer into a temporary array and run a simple in-place sort; the median is BUFFER[CONST_PERIOD / 2]. Useful when one sensor occasionally produces an outlier.
• Standard deviation — store the running sum of squares Σx² in parallel with the running sum Σx. Then σ = sqrt( (Σx² / n) − (Σx / n)² ). The Power Fx aggregate reference also lists StdevP and VarP for population-based dispersion.
• Per-sensor statistics — duplicate the buffer as BUFFER_1 and BUFFER_2 and run two parallel single-pass loops.
• HMI trend — publish Q_MIN, Q_MAX, Q_AV to an HMI tag and graph them on a WinCC / Unified faceplate for operator visibility.
• Alarm on excursion — if Q_MAX − Q_MIN exceeds a process-defined spread (e.g. > 5 °C for a tank), raise a "sensor drift" alarm to drive maintenance.

12. Frequently Asked Questions