Resolving WinCC Flexible I/O Field Not Displaying Negative

Problem Summary

A 4-20 mA pressure transmitter with a calibrated range of -10 to +10 PSI is wired to a Siemens S7 analog input module and scaled in the PLC using FC105 (SCALE). The scaled real value is displayed in a WinCC Flexible I/O field on a panel, but the HMI behaves as follows:

• At 4 mA (lowest current, should map to -10 PSI) the I/O field shows 0.
• At 20 mA (highest current, should map to +10 PSI) the I/O field shows the correct positive value.
• Negative values are never visible on the HMI even though the PLC tag temporarily shows the correct negative number during simulation.

The issue has been observed on WinCC Flexible 2008 SP3 / SP5 panels in combination with S7-300 / S7-400 CPUs using FC105/FC106 from the Standard Library. The same root cause applies to migrated projects on TIA Portal WinCC Comfort/Advanced when the legacy FC105 block is reused.

Root Cause Analysis

There are two independent failure layers, and both must be fixed to recover the full bipolar display.

Layer 1 — FC105 BIPOLAR input set to 1

The PLC engineer used the Standard Library block FC105 SCALE to convert the raw integer from the analog input card (0 to 27648) into the engineering range -10.0 to +10.0 PSI. The engineer mistakenly set the BIPOLAR input to 1 because the engineering range itself is bipolar (contains negative values).

When BIPOLAR = 1, FC105 expects the raw input integer in the range -27648 to +27648 and treats the half-scale point (0) as -10 PSI. The raw integer 0 (4 mA) therefore produces an output of -10 PSI in floating point, but the sign of the raw value is interpreted differently in the conversion math, and depending on the calling logic the value can clamp to zero or wrap, producing the observed behavior where 4 mA reads as 0 PSI on the HMI and the negative half of the range is never reached.

Layer 2 — WinCC Flexible I/O field format pattern

Even when the PLC tag holds the correct negative floating-point value, the I/O field will not render the minus sign unless the format pattern is signed. A pattern of 9999.9 is unsigned; the leading negative sign is suppressed, and a value of -10.0 is displayed as 10.0 (or as zero if the mask width is exceeded). The pattern must be s9999.9.

FC105 and FC106 Technical Reference

FC105 (SCALE) and FC106 (UNSCALE) are the legacy Siemens Standard Library blocks used on S7-300/S7-400 to linearize an analog raw value. They are documented in the Siemens Online Help entry Standard Library > TI-S7 Converting Blocks and the function manual S7-300/400 Standard and System Functions.

FC105 interface

FC106 interface

FC106 performs the inverse operation. It is used when writing a real engineering value to an analog output.

FC105 scaling equation

For BIPOLAR = 0 (unipolar input 0 to 27648):

For BIPOLAR = 1 (bipolar input -27648 to +27648):

If the bipolar equation is applied to a unipolar 0-27648 input, the midpoint (0 → -10 PSI) shifts and the low end clips to 0, which exactly matches the field symptom.

Step-by-Step Resolution

1. Open the S7 program (STEP 7 V5.5 or TIA Portal) and locate the call instance of FC105 that drives the pressure I/O field.
2. Change the BIPOLAR input constant from TRUE/1 to FALSE/0. Leave LO_LIM = -10.0 and HI_LIM = 10.0 unchanged.
3. Compile and download the program to the CPU. Confirm that the FC105 call instance now shows BIPOLAR = 0 in online view.
4. Open the WinCC Flexible project and locate the I/O field on the pressure screen.
5. In the I/O field Properties > General, set:
   • Tag: the same DB/real tag that receives FC105.OUT (e.g. DB100.DBD0 REAL).
   • Mode: Output (or Input/Output if the operator may override).
   • Format Type: Decimal
   • Format Pattern: s9999.9
   • Decimal Places Displayed: 1
6. Compile and transfer the WinCC Flexible project to the panel.
7. Force the analog input to 4 mA with a calibrated current source and verify the I/O field shows -10.0.
8. Force the input to 20 mA and verify the I/O field shows +10.0.
9. Force the input to 12 mA (mid-scale) and verify the I/O field shows 0.0.

Verification Procedure

After making the two changes, perform a four-point calibration check on the loop with a precision current calibrator (Fluke 754, Beamex MC6, or equivalent with ±0.025 % accuracy on current source):

Use the S7 Monitor/Modify tool to confirm the OUT real value of FC105 updates in real time. If the HMI field is correct but the PLC tag is wrong, the problem is upstream of FC105. If the PLC tag is correct but the HMI field is wrong, the problem is the HMI tag connection or the format pattern.

PLC Tag Verification

Open the tag list inside the WinCC Flexible project (Project > Communication > Tags) and confirm:

• The tag type is set to 32-bit IEEE float, not Integer or 16-bit.
• The PLC address points to the correct DB and offset (e.g. DB100.DBD0 for the FC105 output).
• The Update cycle is appropriate (default 1 s is fine for pressure).
• The Acquisition mode is Cyclic continuous, not On demand.

