Siemens TIA Portal I/O Field: Dynamic Decimal Place Formatting

Overview

Siemens TIA Portal HMI I/O fields display process values in a fixed format defined at engineering time. The Display format or Representation property sets the decimal count statically (for example, 999, 999.9, 999.99, 999.999). When an operator must be able to change the resolution of an analog readout at runtime — selecting 0, 1, 2, or 3 decimal places from a separate I/O field — a static format string is no longer sufficient.

This article covers three production-ready techniques for the TP 1200 Comfort (WinCC Comfort / Advanced) and the WinCC Unified runtime, plus a structured assessment of the legacy "four I/O fields with visibility animation" workaround that is currently deployed at the customer site. Each technique is documented with the exact configuration path, the VBScript snippet to use where applicable, and the verification steps required at HMI commissioning.

Prerequisites

Technique A — VBScript with FormatNumber on WinCC Comfort / Advanced

This is the canonical solution for a TP 1200 Comfort panel. The decimal selection is implemented in a single HMI tag of type Integer. A VBScript is fired on the ValueChange event of the selector I/O field. The script reads the source analog value, applies FormatNumber, and writes the result to a display HMI tag of type String. The visible I/O field is bound to the string tag, not the raw process tag, so the format can change every time the operator modifies the selector.

Step 1 — Declare the three HMI tags

1. In the TIA Portal project tree, open HMI tags on the Comfort panel.
2. Create tag ProcessValue, type Real, connection to the PLC tag that carries the analog value (for example, DB101.DBD0).
3. Create tag NumberOfDecimals, type Int (or USInt), range 0 – 3. This is the user-selectable parameter.
4. Create tag ProcessValueText, type WString (recommended for Unicode-safe logging) or String. The visible I/O field is bound to this tag.

Step 2 — Create the VBScript

Under Scripts > VBScripts on the HMI, add a new function. WinCC Comfort / Advanced exposes the legacy SmartTags() collection, but in TIA Portal V17 and later the recommended access pattern is the HMIRuntime.Tags API. The snippet below is fully TIA Portal V17–V20 compatible.

' VBScript: FormatProcessValue
' Trigger: ValueChange event of HMI tag "NumberOfDecimals"

Dim dec
Dim raw
Dim txt
Dim srcTag, dstTag

Set srcTag = HMIRuntime.Tags("ProcessValue")
Set dstTag = HMIRuntime.Tags("ProcessValueText")

srcTag.Read
dec = CInt(HMIRuntime.Tags("NumberOfDecimals").Read)

' Clamp the value to the allowed 0..3 range to avoid FormatNumber errors
If dec < 0 Then dec = 0
If dec > 3 Then dec = 3

raw = CDbl(srcTag.Value)
txt = FormatNumber(raw, dec, vbTrue, vbFalse, vbFalse)

dstTag.Write txt

Step 3 — Wire the script to the event

1. Select the decimal-selector I/O field on the screen.
2. Open Properties > Events > ValueChange.
3. Add a new function list, drop in the FormatProcessValue VBScript, and close the dialog.

Step 4 — Bind the display I/O field

1. Drop the analog display I/O field on the screen and set Mode to Output.
2. Set Process value to the tag ProcessValueText (String / WString).
3. Set Display format to String so the I/O field does not try to re-parse the formatted text as a number.

Step 5 — Bind the selector I/O field

1. Set the selector I/O field's Process value to NumberOfDecimals (Int).
2. Set Mode to Input/Output so the operator can edit the value at runtime.
3. Optional: configure the Lower limit = 0 and Upper limit = 3 to prevent out-of-range entries.

Technique B — WinCC Unified "Shift decimal places" property

WinCC Unified runtime (Unified Comfort Panels, Unified PC Runtime) added a Shift decimal places property to the I/O field object starting with TIA Portal V18, and the property was refined in the V20 Update 3 release notes. According to the official Siemens documentation at SIEMENS Support entry 109816808, the property automatically multiplies or divides the integer tag by a power of 10 at the screen-object level, so a tag holding the raw integer 12345 can be displayed as 1.2345 simply by setting the shift value to 4. The feature is described in the TIA Portal V20 release notes at I/O field: 'Shift decimal places' property.

Step 1 — Configure the I/O field

1. Select the I/O field on the Unified screen.
2. Open the Properties &strong> inspector > General.
3. Set the bound tag to an integer value (the raw process value scaled by 10ⁿ).
4. Set the new Shift decimal places property to the desired shift count (for example, 2 to display 12345 as 123.45).

Step 2 — Drive the shift at runtime

Because the shift is still a compile-time property, dynamic runtime change still requires a script. The Unified VBScript syntax differs from WinCC Comfort / Advanced: tags are accessed through the HMIRuntime object and screen-object properties are set via the Screen.Items(...) collection.

