Siemens S7 PLC: Convert Percentage to 4-20mA Analog Output

Overview

On a Siemens SIMATIC S7 PLC, every analog output is written to the process image as a 16-bit integer between 0 and +27648 (unipolar) or -27648 and +27648 (bipolar). A percentage entered on an HMI – for example 80 % – must therefore be converted into that raw integer before it is transferred to the peripheral output word PQW. Once the correct integer sits in PQW, the analog module (SM332, SM334, SM 6ES7532, ET 200SP AQ, etc.) converts the integer to a current on the field terminals:

The 4 mA offset is a property of the analog output channel configuration (not a constant you subtract in code). Once the channel is configured for "4..20 mA" in HW Config / the device configuration, the value 0 in PQW automatically produces 4 mA on the terminals and 27648 produces 20 mA. Your only job in the PLC program is the percent-to-count conversion.

Prerequisites

1. STEP 7 V5.x (for S7-300/400) or TIA Portal V15 or later (for S7-1200/1500) installed and licensed.
2. An analog output module inserted in the hardware configuration. Common catalog numbers: 6ES7332-5HD01-0AB0 (SM 332, 8 AO, 12 bit), 6ES7332-8TF01-0AB0 (SM 332, 8 AO, 16 bit), 6ES7334-0CE01-0AA0 (SM 334), 6ES7532-5HF00-0AB0 (SM 532 for S7-1500), 6ES7135-6HD00-0BA1 (ET 200SP AQ).
3. The analog output channel configured for the current range 4..20 mA (not 0..20 mA, unless you specifically want zero-lifted to start at 0 mA). Without this configuration, value 0 yields 0 mA regardless of code.
4. An HMI tag of data type REAL (0.0 – 100.0) or INT (0 – 100) wired to the PLC.
5. For S7-300/400: the FC106 "UNSCALE" block from the "Standard Library → TI-S7 Converting Blocks" palette, or a copy from the Siemens FAQ linked below.
6. For S7-1200/1500: the system blocks NORM_X (FC in the "Scaling" folder) and SCALE_X – both are IEC 61131-3 native blocks, no library install required.

Analog Output Fundamentals on S7

Siemens analog output modules are essentially 12- to 16-bit DACs driving a current or voltage output stage. The full theory of digital-to-analog conversion is covered in Maxim/Analog Devices' primer "Digital-to-Analog Converters Are a 'Bit' Analog". What the PLC programmer must know is that:

• The value placed in PQW nnn is the binary code the DAC converts to the output current.
• The range 0…+27648 is fixed by Siemens firmware and corresponds exactly to the nominal range of the configured output. The number 27648 = 2^14 × 1.6875 reflects the 14-bit-plus-sign internal resolution that all standard S7 analog modules expose to the user program.
• Out-of-range values (negative for unipolar, > 32511, overflow bit 32767) are clamped or flagged by the module – see the SIMATIC S7-300/400 Standard Software – Converting Blocks FC105/FC106 manual, section 5.

The Core Formula: Percentage to Raw Count

For a 0..100 % input and a 0..27648 unipolar output (configured for 4..20 mA):

RawCount = ROUND( Percentage × 27648 ÷ 100 )

For 80 %: 80 × 27648 / 100 = 22118. Writing 22118 to the configured PQW gives 4 + (22118/27648) × 16 mA = 16.8 mA, which is 80 % of the 16 mA span (4 → 20 mA).

Implementation on S7-300/400 with FC106 UNSCALE

FC106 "UNSCALE" is the canonical block for converting a real engineering value (the percentage) into the integer the output module expects. It is the inverse of FC105 "SCALE", which is used on the input side. The block ships in the STEP 7 standard library; full documentation is in the official FAQ 14431100.

FC106 Pin Assignment

STL Snippet (matches the example in the original question)

// --- Percentage → raw count, then write to PQW 288 ---
      L     "HMI_Percentage"      // REAL, e.g. 80.0
      L     100.0
      /R                             // clamp/normalize: pct/100
      L     27648.0
      *R
      RND                             // round to DINT
      T     "RawCount_INT"

      L     "RawCount_INT"
      T     PQW 288                   // 4..20 mA channel of SM332

If you prefer the FC106 block path, call it once in OB1 / OB35:

CALL "UNSCALE"
     IN     := "HMI_Percentage"      // REAL 0.0..100.0
     HI_LIM := 100.0
     LO_LIM := 0.0
     BIPOLAR:= FALSE
     RET_VAL:= "fc106_err"
     OUT    := PQW 288

Implementation on S7-1200/1500 with NORM_X and SCALE_X

On S7-1200 and S7-1500, FC105/FC106 are deprecated. Siemens replaced them with two IEC blocks: NORM_X (normalizes an engineering value to a 0.0 – 1.0 REAL fraction) and SCALE_X (scales the normalized fraction to the integer output range). The official procedure is in the FAQ 39334504 – Scale integer values to real numbers and vice versa on S7-1200/1500 and the broader 77316903 – Standardize and destandardize analog values in TIA Portal.