To isolate the HMI rendering from the PLC value, temporarily add a second I/O field bound to a constant tag with a hard-coded -10.0. If the second field still drops the sign, the format pattern is the issue and not FC105.

WinCC Flexible I/O Field Format Pattern Reference

WinCC Flexible format patterns are described in the WinCC Flexible Online Help > Configuring Screens > Working with I/O Fields > Format Pattern section. The leading s denotes a signed mask, allowing leading minus characters. Mask width must be wide enough to hold the longest expected negative value, including the minus sign.

Analog Input Module Wiring and Signal Conditioning

Verify the hardware layer before assuming software misconfiguration.

• Confirm the analog input module is configured for 4-20 mA, not 0-20 mA or ±10 V. For S7-300 SM331 (6ES7331-7KF02-0AB0 and similar), use STEP 7 HW Config > Properties > Inputs > Measurement type = Current (4-wire transducer) or Current (2-wire transducer) as required by the transmitter.
• Wire a 2-wire loop transmitter with the module's 24 V sensor supply, or a 4-wire transmitter from an external 24 VDC source, with the signal return on the module's M terminal.
• Check the unused channels of the same module are configured as deactivated or shorted, otherwise a floating input can inject noise that biases the scaled value.
• On SM331, set the measuring-range module (the small red jumper block on the side of the module) to the D position for 4-20 mA. This is the most common field error and produces raw values that do not start at zero.

Migration to TIA Portal / S7-1200 / S7-1500

On S7-1200 and S7-1500, FC105/FC106 are not available in the same form. The equivalent instructions are NORM_X and SCALE_X in the Convert folder of the Instructions task card. The conceptual flag is replaced by the type input of SCALE_X (BOOL: 0 = unipolar INTEGER / 1 = bipolar INT). For floating-point scaling, use SCALE_X with INT tag and a normalized MIN/MAX in the range 0.0 to 1.0 (unipolar) or -1.0 to +1.0 (bipolar). For an analog input raw 0-27648 mapped to engineering -10.0 to +10.0 on S7-1500:

// LAD: SCALE_X (EN, MIN, MAX, VALUE, RET, OUT)
// VALUE = %IW96 (raw 0-27648)
// MIN   = -10.0  (REAL at MIN of input 0)
// MAX   = +10.0  (REAL at MAX of input 27648)
// OUT   = "Pressure_PSI"  (REAL)

The same FC105-style BIPOLAR pitfall exists: if the engineer treats the engineering range sign as the input sign, the same zero-clip behavior is observed.

Field Commissioning Checklist

1. Confirm transmitter calibration certificate matches -10 to +10 PSI at 4-20 mA.
2. Loop-check the mA signal at the transmitter terminals with a clamp-on or in-line mA meter.
3. Verify the measuring-range jumper on SM331 is in the correct position.
4. In STEP 7 HW Config, set the analog input channel to 4-20 mA, Integration time = 20 ms (or 16.67 ms if line frequency is 60 Hz), Smoothing = none for first test.
5. Compile and download the STEP 7 program with FC105 BIPOLAR = 0.
6. Compile and transfer the WinCC Flexible project with format pattern s9999.9.
7. Apply 4 / 8 / 12 / 16 / 20 mA and record the HMI display at each point.
8. Enable FC105 RET word monitoring — values other than 0 indicate scaling errors (e.g. limits inverted, IN out of range).
9. Document the final tag address, scaling constants, and format pattern in the project functional specification.

Troubleshooting Matrix

Common Pitfalls and Field-Proven Caveats

• Reusing an FC105 instance from another project: the BIPOLAR constant may have been compiled for a different signal type. Always re-verify the constant after copy/paste.
• On the 4 mA low end, the raw integer is 0, not 1. Some transmitters with a 3.6 mA low-rail-health fault sit at 553 raw; this is normal and does not require scaling changes.
• If the transmitter is 4-20 mA but the HMI range needs to include a 0-only deadband, do not bias the scaling — use a separate alarm tag and keep FC105 mathematically clean.
• WinCC Flexible 2008 SP3 has a known issue where adding a s to a format pattern does not always trigger a recompile warning. Force a full project recompile (Project > Compiler > Rebuild All) after the change.
• On OP177B and TP170A panels, the I/O field only supports the legacy format patterns (no s). Migrate the panel to a TP177B or KTP series if signed display is required.

Related Siemens Documentation

For deeper background, consult the following official Siemens documents:

• Siemens Online Help: S7-300/400 Standard and System Functions Reference Manual — sections on FC105 SCALE and FC106 UNSCALE.
• Siemens Application Note: Analog value processing for S7-300/S7-400 (entry ID 23906620 on the Siemens Industry Online Support portal).
• Siemens Industry Online Support — search for "FC105 bipolar" to find migration notes to NORM_X / SCALE_X.
• WinCC Flexible 2008 SP5 Online Help: I/O field > Properties > Format.

FAQ