' Unified VBScript: SetShift
' Trigger: ValueChange of the decimals selector tag

Dim n
n = CInt(HMIRuntime.Tags("NumberOfDecimals").Read)
If n < 0 Then n = 0
If n > 3 Then n = 3

' Apply shift to the named I/O field on the active screen
Screen.Items("IOField_ProcessValue").ShiftDecimalPlaces = n

Technique C — Multiple I/O fields with visibility animation (legacy workaround)

This is the technique the customer is currently using. It is fully supported on Basic Panels as well, where VBScript and the Unified shift property are not available. The implementation cost scales linearly with the number of source values and is the main reason engineers search for alternatives: 30 analog values × 4 decimal formats = 120 I/O fields.

Step 1 — Pre-build the four format variants

1. For each analog source tag, create four I/O fields on the screen.
2. Set their Display format strings to 999, 999.9, 999.99, and 999.999 respectively.
3. Stack the four I/O fields at the same screen coordinates so they visually overlap.

Step 2 — Animate visibility

1. Open the Animations > Visibility dialog of the first I/O field.
2. Add a new animation that evaluates the value of the HMI tag NumberOfDecimals.
3. Use the table-driven form (PLC tag = value) or an expression such as NumberOfDecimals == 0 to map the tag value to the corresponding I/O field's visibility.

Step 3 — Repeat for the other three formats

Map the same NumberOfDecimals tag to the visibility of the remaining three I/O fields with the conditions == 1, == 2, and == 3. At any moment exactly one of the four fields is visible.

Cross-project configuration — TIA Portal HMI + SIMATIC Manager PLC

A frequently-asked follow-up in the field is whether the VBScript approach works when only the HMI has been migrated to TIA Portal while the PLC is still programmed in STEP 7 V5.x (SIMATIC Manager). The answer is yes, with two important constraints.

Constraint 1 — Tag access direction

The HMI accesses the SIMATIC Manager PLC through an HMI connection configured with the PLC's MPI / PROFIBUS / Ethernet address. Tags are read-only mirrors of PLC addresses; the script cannot write back to a DB in the S7-300 / S7-400 project from a Comfort panel. The script must read the analog value and write the formatted text to a local HMI tag on the panel.

Constraint 2 — Tag type and HMI-side storage

Local HMI tags are stored in the panel's internal tag memory and survive power cycles only if the tag is marked as Persistent. The ProcessValueText tag from Technique A is a String / WString of length 16, which is well within the Comfort panel's tag memory budget. The persistence flag is not required for the formatted text since it is recomputed on every ValueChange of the selector.

Code that works across the two projects

' TIA Portal HMI, VBScript — usable when the PLC lives in SIMATIC Manager
' The PLC connection is an HMI connection of type "SIMATIC S7 300/400"

Dim src, dec, txt

' src.Value is automatically updated by the HMI's polling cycle
src = SmartTags("ProcessValue")
dec = SmartTags("NumberOfDecimals")

If dec < 0 Then dec = 0
If dec > 3 Then dec = 3

txt = FormatNumber(src, dec, vbTrue, vbFalse, vbFalse)
SmartTags("ProcessValueText") = txt

Both SmartTags("ProcessValue") (the S7-300 / S7-400 DB mirror) and SmartTags("ProcessValueText") (the local HMI tag) are visible in the VBScript because the script runs inside the TIA Portal HMI project; the script does not care which project the source PLC tag was originally configured in.

Solution comparison

Verification and commissioning

1. Compile and download the HMI project to the TP 1200 Comfort. Start the runtime.
2. Force the PLC tag ProcessValue to four representative values: 0, 1.2345, -1.2345, and 99999.9999 (clamp to the configured display range).
3. Cycle the selector I/O field through 0, 1, 2, 3 and confirm the visible readout matches the requested precision.
4. Enter a value of 5 into the selector and confirm the clamping logic falls back to 3 decimal places.
5. Open the HMI's diagnostic view (Control Panel > System > Runtime Logs) and verify that the FormatNumber script has not raised an exception.
6. Power-cycle the panel. The selector value should be retained if the tag is marked as Persistent; the formatted text will be recomputed on the first ValueChange.

Troubleshooting matrix

Field-proven caveats

• Logarithmic displays: FormatNumber does not respect the Display format property of the I/O field. If the HMI screen uses a five-digit 99999 format and the process value exceeds it, FormatNumber will still return the full text. Pad the display I/O field with a 16-character String tag to be safe.
• Engineering language: The script editor of WinCC Comfort / Advanced is English-only; comments inside the script must use English identifiers even on a Chinese or German localization. vbTrue and vbFalse are the locale-independent constants.
• Alarm acknowledgment: Alarms that reference the raw process tag remain triggered by the original value, not by the formatted string. The string tag is purely a display artifact.
• Audit trail: On regulated sites, the formatted string is not acceptable as a logged value. Always log the raw Real tag, never the formatted String tag.