NORM_X / SCALE_X Parameters

SCL Implementation (S7-1500)

// "HMI_Percentage" : REAL;   // 0.0 .. 100.0
// "NormValue"      : REAL;
// "RawCount"       : INT;    // 0 .. 27648

IF "HMI_Percentage" < 0.0 THEN
    "HMI_Percentage" := 0.0;
ELSIF "HMI_Percentage" > 100.0 THEN
    "HMI_Percentage" := 100.0;
END_IF;

"NormValue" := NORM_X(MIN := 0.0,
                      MAX := 100.0,
                      VALUE := "HMI_Percentage");

"RawCount" := REAL_TO_INT(SCALE_X(MIN := 0.0,
                                   MAX := 27648.0,
                                   VALUE := "NormValue"));

%QW288 := "RawCount";          // SM 532 / 6ES7532 channel 0

Ladder / FBD Alternative

Place an NORM_X box with MIN=0.0, MAX=100.0, VALUE="HMI_Percentage". Connect its OUT to the VALUE input of a SCALE_X box with MIN=0.0, MAX=27648.0. Connect the SCALE_X OUT to a MOVE box that writes to %QW288. Add a LIMIT (0, 100, "HMI_Percentage") upstream so out-of-range HMI values are clamped, not just sign-flipped.

HMI Configuration Considerations

• Tag data type: REAL (0.0 – 100.0) is the most flexible because both FC106 and NORM_X expect REAL inputs. Integer percentages (0 – 100) work but require a DINT-to-REAL conversion in the PLC.
• IO field limits: configure the HMI input field with min=0.0, max=100.0, scaling 0.0–100.0. This prevents the operator from typing negative or out-of-range values.
• Display formatting: if the operator enters "80 %", the HMI tag value is 80.0 (REAL) or 80 (INT). The conversion is identical, only the data type changes.
• Bar graph / gauge on the HMI side: link to the same tag, configure min=0, max=100. The HMI never sees the raw count, so the operator cannot break the scaling by reading the PQW directly.

Verification and Commissioning Steps

1. Open the hardware configuration and confirm the analog output channel is set to "4..20 mA" (not 0..20 mA). The channel's measuring range is selected via the "Output" drop-down in the module properties; the choice is stored in the SDD of the module, not in the program.
2. Force the HMI tag to 0.0 %; measure the loop current with a calibrated multimeter in series with the load. Expected: 4.000 mA ± module accuracy.
3. Force the HMI tag to 50.0 %; expect 12.000 mA.
4. Force the HMI tag to 100.0 %; expect 20.000 mA.
5. Force 25.0 % and 75.0 % to verify linearity: expect 8.000 mA and 16.000 mA respectively.
6. Monitor PQW 288 in the watch table – the integer must equal ROUND(percentage × 276.48). 80 % should show 22118.
7. Disconnect the field wire and verify the wire-break diagnostic (only available on modules that support it, e.g. SM 332 6ES7332-8TF01-0AB0).

Troubleshooting Matrix

Specifications of Common Siemens Analog Output Modules

All modules above use the same user-side scaling 0..27648, regardless of internal resolution. The extra bits inside the module are used for diagnostics and noise-shaping, not for the user-program interface.

Edge Cases and Field Tips

• Bipolar 0–100 % with offset: If the field device expects -100 %…+100 % and the HMI is 0–100 %, scale the HMI tag to -100…+100 in the PLC, then call FC106 with HI_LIM=100.0, LO_LIM=-100.0, BIPOLAR=TRUE. The result fills -27648…+27648.
• Reversed output (valve close at 100 %): Subtract from 27648: PQW := 27648 - RawCount; or set NORM_X MIN/MAX to 100.0/0.0 so the mapping flips natively.
• Cascaded loops: If the percent input is itself the output of a PID block (e.g. a valve position controller), use the PID's LMN output directly as the engineering value – it is already 0..100 % REAL on S7-300/400 and -100..+100 % on S7-1500 PID_Compact.
• Cold-junction / channel diagnostics: On 16-bit modules, use SFC51 "SSL_PARTIAL_LIST" to read the channel diagnostic bytes if the output does not match the expected current – the module may have raised an "overload" or "wire break" diagnostic that the controller does not propagate automatically.
• Watchdog / CPU STOP behavior: Configure the module's substitute value behavior in the device configuration. If you set "Keep last value", the PQW retains the last integer when the CPU goes to STOP; with "Substitute value" you can pre-define a safe current (e.g. 4 mA).

Frequently Asked Questions