Calculating Values in WinCC 7.0 SP3: Divide PLC Tags Dynamically

Overview

In WinCC 7.0 SP3 the runtime database stores every external variable (process tag) as a raw value supplied by the AS (Automation Station). When the HMI application needs a derived value - for example a length in millimeters divided by a constant, or a count scaled to engineering units - the calculation can be performed at three distinct layers:

1. Inside the I/O Field using Dynamic Dialog (presentation layer).
2. Inside the Tag Management / Channel using Linear Scaling (acquisition layer).
3. Inside a VBScript action or global script cycle (logic layer).

Each approach has different performance, scalability, and licensing implications. This reference documents all three methods for a recurring requirement: receive a numeric value from the PLC, divide it by a constant (e.g. 300), and expose the result either on the screen or as a new internal tag that other graphics or archives can consume.

Prerequisites

Before configuring any of the three methods, verify that the engineering station meets the following requirements:

Open the project in the WinCC Explorer, then launch Graphics Designer for screen-level methods or stay in the Explorer for channel-level scaling.

Method 1 - Dynamic Dialog inside an I/O Field

This is the fastest method for a single graphic object. It does not require an internal tag; the I/O field evaluates the formula on each redraw cycle.

Step-by-step

1. In the Graphics Designer, open the target picture (e.g. NewPdl.pdl).
2. From the Standard object palette, drop an I/O Field onto the canvas.
3. Right-click the I/O Field and select Properties > Output/Input.
4. In the Output Value cell, click the small bulb icon and choose Dynamic Dialog.
5. The Dynamic Dialog wizard opens. Configure the four fields as follows:

6. Click Apply, then OK. The dialog closes and the I/O field now displays the computed value at runtime.
7. Save the picture and activate the project (File > Activate in WinCC Explorer).

Syntax rules for Dynamic Dialog formulas

• Tag names must be wrapped in single quotes: 'MyTag'. Long tag prefixes (WinCC 7.4+) use the form 'Siemens\::MyTag'.
• Constants can be integer (300) or floating point (300.0). Mixing an integer source with an integer divisor returns an integer - use 300.0 to force floating-point division.
• Supported operators: +, -, *, /, % (modulo), unary minus.
• Bitwise operations (&, |, ^) are not available in Dynamic Dialog; use VBScript for these.
• The dialog rejects formulas longer than 256 characters and silently truncates results that exceed the chosen datatype range.

Method 2 - Linear Scaling at the Tag / Channel layer

Linear Scaling is the cleanest approach when the same conversion must be applied globally (e.g. a 0-30 000 raw count from a flow meter is always shown as 0-100 % on every screen). The conversion happens once per acquisition cycle, the result is a normal WinCC tag, and every consumer (I/O fields, trends, alarms, scripts) reads the already-scaled value.

Configuration path

1. In WinCC Explorer, right-click Tag Management and open the relevant channel (e.g. SIMATIC S7 PROTOCOL SUITE > TCP/IP).
2. Select the connection (e.g. S7-1200/1500) and click New Tag.
3. Name the tag (e.g. FlowRate_Eng), set datatype to Floating-point 32-bit IEEE 754, and click Select to choose the PLC address (e.g. DB10.DBD0).
4. Click the Scaling button. Enable Linear scaling and enter:

5. Apply. The formula applied internally is:

HMI = HMI_low + ( Raw - Raw_low ) * ( HMI_high - HMI_low ) / ( Raw_high - Raw_low )

This is a two-point linear interpolation. For a pure division by a constant (e.g. raw 0-300 000 → engineering 0-1 000), set PLC raw end = 300 000 and HMI end = 1 000. The WinCC tag FlowRate_Eng will then automatically contain the divided value, and any I/O field bound to it displays the engineering units directly.

Method 3 - VBScript calculation into an internal tag

When the formula is too complex for Dynamic Dialog - conditional scaling, multi-tag averaging, unit conversion tables - use a VBScript action. This method mirrors the pattern described in the Microsoft Visual Basic documentation for calculating numeric values: combine literals, constants, and variables in a numeric expression.

Creating the internal tag

1. WinCC Explorer → Internal Tags → right-click → New Tag.
2. Name: RawDiv300, Datatype: Floating-point 64-bit IEEE 754 (highest precision available).
3. Click OK. The tag is now visible to every script and screen.

