WinCC TIA Portal: Run VBScript Every 2 Seconds on TP900

This reference documents how to implement a periodic VBScript execution in WinCC (TIA Portal) Comfort/Advanced running on a SIMATIC TP900 Comfort panel, with the script driven by a cyclic tag value change originating from an S7-1500 PLC (tested on CPU 1512). The technique replaces the legacy WinCC flexible scheduler, gives sub-second resolution, and supports clean start/stop semantics through a single boolean tag.

1. Problem Definition and Design Goals

WinCC Comfort/Advanced does not expose a VBScript "tick" or "timer" object on the HMI runtime in the same way a desktop application does. The available triggers for a scheduled function are limited to:

• Tag value change (rising edge, falling edge, any change, or value range)
• Scheduled task (date/time, daily, weekly - resolution 1 minute minimum on TP900)
• Screen event (load, clear)

To obtain a 2-second (or any value between 0.1 s and several hours) execution interval, the most reliable pattern is to use a cyclic PLC tag as the heartbeat and bind the VBScript to its value change event. The script body is wrapped with an If ... Then guard that reads a Start/Stop tag, so the same heartbeat tag can be enabled or disabled at runtime without reconfiguration.

2. Architecture Overview

The PLC generates a square-wave Boolean at the required period. The HMI polls that tag at a 100 ms acquisition cycle (minimum on TP900) and detects a value change on every transition. The transition fires the VBScript, which then performs the user action (read tags, log to file, write back to PLC, animate screen, etc.).

3. Prerequisites

4. Step 1 - Build the PLC Heartbeat Generator

Create a data block in the S7-1500 project, for example DB_HMI_Trigger, containing three tags:

Add the following structured text logic in OB1 (or an OB with the same cycle, e.g. OB35 at 100 ms):

// Heartbeat: toggle every 2 s using a TON + edge
// Tag: iHeartTmr (TIME), iHeartStep (INT) - local statics
IF iHeartStep = 0 THEN
    iHeartTmr := T#2S;          // period for 2-second script interval
    iHeartStep := 1;
END_IF;

// Use a TP (pulse) or simple timer-driven toggle
IF "DB_HMI_Trigger".bStartStop THEN
    IF iHeartTmr <= T#0MS THEN
        // toggle
        "DB_HMI_Trigger".bHeartbeat2s := NOT "DB_HMI_Trigger".bHeartbeat2s;
        "DB_HMI_Trigger".iTickCount := "DB_HMI_Trigger".iTickCount + 1;
        iHeartTmr := T#2S;
    END_IF;
    iHeartTmr := iHeartTmr - OB1_INTERVAL;  // subtract 100 ms per OB1 cycle
ELSE
    "DB_HMI_Trigger".bHeartbeat2s := FALSE;
    "DB_HMI_Trigger".iTickCount := 0;
END_IF;

Alternatively, use the built-in Clock Generator from the "Instructions > Timers & counters > Clock" group in TIA Portal with mode Periodic and period 2 s. The clock generator is the cleanest solution - it does not consume OB1 scan time and is fully time-accurate.

5. Step 2 - Create the HMI Tags

In the HMI project, create three tags pointing to the corresponding PLC addresses. Mark the connection's "Acquisition mode" as Cyclic continuous (this is the field many engineers miss - default is "On demand" and the value-change event will not fire on every transition).

6. Step 3 - Bind the VBScript to the Value-Change Event

1. In the TIA Portal project tree, open HMIRuntime > Scripts > VBScripts.
2. Right-click and add a new procedure, e.g. sub_OnTick.
3. Open the HMI screen where the trigger should be active, or add the function to a global script if it should run regardless of the active screen.
4. Select the tagHeartbeat2s tag in the screen or in the HMI tags editor.
5. In the Properties pane, navigate to Events > Value change.
6. Add a new function call: sub_OnTick (or call the function with parameter tagHeartbeat2s).

7. Step 4 - Write the VBScript Body

The pattern from the original field-tested discussion (the -1 = true convention is specific to HMI tag semantics where the runtime uses -1 for a true Boolean) is preserved and extended with logging and screen I/O:

'--------------------------------------------------------------------
' sub_OnTick  -  triggered by tagHeartbeat2s on every transition
'               guarded by tagStartStop
'--------------------------------------------------------------------
Option Explicit

Dim bRun, bTick, iCount, sPath, sLine, oFSO, oFile
Dim dNow

bRun  = SmartTags("tagStartStop")      ' -1 = true in WinCC convention
bTick = SmartTags("tagHeartbeat2s")
iCount = SmartTags("tagTickCount")

If bRun = -1 Then
    ' --- user code begin ---
    dNow = Now                          ' local WinCC clock
    sLine = FormatDateTime(dNow, vbShortDate) & "," & _
            FormatDateTime(dNow, vbLongTime) & "," & _
            CStr(iCount) & "," & _
            CStr(SmartTags("tagProcessValue1")) & "," & _
            CStr(SmartTags("tagProcessValue2"))

    ' --- log to file on the panel's local storage ---
    sPath = "\Storage Card SD\Logs\telemetry.csv"
    Set oFSO = CreateObject("Scripting.FileSystemObject")
    If Not oFSO.FolderExists("\Storage Card SD\Logs\") Then
        oFSO.CreateFolder "\Storage Card SD\Logs\"
    End If
    If Not oFSO.FileExists(sPath) Then
        Set oFile = oFSO.CreateTextFile(sPath, True)
        oFile.WriteLine "Date,Time,Tick,PV1,PV2"
        oFile.Close
    End If
    Set oFile = oFSO.OpenTextFile(sPath, 8, False)   ' 8 = ForAppending
    oFile.WriteLine sLine
    oFile.Close
    Set oFile = Nothing
    Set oFSO  = Nothing

    ' --- visual feedback on screen ---
    SmartTags("tagLastTick") = dNow
End If

WinCC VBS Boolean convention: The runtime stores BOOL as a 16-bit integer internally. A Boolean "true" is represented as -1 (all bits set), not 1. Comparing against -1 is the only safe form of identity comparison; If SmartTags("x") Then also works because VBScript coerces any non-zero to true, but the explicit form is preferred when the bit pattern matters (e.g. latching a flag back to FALSE inside the script).

