Reversing 1-10V Analog Input on Siemens S7-1200 with NORM_X

Overview

Many industrial sensors present their output in an inverted manner when paired with a 4–20 mA loop resistor and 0–10 V analog input. A common case is the VEGA VEGAPULS WL 61 ultrasonic level sensor: at 0 m measuring distance the output sits near 9.95 V, and the voltage falls as the level rises. The PLC does not work in volts — it works in raw integer counts — so the engineer must rescale and mathematically reverse the relationship between the raw count and the engineering unit (meters or millimeters).

This article walks through three production-ready methods for the Siemens SIMATIC S7-1212C AC/DC/Rly using the default SM 1231 analog input signal board (or signal module) and TIA Portal ladder logic:

1. Direct integer inversion by subtracting from 27648.
2. Two-step NORM_X + SCALE_X rescaling with sign inversion.
3. Single CALCULATE block using the linear equation y = mx + b.

The recommended final stage depends on whether you need only the polarity inverted (Method 1 or 2) or both inverted and rescaled to engineering units (Method 3).

Prerequisites

Hardware

• Siemens SIMATIC S7-1212C CPU (article number 6ES7212-1BE40-0XB0 or compatible AC/DC/Rly variant).
• Signal board SB 1231 (e.g., 6ES7231-4HA30-0XB0, 1 AI, 0–10 V / ±10 V / 0–20 mA) or signal module SM 1231 (e.g., 6ES7231-4HF32-0XB0, 4 AI).
• VEGA VEGAPULS WL 61 ultrasonic sensor with 4–20 mA current output.
• 500 Ω precision sense resistor (≤ 0.1 % tolerance, ≥ 0.25 W) wired across the analog input terminals to convert 4–20 mA to 2–10 V, or a 250 Ω resistor to convert to 1–5 V. The original application uses a 500 Ω resistor, which produces a 2–10 V signal under the 4–20 mA loop.
• 24 VDC loop power supply, shielded two-wire cable, and an earth-grounded shield termination at the cabinet entry.

Software

• TIA Portal V16 or newer (V17/V18 supported for the same instructions).
• S7-1200 CPU firmware V4.2 or newer (NORM_X / SCALE_X are available from firmware V4.0 onwards).
• CPU configuration: analog input channel configured for voltage measurement, 0–10 V range, no smoothing filter (or a filter of 1–4 cycles if the level surface is turbulent).

Understanding the Raw Analog Value

For the SIMATIC S7-1200 SM/SB 1231 in the unipolar 0–10 V range, the conversion formula from the official Siemens manual is:

Digital value = (Voltage / 10 V) × 27648

So 0 V → 0, 10 V → 27648, with 9.95 V → 27504. For the VEGAPULS WL 61 in this application the relationship between measured distance and raw count is therefore:

By default the CPU places the input word in %IW64 for the SB 1231 in slot 1 of the CPU. Verify the address in Device View → Analog inputs → Properties → I/O addresses; the system manual S7-1200 System Manual (entry ID 39334504) documents the default addresses per slot.

Method 1 — Direct Integer Inversion

The fastest path: subtract the raw count from the full-scale integer 27648. This gives a value that is large at far distance (10 V → 0) and small at near distance (0 V → 27648), but it is unsigned and not scaled to engineering units. Use it only as an intermediate or when you only need a polarity-flipped count.

// LAD network 1
  +----[ %IW64 ]----[ MOVE ]----+      // read raw input
  |                              |
  +----[ 27648 ]----[ SUB ]------+      // 27648 - %IW64
                | |
                | +----[ %QW80 ]+        // optional: write to analog output
                +----( Distance_Raw )   // tag of type INT

In ladder: place a SUB (SUB_I) block, set IN1 = 27648, IN2 = %IW64, OUT = a tag such as Distance_Raw of type INT. The result is a value 0 at full-scale and 27648 at zero. Wire the negated count to a SCALE_X stage if you need engineering units.

Method 2 — NORM_X then SCALE_X with Sign Inversion

This is the canonical Siemens pattern documented in the TIA Portal help (instruction NORM_X / SCALE_X). The chain is: raw integer → normalized REAL 0.0–1.0 → engineering unit with reversed slope.

