Analog Scaling in Siemens PLC & Building Real-Time Trends in WinCC

• Confirm the analog module measuring range before assuming a raw range.
• Use NORM_X to normalize the configured raw span and SCALE_X to convert it to engineering units.
• Trend the engineering value, not an unexplained raw count, unless raw diagnostics are intentionally required.
• Choose archive cycle based on process dynamics and reporting need, not the fastest possible interval.

1. Analog Signal Path: Sensor → PLC → WinCC

The complete path is Transmitter → Analog Input Module → Raw PLC Value → Normalized Value → Engineering Unit → WinCC Tag → Archive → Trend. Each stage should have a documented range. If the transmitter is 0–100 °C, the SCADA engineer should not have to guess what a raw value such as 13824 means.

2. Confirm Raw Range and Module Configuration

Many Siemens analog examples use a nominal raw range of 0…27648 for a unipolar configured measuring range. However, diagnostic overrange/underrange behavior and exact representation depend on the module and selected range. Always check the hardware configuration and module documentation for the channel you are commissioning.

3. Normalize the Raw Value with NORM_X

NORM_X converts a value between a defined minimum and maximum into a normalized REAL value, typically 0.0 to 1.0 for an in-range signal.

// Example: raw input represented from 0 to 27648
#Normalized := NORM_X(
    MIN   := 0,
    VALUE := #AI_Raw,
    MAX   := 27648
);

Monitor the normalized value online. At approximately half scale, it should be close to 0.5. If it is negative or above 1.0 under normal conditions, verify wiring, measuring range and constants before continuing.

4. Convert to Engineering Units with SCALE_X

SCALE_X maps the normalized value to the required engineering range. For a 0–100 °C process:

#Temp_C := SCALE_X(
    MIN   := 0.0,
    VALUE := #Normalized,
    MAX   := 100.0
);

For a pressure transmitter of 0–16 bar, change the engineering limits to 0.0 and 16.0. Keep the engineering unit in the tag name or documentation so a later SCADA/report engineer does not misinterpret the number.

5. Add Validation, Limits and Signal Fault Handling

Scaling is only one part of analog engineering. Add plausibility checks and decide what WinCC should display if the input channel reports a diagnostic condition.

#AI_Valid := (#AI_Raw >= #RawValidMin) AND (#AI_Raw <= #RawValidMax);

IF #AI_Valid THEN
    #Temp_QualityOK := TRUE;
    #Temp_PV := SCALE_X(MIN := 0.0, VALUE := #Normalized, MAX := 100.0);
ELSE
    #Temp_QualityOK := FALSE;
END_IF;

The exact diagnostic strategy should use the module status available in your hardware. The key point is to expose a separate quality/validity status instead of silently presenting a faulty signal as a believable process value.

6. Map the Engineering Value to WinCC

Create a WinCC process tag mapped to the final engineering value, such as Furnace.Temp_PV_C, and if useful a second Boolean such as Furnace.Temp_QualityOK. Use the engineering tag for numeric displays, alarms and trends.

7. Configure WinCC Tag Logging / Historical Acquisition

In WinCC Classic, historical process values are configured through Tag Logging archives. In TIA-integrated WinCC products, configure historical tags/archive acquisition using the tools available in that generation. Define the acquisition mode and cycle from process requirements.

• Fast-changing motion/process loops may need sub-second or short-cycle visualization.
• Slow furnace temperature may be adequately archived every few seconds.
• Production reports may use event-based or longer-cycle values.
• Do not archive every tag at the fastest rate “just in case”.

8. Build the Real-Time Trend in WinCC

1. Insert the appropriate trend control on a diagnostic/process screen.
2. Add Temp_PV_C as the primary pen.
3. Add Temp_SP_C as a second pen for setpoint comparison.
4. Select online/live or archived data source as required.
5. Configure Y-axis engineering range and units.
6. Choose a meaningful time window.
7. Verify cursor/readout functions for commissioning.

A useful trend tells a process story. For temperature control, plotting PV and SP together is more valuable than plotting PV alone.

9. Sampling, Archive Size and Performance

Archive load is approximately proportional to tag count and acquisition frequency. A 250 ms archive cycle generates four samples per second per tag, while a 5 s cycle generates one sample every five seconds. Choose the cycle from process dynamics, investigation needs and retention requirements.

10. Troubleshooting Analog Scaling and WinCC Trends

11. Hands-On Lab: Temperature PV + SP Trend

Simulate a raw value from 0 to 27648, scale it to 0–100 °C with NORM_X and SCALE_X, map the engineering value to WinCC, archive PV and SP, and display both on a trend. Step the setpoint and observe process response.

Lab Manual: From Transmitter to WinCC Trend in Six Stages

This lab turns the sections above into the six-stage pattern of the series, with the scaling written in both ladder and SCL.

Stage 1 – Field wiring and PLC tag

Stage 2 – Input mapping and scaling in ladder and SCL

The same in SCL:

// FC_InputMapping – SCL equivalent of the ladder network
"DB_IO".PT601_Valid := "PT601_Raw" >= -4864 AND "PT601_Raw" <= 32511;
IF "DB_IO".PT601_Valid THEN
    "DB_IO".PT601_bar := SCALE_X(MIN := 0.0, MAX := 16.0,
                         VALUE := NORM_X(MIN := 0, VALUE := "PT601_Raw", MAX := 27648));
END_IF;
"DB_Alarm".PT601_High := "DB_IO".PT601_Valid AND "DB_IO".PT601_bar > 12.0;   // limit also usable in WinCC

Stage 3 – Logic: validity and limit

The validity bit stops a wire break from being archived as 0 bar, and the limit bit can feed the PLC alarm word – see WinCC alarm management. Analog scaling theory: PLC analog input scaling.

Stage 4 – Link PLC tags to WinCC

In Tag Management add the SIMATIC S7-1200, S7-1500 Channel, create a connection with the CPU IP address, then use AS Symbols to load the PLC symbols and drag the DB members you need into a tag group. The DB members must have Accessible from HMI ticked in TIA Portal. Names and menus can differ slightly between WinCC versions.

Stage 5 – Tag Logging and trend graphics

Create process value archive PVA_Pressure with acquisition 1 s and archiving 2 s (mean), place a WinCC OnlineTrendControl on Pressure.pdl with the archived tag, a fixed 0 – 16 bar value axis and a limit line at 12 bar. Cycle and compression choices are compared in WinCC Tag Logging and trends.

Stage 6Perform and Test: Scaled Pressure into a WinCC Trend

Move the 4–20 mA signal, change the WinCC archiving cycle and break the wire. The PLC scales and validates; WinCC archives and draws only valid engineering values.