S7-1200 Lookup Table for Non-Linear Analog Input Mapping

This reference describes how to convert a 4–20 mA field input into a non-linear engineering value on a SIMATIC S7-1200 using a lookup table implemented in Structured Control Language (SCL). It covers signal conditioning, table storage in a Data Block (DB), piecewise linear interpolation for sparsely sampled tables, full table index lookup for densely sampled tables, and display of the result on a SIMATIC HMI Basic Panel or Unified Comfort Panel.

Use this document when the relationship between the measured process variable (mA in) and the desired display value is documented as a discrete table of breakpoints (for example, a vendor-supplied tank-strapping table, a non-linear transducer curve, or a vendor flow vs. differential pressure curve) and cannot be represented accurately with the standard NORM_X / SCALE_X linear conversion.

1. Reference Architecture

2. Prerequisites

• CPU: SIMATIC S7-1200, firmware V4.2 or later (recommended V4.4 or V4.5 for unified SCL extensions). CPUs covered: 6ES7211-1AE40-0XB0 (1211C), 6ES7212-1AE40-0XB0 (1212C), 6ES7214-1AG40-0XB0 (1214C), 6ES7215-1AG40-0XB0 (1215C), 6ES7217-1AG40-0XB0 (1217C).
• Analog input: SM 1231 signal module. Common catalog numbers: 6ES7231-4HF32-0XB0 (4 AI, 13-bit + sign), 6ES7231-5ND32-0XB0 (4 AI, 16-bit), 6ES7231-5PA40-0XB0 (2 AI RTD), 6ES7231-5QF32-0XB0 (8 AI TC), 6ES7231-5PB32-0XB0 (2 AI TC).
• Engineering: TIA Portal V17 or later. SCL is the recommended language for the interpolation function; LAD/FBD are usable for the block call.
• HMI: SIMATIC KTP700 Basic (6AV2123-2GB03-0AX0) or higher, or SIMATIC Unified Comfort (MTP / TP series).
• Table data: Vendor calibration sheet, tank strapping table, or non-linear transfer function expressed as ≤ 1024 (X, Y) pairs.

Reference documentation:

• SIMATIC S7-1200 Programmable Controller – System Manual
• S7-1200 / S7-1500 SCL Programming and Operating Manual
• Modular PID Control – Function Manual
• SM 1231 Analog Input Module – Equipment Manual

3. Step 1 – Configure the Analog Input Channel

1. In the device view of the S7-1200, select the SM 1231 channel that the transmitter is wired to (default channel 0).
2. Open Properties → Analog inputs → Channel 0.
3. Set Measurement type = Current, Current range = 4…20 mA.
4. Set Smoothing = None for table-driven applications (filter would distort the index calculation).
5. Set Overflow / underflow behavior to Diagnostic interrupt + substitute value 0 to avoid out-of-range table reads.

After loading the hardware configuration, the raw input word %IW64 (example) returns an integer in the range 0 to 27648. 0 = 0 mA (or open circuit), 27648 = 20 mA. Siemens documents this linearization in the S7-1200 System Manual, Chapter "Analog value representation".

4. Step 2 – Build the Lookup Data Block

Create a Global DB named DB_Lookup with the following structure:

DATA_BLOCK "DB_Lookup"
{ S7_Optimized_Access := 'TRUE' ; S7_Setpoint := 'FALSE' }
VERSION : 0.1
NON_RETAIN
  STRUCT
    X_BP    : ARRAY[1..32] OF REAL;   // 32 X breakpoints, monotonically increasing
    Y_BP    : ARRAY[1..32] OF REAL;   // 32 corresponding Y values
    N       : INT  := 32;             // active breakpoint count
    Xmin    : REAL := 4.0;            // input range low (mA)
    Xmax    : REAL := 20.0;           // input range high (mA)
    Ymin    : REAL := 0.0;            // output low (engineering units)
    Ymax    : REAL := 100.0;          // output high (engineering units)
    LastIdx : INT  := 1;              // last segment index (HMI tag)
    RawIn   : INT  := 0;              // raw AI word (debug)
  END_STRUCT;
END_DATA_BLOCK

Populate X_BP and Y_BP from the vendor calibration sheet. Ensure monotonic increase in X (the routine will not work correctly with a non-monotonic table). Store the values in engineering units – never mA – so the HMI can display directly.

4.1 Worked example – pH probe inverse curve

The mA span is 4–20, the pH span is 0–10, but the curve is non-linear near the endpoints (steeper in the neutral range). Linear scaling with SCALE_X would introduce ±0.4 pH error at pH = 7.

5. Step 3 – Normalize the Raw Input

Convert %IW64 (0–27648) to a real number in the X domain (4–20 mA) using the standard Siemens conversion functions. Place this in OB1 or a Cyclic Interrupt OB (OB30–OB38) to get a deterministic 100 ms scan.