Creating the action

1. WinCC Explorer → Global Scripts → right-click → New > Action.
2. Choose trigger: Tag trigger on the source PLC tag RawValue (right-click the line, select Trigger, browse the tag).
3. Insert the following code body:

' WinCC VBS action - divide incoming PLC tag by 300
Option Explicit

Dim rawVal
Dim scaledVal

' Read external tag (auto-quotes allowed in HMIRuntime.Tags path syntax)
rawVal = HMIRuntime.Tags("RawValue").Read

' Guard against divide-by-zero and invalid float
If IsNumeric(rawVal) Then
    scaledVal = CDbl(rawVal) / 300.0
    HMIRuntime.Tags("RawDiv300").Write scaledVal
Else
    ' keep last good value, optionally raise a bit in an alarm tag
    HMIRuntime.Trace "RawValue not numeric - division skipped" & vbCrLf
End If

4. Compile (Ctrl+F7) and confirm Script successfully compiled.
5. Save the action. Activation is automatic when the runtime starts.

Why use VBScript instead of Dynamic Dialog

• Supports CDbl, CLng, Round, FormatNumber - higher precision than the I/O field engine.
• Conditional logic (If...Then, Select Case) allows range-dependent scaling or fallback values.
• One script scales one tag but can also fan out to multiple internal tags in a single execution.
• Errors can be logged with HMIRuntime.Trace to the WinCC diagnosis file WinCC_Sys_xx.log.

Data Type Handling and Conversion Pitfalls

The most frequent cause of "I am not receiving the answer" in WinCC 7.0 SP3 is datatype mismatch between the PLC value and the HMI datatype declared on the tag.

If the PLC side is REAL and the WinCC tag is Signed 32-bit, the value is truncated to integer before the formula runs. Always match the WinCC datatype to the PLC datatype when first commissioning the tag.

Common Errors and Fixes

Verification Procedure

After applying any of the three methods, validate the result with the following checklist before handing the project to operations.

1. Open WinCC Explorer and confirm the source tag shows a Quality Code of 0xC0 (Good) in the diagnostics view (right-click tag → Properties > Statistics).
2. Force a known value from the PLC (e.g. write 30000 to the source DB word in STEP 7 / TIA Portal) and observe the I/O field or internal tag.
3. Cross-check the displayed value with a handheld calculation: 30000 / 300 = 100. If the HMI shows 99.99, the value was truncated at the integer boundary - switch the tag to Float.
4. Force a value that exercises the upper range (e.g. 300000) and confirm the result equals the configured upper engineering value.
5. Force a negative value if the engineering range allows it; confirm two's-complement behaviour matches the PLC.
6. Restart the WinCC runtime (Start > Programs > Siemens Automation > WinCC > WinCC Runtime Stop / Start) and verify the scaled tag retains its last value or starts at the configured start value.
7. Inspect the WinCC diagnosis file <Project>\Diagnostics\WinCC_Sys_01.log for any VBScript error entries.

Method Selection Matrix

Performance and Commissioning Tips

• Push the maths down to the PLC. A S7-1500 CPU executes a single /R instruction in < 1 µs and the result is available to WinCC as a normal tag - no licensing, no script overhead, no maintenance burden. Reserve HMI-side calculation for values that genuinely depend on HMI state (e.g. operator-selected units).
• Avoid cyclic float division in VBS. If a value must be divided every cycle, do it once on the AS and transmit the pre-scaled value.
• Use bit-triggered actions instead of time-triggered ones when the source tag rarely changes - saves CPU.
• Document the formula in the tag comment. WinCC Explorer allows a free-text comment per tag. Recording "RawValue / 300 = engineering mm" prevents the next engineer from re-deriving the scaling factor.
• Centralise scripts in project functions. If ten tags need the same divide-by-300, create one project function fncScaleBy300(raw) and call it from each action - simplifies maintenance and version control.

Safety and Archiving Considerations

Scaled values that are archived in Tag Logging must be added to an archive tag manually; the archive does not inherit the scaling from the source tag. Configure the archive cycle independently - typically 1 s for trending and 100 ms for fast process values. For redundant WinCC servers, ensure the scaled internal tag is configured identically on the standby server and that VBS actions are exported/imported via the Project Duplication tool.

Alarms that trigger on the scaled value use the same bit or limit logic as on the raw value; re-evaluate alarm limits in engineering units, not raw units, otherwise the alarm will fire at the wrong point. For example, an alarm on a raw count of 30 000 corresponds to an engineering value of 100, not 30 000.