Configuring Fractional Decimal Precision for Process Trends in WinCC and PCS 7

In SIMATIC PCS 7 V8.2, online process trend curves displaying continuous analog variables (such as pressure in bar) strictly as coarse integers (e.g., 1, 2, 3) rather than fractional real numbers (e.g., 1.1, 1.2, 1.3) is caused by display formatting masks within the WinCC Trend Control, not by archive data rounding. WinCC Tag Logging archives floating-point numbers in full 32-bit or 64-bit IEEE 754 precision without truncation. However, if the WinCC Trend Control's Value Axis or ruler formatting is set to an integer format mask (%d or decimal places set to 0), the control visually truncates fractional values. Configuring the Value Axis format to floating-point with decimal places (%.2f) restores fractional visibility immediately.

Floating-Point Archiving in WinCC Tag Logging

A common misconception among process control engineers observing integer-stepped trend curves is that the Tag Logging archive compression algorithm (such as swinging door or deadband compression) is rounding floating-point data to integers to save SQL database disk space.

This is technically incorrect:
* When an analog process value (REAL) is archived in SIMATIC PCS 7, WinCC Tag Logging stores the exact IEEE 754 floating-point representation (32-bit float or 64-bit double).
* The underlying Microsoft SQL Server archive database (TagLoggingFast / TagLoggingSlow) preserves full fractional decimal precision.
* The truncation to integer steps is purely an optical presentation issue occurring inside the WinCC Trend Control graphical rendering engine.

Why Trend Controls Display Floating-Point Data as Integers

The visual representation of values in the WinCC Online Trend Control and WinCC Online Table Control is governed by formatting properties:

Formatting PropertyLocationDefault / Erroneous SettingRequired Setting for Tenths of a BarValue Axis Number FormatTrend Control Properties -> Value AxesFixed / Integer or %dDecimal / Floating Point (%.1f or %.2f)Decimal Places SettingTrend Control Properties -> Value Axes0 Decimal Places1 or 2 Decimal PlacesRuler Value WindowTrend Control Properties -> Ruler Window%d Format Mask%f Format Mask with precisionArchive Tag Data TypeWinCC Tag Logging -> Archive TagFloating-point number 32-bit IEEE 754Must match 32-bit float

If an engineer creates a trend window or copies an existing typical whose Y-axis decimal places property is configured as 0, WinCC snaps the vertical grid labels and ruler cursor readouts to whole integer values, giving the false impression that fractional process changes are lost.

Configuring Value Axis and Decimal Formatting in Trend Control

To restore fractional decimal display on a process trend:

Open WinCC Graphics Designer.
Open the process picture (.pdl) containing the affected trend display.
Double-click the WinCC Online Trend Control to open its Configuration Dialog.
Select the Value Axes tab.
In the axis selection tree, select the active Y-axis assigned to the pressure curve.
Under Format, change the display mask:
- In the Decimals field, change 0 to 1 (for tenths, e.g., 1.5 bar) or 2 (for hundredths, e.g., 1.55 bar).
- Alternatively, enter a standard C-format printf string: %.2f.
Select the Ruler Window tab; verify that the readout format for trend cursors also specifies %.2f.
Click Apply and OK, then save the process picture.

Verifying Tag Logging Data Types and Linear Scaling

If formatting is verified but curves still appear stepped:
1. In WinCC Explorer, open Tag Logging.
2. Locate the specific archive tag (e.g., PT101/PV.Value#Archive).
3. Check the Process Tag properties:
- Ensure the Data Type indicates Floating-point number 32-bit IEEE 754.
- If accidentally mapped to an integer data type (such as Signed 16-bit value), change it to 32-bit float.
4. Check Linear Scaling: verify that raw counts from the AS are not being truncated by an integer scaling formula.

Verification

To verify fractional trend rendering:
1. Start WinCC Runtime on the OS Client.
2. Open the modified process trend picture.
3. Observe the Y-axis labels; verify that tick marks reflect decimal divisions (e.g., 1.0, 1.5, 2.0, 2.5).
4. Activate the Trend Ruler tool and drag the cursor across historical pressure data; verify that the floating ruler box displays fractional values with tenths and hundredths of a bar (e.g., 2.34 bar).

Limitations and Operational Precautions

Increasing display precision to 3 or 4 decimal places increases visual clutter on small trend window viewports. Standard practice for industrial pressure and temperature loops is 1 or 2 decimal places. Ensure that excessive archiving deadbands are not configured in Tag Logging: if an archive deadband of 1.0 bar is configured, the database will not log changes smaller than 1.0 bar regardless of display formatting.

Engineer FAQ

Does Tag Logging compress data by rounding off decimals?

No. Tag Logging compression algorithms (such as deadband compression) discard redundant identical points, but the points that are recorded retain their exact IEEE 754 floating-point precision.

Can I configure different decimal places for different curves on the same trend window?

Yes. WinCC Online Trend Control supports multiple independent Value Axes, allowing each curve to have its own dedicated Y-axis with custom ranges and decimal formatting.