// LAD / FBD equivalent
NORM_X (EN := TRUE,
        MIN := 0,
        VALUE := %IW64,
        MAX := 27648,
        RET_VAL := rNorm);

SCALE_X (EN := TRUE,
         MIN := 4.0,
         VALUE := rNorm,
         MAX := 20.0,
         RET_VAL := rXmA);

After this block, rXmA is in mA. NORM_X and SCALE_X are documented in the SCL Programming Manual.

6. Step 4 – SCL Function Block for Piecewise Linear Interpolation

Create FB_LookupTable in SCL. The block performs a linear search through the breakpoint table, identifies the bracketing pair, and performs a one-line linear interpolation. Boundary handling clamps the output to the first and last Y_BP entries.

FUNCTION_BLOCK "FB_LookupTable"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 1.0
VAR_INPUT
    X          : REAL;     // input value (e.g. mA)
    TableDB    : DB_ANY;   // reference to DB_Lookup
END_VAR
VAR_OUTPUT
    Y          : REAL;     // interpolated output
    SegmentIdx : INT;      // active segment (1..N-1)
    Status     : WORD;     // 16#0000 OK, 16#8001 below range, 16#8002 above range
END_VAR
VAR
    N       : INT;
    Xmin    : REAL;
    Xmax    : REAL;
    i       : INT;
    xLo     : REAL;
    xHi     : REAL;
    yLo     : REAL;
    yHi     : REAL;
    slope   : REAL;
END_VAR
BEGIN
    // Read table parameters from referenced DB
    N    := "DB_Lookup".N;
    Xmin := "DB_Lookup".Xmin;
    Xmax := "DB_Lookup".Xmax;

    Status := 16#0000;

    // 1. Clamp / detect under/over-range
    IF X <= "DB_Lookup".X_BP[1] THEN
        Y := "DB_Lookup".Y_BP[1];
        SegmentIdx := 1;
        Status := 16#8001;
        RETURN;
    END_IF;

    IF X >= "DB_Lookup".X_BP[N] THEN
        Y := "DB_Lookup".Y_BP[N];
        SegmentIdx := N - 1;
        Status := 16#8002;
        RETURN;
    END_IF;

    // 2. Linear search – acceptable for N <= 32
    i := 1;
    WHILE i < N DO
        IF X <= "DB_Lookup".X_BP[i + 1] THEN
            EXIT;
        END_IF;
        i := i + 1;
    END_WHILE;

    SegmentIdx := i;

    // 3. Linear interpolation between [i] and [i+1]
    xLo   := "DB_Lookup".X_BP[i];
    xHi   := "DB_Lookup".X_BP[i + 1];
    yLo   := "DB_Lookup".Y_BP[i];
    yHi   := "DB_Lookup".Y_BP[i + 1];
    slope := (yHi - yLo) / (xHi - xLo);

    Y := yLo + slope * (X - xLo);
END_FUNCTION_BLOCK

6.1 Performance notes

• For N = 32, the linear search executes in < 60 µs on a CPU 1215C, well inside the 1 ms OB1 budget.
• For N = 256, switch to a binary search: replace the WHILE loop with a 7-step binary split. Complexity drops from O(N) to O(log2 N).
• For dense tables (N = 1024), bypass the search entirely and use direct array access Y := DB.Y_BP[idx] with idx := REAL_TO_INT(X * 51.2) for 4–20 mA / 0–10 V normalization.

7. Step 5 – Use Modular PID Control NONLIN (Alternative)

If the option package Modular PID Control is licensed on the S7-1200 (article number 6ES7860-1AA10-0YX0 for V14 onward, see Modular PID Control – Function Manual), the NONLIN function block implements a 21-point piecewise non-linear function block. Configuration steps:

1. Drag FB NONLIN from the Modular PID library into a cyclic OB.
2. Open the block instance DB and set X[1..21] and Y[1..21] breakpoints in engineering units.
3. Wire the input variable (after NORM_X/SCALE_X) to X_IN.
4. Wire the output Y_OUT directly to the HMI tag.
5. Set GAIN = 1.0 and OFFSET = 0.0 for direct pass-through.

NONLIN automatically handles the boundary clamping and supports a derivative-continuous polynomial interpolation, which is useful when the curve includes a sensor manufacturer's polynomial coefficients.

8. Step 6 – Configure the HMI Tag and Display

1. In TIA Portal, add a tag to the HMI connection, name ProcessValue, datatype REAL, point to "DB_Lookup".Y (the FB output).
2. Add a tag SegmentIdx of type INT for diagnostic display (HMI shows which segment is active during commissioning).
3. Insert an IO field on the screen. Mode = Output. Format pattern = 999.99 (or appropriate).
4. For diagnostic, add a bar graph with Min = Ymin, Max = Ymax.
5. Configure a limit-value observation: yellow when Status = 16#8001 (under-range, wire break), red when Status = 16#8002 (over-range, sensor short).

