PLC Analog Input Scaling in TIA Portal: Formula and Examples

• Configure the channel signal type correctly before writing scaling logic.
• Use the raw range documented for the exact module and configured measuring range.
• Convert to REAL for calculation to avoid unwanted integer rounding.
• Validate zero, midpoint and full-scale values before trusting the measurement in control logic.

1. What Is PLC Analog Input Scaling?

An analog input channel converts an electrical measurement into a digital number. The PLC cannot inherently know that a particular raw value means 6.2 bar, 143 °C or 72% tank level. The program must map the digital range to the physical engineering range.

Scaling is therefore a linear mapping from one range to another. If both ranges are linear, the same percentage position in the raw range must produce the same percentage position in the engineering range.

2. Signal Flow: Sensor → PLC → Engineering Units

For example, a 0–10 bar pressure transmitter may output 4–20 mA. The analog module digitizes the current and the PLC scales the resulting raw value to 0.0–10.0 bar. The scaled value can then be used by HMI displays, alarms, trends, recipes or PID control.

3. Understand Siemens Analog Raw Values

Siemens standardizes the nominal analog representation so the application logic is not tied directly to the hardware converter resolution. For general-purpose S7-1200/S7-1500 analog processing, the rated current range is represented by 0 to 27648, while bipolar voltage ranges use -27648 to +27648. Values beyond the nominal range can indicate overrange or underrange depending on module type and configuration.

*Always verify the exact analog module and measuring-range table. Siemens families and modules differ in supported voltage/current ranges and diagnostic regions.

4. Universal PLC Analog Scaling Formula

The general linear equation maps a raw input range to an engineering range:

Where:

For a standard current channel where RawMin = 0 and RawMax = 27648, the equation becomes:

5. Example 1: 0–10 V Pressure Transmitter, 0–10 bar

Assume the configured analog range gives 0...27648 across the transmitter span and the engineering range is 0.0 to 10.0 bar. At half scale, Raw = 13824.

6. Example 2: 4–20 mA Temperature, 0–200 °C

When the analog module is configured for a 4–20 mA measuring range, the rated current span is represented from raw minimum to raw maximum. Therefore 4 mA represents the engineering minimum and 20 mA represents the engineering maximum.

This is different from connecting a 4–20 mA transmitter to a channel configured as 0–20 mA. In that case, 4 mA is already 20% of the electrical range, so the raw value at the transmitter’s zero point is not zero. The hardware configuration and scaling equation must agree.

7. Example 3: -10...+10 V Position, -100...+100 mm

For a bipolar voltage range with raw values -27648...+27648, let the engineering range be -100.0 to +100.0 mm.

8. NORM_X and SCALE_X in TIA Portal

Siemens recommends a two-stage method for most PLC applications:

NORM_X

NORM_X determines where the input sits between the selected minimum and maximum. A halfway raw value produces approximately 0.5.

SCALE_X

SCALE_X converts the normalized value into the required engineering range.

Keeping normalization and engineering scaling separate makes the program easier to reuse: the same normalized signal can be mapped to different display, command or diagnostic ranges.

9. SCL Example: Analog Input Scaling

For a 0–10 bar pressure transmitter using a nominal raw span of 0...27648, an SCL implementation can be written as:

VAR
    AI_Raw          : INT;
    AI_Normalized   : REAL;
    Pressure_bar    : REAL;
END_VAR

AI_Normalized := NORM_X(
    MIN   := 0,
    VALUE := AI_Raw,
    MAX   := 27648
);

Pressure_bar := SCALE_X(
    MIN   := 0.0,
    VALUE := AI_Normalized,
    MAX   := 10.0
);

You can also implement the equation explicitly. Convert the raw integer to REAL before multiplication and division so the calculation keeps fractional precision.

Pressure_bar := 0.0
    + (INT_TO_REAL(AI_Raw) - 0.0)
    * (10.0 - 0.0)
    / (27648.0 - 0.0);

10. LAD / FBD Workflow with NORM_X and SCALE_X

11. Configure the Analog Channel Before Scaling

Correct logic cannot repair an incorrectly configured analog module. In TIA Portal, open Device configuration, select the analog module/channel and verify the supported measuring type and range.

• Voltage or current input selected correctly
• 0–20 mA vs 4–20 mA selected correctly
• Bipolar voltage range selected when negative values are required
• Channel address matches the PLC tag
• Wire type and terminal connection match the module manual
• Diagnostics are enabled where useful
• Transmitter supply and loop power are correct
• Shielding and grounding follow plant standards

After downloading the hardware configuration, monitor the raw value first. Only after the raw channel behaves correctly should you troubleshoot the scaling math.

12. Overrange, Underrange and Diagnostic Values

The nominal range is not the only possible integer region. Analog modules can provide overrange/underrange information and may report substitute or diagnostic states depending on hardware and configuration. Therefore a robust program should not assume that every raw value is valid process data.

Use module-specific diagnostics for wire break, overflow, underflow and configuration faults where supported. The exact thresholds differ by module, so use the device manual rather than hard-coding one universal diagnostic number.

13. Analog Noise, Smoothing and Filtering

Scaling changes units; it does not remove electrical noise. If the raw value fluctuates, first check the physical installation: transmitter stability, cable routing, shield termination, grounding, power supply and channel integration/filter settings.

Software filtering can be added after the measurement chain is healthy. Common approaches include a moving average, first-order low-pass filter or deadband. The filter time constant must suit the process: a fast pressure-control loop needs a different response than a slowly changing tank level.

14. Common PLC Analog Scaling Problems

15. Best Practices for Siemens Analog Scaling

• Document signal type beside every analog tag
• Keep raw value and scaled value as separate tags
• Use descriptive engineering units in tag comments
• Use REAL for normalized and engineering calculations
• Build one reusable scaling FC/FB instead of repeating arithmetic
• Validate zero, 25%, 50%, 75% and 100% where practical
• Handle module diagnostics separately from process scaling
• Do not silently mask sensor faults with clipping
• Use a simulator/calibrator for commissioning
• Trend both raw and scaled values during troubleshooting