Network 1: NORM_X

Network 2: SCALE_X (inverted)

Subtracting the NORM_X output from 1.0 inverts the slope, then the SCALE_X block maps the inverted normalized value into the engineering range (0 m to 5 m in this example). Adjust MIN/MAX to your tank geometry.

// ST equivalent for Method 2
NormValue  := NORM_X(MIN := 0,    VALUE := %IW64, MAX := 27648);
Distance_m := SCALE_X(MIN := 5.0, VALUE := 1.0 - NormValue, MAX := 0.0);

Method 3 — CALCULATE Block with Linear Equation

For a single network that does everything, use the CALCULATE instruction. Starting from a normalized 0.0–1.0 input (post-NORM_X), the linear equation for an inverted sensor is derived from y = mx + b. The two calibration points given in the application (1 m @ 9.0 V and 2 m @ 8.2 V) yield:

m = (2 − 1) / (0.82 − 0.90) = −12.5 m per unit

Flipping the sign and solving for the offset that gives 0 m at 0.995 (9.95 V):

Distance_m = 12.5 × NormValue − 12.4375

CALCULATE configuration

// CALCULATE block: y = 12.5 * x - 12.4375
// IN1 = NormValue (REAL, 0.0 to 1.0)
// OUT = Distance_m (REAL)
// Expression:  12.5 * IN1 - 12.4375

Complete Ladder Example (Method 3)

Network 1: NORM_X
  +----[ %IW64 ]----[ NORM_X ]----+   // MIN=0, MAX=27648
                |                  |
                +---- NormValue ---+   // tag REAL
Network 2: CALCULATE
  +----[ NormValue ]----[ CALCULATE ]----+   // 12.5*IN1 - 12.4375
                |                       |
                +---- Distance_m -------+   // tag REAL
Network 3: Clamp
  +----[ Distance_m ]----[ MIN ]----[ MAX ]----+   // MAX(0, MIN(8, x))
                |                                |
                +---- Level_m -------------------+   // tag REAL for HMI

Hardware Verification

1. With the sensor powered and aimed at the empty vessel, force the watch table to display %IW64. Confirm the value is near 27504 ± 50 counts (9.95 V ± 0.02 V).
2. Move a hard target 1 m from the sensor face. Confirm the count drops to approximately 24883.
3. Observe the Distance_m tag in the watch table — it should read 0.000 at step 1 and 1.000 ± 0.02 at step 2.
4. Trigger an online trace in TIA Portal (Tools → Trace) to log %IW64, NormValue, and Distance_m while the level changes. Verify monotonic, linear behavior with no ringing.
5. Compare against an independent reference (e.g., a tape measure) over the full 0–5 m range. Acceptable error band for the WL 61 is ±0.2 % of full scale per the manufacturer datasheet.

Troubleshooting Matrix

Field Tips

• Sense resistor tolerance. A 0.1 % 500 Ω resistor produces a 0.05 % full-scale error contribution — an order of magnitude better than a 1 % resistor. For the WL 61 this is the dominant scaling error source.
• Block dead-band. Configure the SB/SM 1231 with a 50/60 Hz hardware filter at 50 Hz (line frequency in EU/Asia) to suppress mains pickup on long cable runs.
• Wire length. Keep the sensor cable shield continuous through the cabinet wall; the VEGAPULS WL 61 datasheet recommends ≤ 250 m with a 0.5 mm² shielded two-wire cable.
• Overflow diagnostic. Wire %IW64 into a LIMIT block so the HMI displays "Sensor fault" if the value falls below 0 or exceeds 27648 (overrange bit 32767).
• Multiple calibration points. For highest accuracy, perform a 2-point calibration in the actual process medium (water vs. oil changes the speed of sound) and replace 12.5 and 12.4375 with the measured coefficients.

Standards and References

For engineers documenting the design, the relevant signal ranges, error bands, and EMC requirements are governed by:

• Siemens SIMATIC S7-1200 Programmable Controller System Manual (entry ID 39334504) — analog input wiring, default addresses, and 0–10 V scaling table.
• IEC 61131-2:2017 — Programmable controllers, equipment requirements and tests, including analog signal levels.
• IEC 60584 / NAMUR NE 43 — 4–20 mA current loop conventions and fault current levels (3.6 mA / 21 mA for sensor diagnostics).
• Balluff: Analog signals 0…10 V vs. 4…20 mA — guidance on choosing voltage vs. current loop for industrial position sensors.

FAQ