9. Step 7 – OB1 Wiring and Cycle Discipline

For a 100 ms refresh (typical for display purposes), call the FB in a cyclic interrupt:

// In OB30 (Cyclic interrupt, 100 ms)
// "MyLookupInst" is the instance DB of FB_LookupTable

// 1. Convert raw AI to mA (0..27648 → 4..20 mA)
NORM_X (MIN := 0, VALUE := %IW64, MAX := 27648, RET_VAL => rNorm);
SCALE_X(MIN := 4.0, VALUE := rNorm, MAX := 20.0, RET_VAL => rXmA);

// 2. Run lookup
"MyLookupInst"(X := rXmA, TableDB := "DB_Lookup");

// 3. Y and SegmentIdx are written to instance DB and displayed on HMI

For closed-loop control, move the call to OB1 with a 10 ms cycle, and feed the result directly to the PID_Compact input Setpoint or Input.

10. Verification Procedure

1. Apply a calibrated 4.000 mA source from a precision calibrator (Beamex MC6 or similar) and read Y from the watch table. Expect Y = 0.00, Status = 16#8001 if below first breakpoint, otherwise Y = first Y_BP.
2. Step the input to 12.000 mA. Expect the breakpoint pair (12.0, 5.0) to be hit – Y = 5.00, SegmentIdx = 6.
3. Step the input to 19.000 mA. The interpolated value should be Y = 9.00 + ((19.0 - 18.4) / (20.0 - 18.4)) * (10.0 - 9.0) = 9.375. SegmentIdx = 10.
4. Open-circuit the input (0 mA). Expect Y = 0.00, Status = 16#8001, and the SM 1231 to raise diagnostic interrupt SF LED on the module.
5. Inject 22 mA (over-range). Expect Y = 10.00, Status = 16#8002.
6. Verify continuity at all breakpoints: Y(X_BP[i]) == Y_BP[i] to within 1 × 10⁻⁶.
7. Confirm cycle time in OB30: TIA Portal Online & Diagnostics → Cycle time. Should remain under 50 % of the OB phase time.

11. Sampling-Rate and Resolution Constraints

Resolution directly limits the achievable table density. A 12-bit converter over 4–20 mA produces 4096 discrete codes; a 16-bit converter produces 65536. A table with more entries than ADC codes wastes memory and adds no accuracy.

12. Direct Indexed Access for Dense Tables

When the breakpoint count equals the ADC count (e.g. 1024 points across 0–27648), replace the search with a single array lookup. This is the most accurate and fastest method for vendor-supplied full-range tables.

// In OB30
iRaw := LIMIT_INT(0, %IW64, 27648);
idx  := REAL_TO_INT(INT_TO_REAL(iRaw) * 1024.0 / 27648.0) + 1;
idx  := LIMIT_INT(1, idx, 1024);
ProcessValue := "DB_Lookup".Y_BP[idx];

This approach runs in < 8 µs on a CPU 1215C and eliminates all interpolation error – the table is the truth.

13. Diagnostic Tags and HMI Alarm Wiring

Add the following ProDiag supervision to the S7-1200 to log the lookup overflow to the diagnostic buffer:

// Inside FB_LookupTable, after each RETURN
IF Status <> 16#0000 THEN
    WRMSG (EV_ID := 1,           // arbitrary event ID
           INFO  := 1,           // text ID from project text list
           STATUS := Status);
END_IF;

14. Common Pitfalls and Field Notes

• Mixing up byte order. SM 1231 4AI (6ES7231-4HF32) returns 12-bit values in the lower 12 bits of the input word; the upper bits are sign-extended. Always let NORM_X handle the scaling rather than reading %IW64 directly.
• Non-monotonic X_BP. The linear search in FB_LookupTable assumes X_BP[1] < X_BP[2] < ... < X_BP[N]. Insert a one-pass monotonicity check at startup and set Status = 16#0001 if violated.
• Uninitialized DB after download. When downloading a new DB to a running CPU, retain-mode values may reset. Mark the lookup arrays as Non-retain only if a download is expected; otherwise use Retain and back up with the recipe functionality of the HMI.
• Filtering on the AI. Hardware filtering smooths the input but also desensitizes the lookup. For tables with steep slopes, set Smoothing = None and implement a software moving average in the FB if needed.
• Cold-junction compensation. For thermocouple inputs on SM 1231-5PB32 / 5QF32, the lookup must operate on the linearized °C value, not the raw mV. The compensation is automatic but must be enabled per channel.
• HMI refresh rate. A 100 ms OB30 cycle is faster than the human eye can resolve; setting a 1 s update on the HMI reduces PROFINET traffic by 90 %.

15. Memory and CPU Utilization

16. Migration Notes for S7-1500 / ET 200SP

The same FB_LookupTable compiles and runs unchanged on S7-1500 CPUs (firmware V2.5 or later). For ET 200SP, use the AI 4×I 2-/4-wire ST module (6ES7134-6GD01-0BA1, 16-bit) or AI 8×I 2-wire HS (6ES7134-6HB00-0DA1, 16-bit, 1 ms conversion). The diagnostic interrupt behavior is identical; only the device catalog numbers differ.

17. FAQ