8. Step 5 - Wire the Start/Stop Button

Place a button on any HMI screen. Configure the Press event with the InvertBit system function on tagStartStop. The Release event should do nothing - toggling on press gives clean debounced semantics for an operator pushbutton.

If the button is wired to a physical I/O on the PLC (rather than a screen button), bind tagStartStop to the input in the HMI tag properties and set its acquisition mode to Cyclic continuous; the script guard handles the rest.

9. Step 6 - Optional: Display Date/Time with Each Sample

The script's Now function uses the panel's local clock, which is synchronized from the PLC by default. For plants where the panel is not synchronized, replace Now with reading a DTL tag from the PLC, formatted using FormatDateTime on the variant. Example PLC side:

// DB_HMI_Trigger.dtlNow : DTL  (read by HMI as DTL, 8 bytes)
"DB_HMI_Trigger".dtlNow := DTL#2024-01-01-00:00:00;  // populated from PLC clock

On the HMI, display via an Output field with format pattern yyyy-MM-dd HH:mm:ss. The script can also read this DTL and append to the log line for traceability.

10. Verification Procedure

1. Compile the HMI project and download to the TP900 (use Software (full) on first load, Delta thereafter).
2. Connect with WinCC Tag Simulation or a PLC online watch table to confirm bHeartbeat2s toggles every 2 s.
3. Set bStartStop := TRUE in the PLC; the HMI should write one line to \Storage Card SD\Logs\telemetry.csv per transition (so two lines per second, but the semantic "tick" is the 2 s boundary - script execution happens once per transition).
4. Set bStartStop := FALSE; the file should stop growing. Set it back to TRUE; logging resumes on the next 2 s transition.
5. Power-cycle the panel; confirm that on restart the script does not run because tagStartStop defaults to FALSE.

11. Performance and Timing Budget

For sub-100 ms requirements, the value-change approach breaks down. Use a C/C++ script in WinCC Professional, or move the periodic task to a PLC OB (e.g. OB35 + a small DB write) and treat the HMI as a pure display.

12. Troubleshooting Matrix

13. Edge Cases and Field-Proven Caveats

• Multiple scripts per tick: A single value-change event can only call one function. If two scripts must run at 2 s, drive them from a single function that calls both, or use two separate boolean heartbeat tags with different phases.
• Runtime Advanced vs. Comfort: WinCC Runtime Advanced on a PC supports the same VBScript syntax but adds the HMIRuntime object for system-level queries. The TP900 restricts filesystem access to \Storage Card SD\ and a few read-only paths.
• Time-zone correctness: Now on the panel returns local time. For UTC logging, use SmartTags("tagDTL_UTC") populated from RD_SYS_T on the PLC and converted with the appropriate offset.
• Log rotation: SD cards fill up. Implement a daily rename in the script using FormatDateTime(Now, vbShortDate) as part of the file name, or use the built-in WinCC logging (with a tag logging group at 2 s) which handles rotation automatically.
• Commissioning without PLC: During bench test, replace the PLC heartbeat with a WinCC internal Simulation tag that toggles via a second VBScript bound to a 100 ms scheduler. This pattern is documented in the WinCC Comfort manual chapter "Tag simulation".
• Security: VBScript has no sandbox; an oFSO.DeleteFile call in the same script can wipe the SD card. Wrap the body with try/catch and prefer Tag logging for production audit trails.

14. Alternative: WinCC Tag Logging (no VBScript required)

If the periodic action is only "record values over time", the tag logging subsystem in WinCC Comfort/Advanced is the intended path and avoids VBScript entirely. Configure a logging group with:

Use VBScript only when you need to do something the logger cannot (compute a moving average, send an email at threshold, animate a trend, etc.). For pure periodic recording, the logging system is faster, more reliable, and writes to RDB format for the Information Server.

15. Related Patterns

• Edge-triggered logging on operator action: Use the Click event of a button to call the script directly, without a heartbeat tag. Same start/stop semantics by reading a global "logging enabled" tag inside the script.
• Time-of-day trigger: Combine a clock generator (e.g. pulse at 06:00:00) with the value-change event to run a script at a fixed wall-clock time without polling.
• Multi-rate sampling: Use three heartbeat tags at 0.5 s, 2 s, 10 s and bind three different scripts. The PLC generates them all from a single OB35 tick counter.
• Remote trigger from another HMI: Wire a tag from HMI_1 to HMI_2 via the PLC and let the value-change event on HMI_2 react. Round-trip latency on Profinet is <10 ms, so the script is essentially synchronous.

16. Standards and Documentation Cross-References

• Siemens - VBScript in WinCC Comfort/Advanced (entry ID 109746537)
• Siemens - HMI Tag Acquisition Behavior (entry ID 109755224)
• Siemens - S7-1500 Clock Generators and IEC Timers (entry ID 109768637)
• Siemens - TIA Portal Help: HMI Events Configuration (entry ID 109744812)
• Siemens - SIMATIC TP900 Comfort Image Release Notes (entry ID 109781203)