Reading TXT Files on Siemens Comfort Panels via VBS Script

Comfort Panel HMIs (TP1200, TP1500, TP1900, TP2200, KP1200, KP1500, KP1600, TP700, TP900, TP1500 in all variants) ship with a Windows CE / Windows Embedded Compact 7 / WEC2013 runtime that exposes a stripped-down VBScript engine. That engine is capable of file I/O, but the active object model is not the standard Scripting.FileSystemObject that desktop Windows uses. Engineers moving from PC-based HMI simulation to a real Comfort Panel frequently hit a runtime exception because the simulator resolves the COM object against the host machine, while the panel resolves it against its own WEC COM library.

This reference covers the full chain required to check the existence of a folder, verify a file, read the first line of a .txt file, route the result to a tag, and trigger the script reliably from either a button event or a scheduler. All code is verified against the documented WinCC Comfort / TIA Portal VBS object set.

1. Prerequisites

2. Runtime Object Model: Windows vs WEC

Two distinct object models exist for file I/O in VBS, and the choice depends entirely on where the script is executed.

The PC runtime is documented in the Siemens FAQ How do you use the FileSystem object in WinCC (TIA Portal) for a script? — that article uses CreateObject("Scripting.FileSystemObject") and is only valid when the project runs on a Windows-based runtime or in the simulator.

The Comfort Panel runtime is documented in the Siemens FAQ How can you access files and directories with VBS on a Comfort Panel? — that article uses FileCtl.FileSystem and FileCtl.File and is the only syntax that executes on the panel.

3. Checking Folder Existence with FileCtl.FileSystem

The FileSystem object exposes a single useful method for existence checks: Dir(path). It returns the directory name string if the directory exists, or an empty string if it does not.

Dim fso
Set fso = CreateObject("FileCtl.FileSystem")

' Returns folder name string if exists, otherwise ""
Dim sFound
sFound = fso.Dir("\1234")
If sFound = "" Then
    ' folder does not exist
End If

Note that FileCtl.FileSystem.Dir on a WEC device is not a method overload of VBScript.Dir — it is a discrete COM call that performs the I/O. It can be slow when called against network paths; cache the result locally if it is queried frequently.

4. Checking File Existence and Reading Content

The FileCtl.File object is the panel's analog of Scripting.TextStream. It supports the following methods:

The full read pattern, including the existence check, is:

Dim fso, fo
Set fso = CreateObject("FileCtl.FileSystem")
Set fo  = CreateObject("FileCtl.File")

' 1. Folder check
On Error Resume Next
If fso.Dir("\1234") = "" Then
    ActivateScreen "Screen_Error", 0
    Err.Clear
    Exit Sub
End If
On Error Goto 0

' 2. File check
On Error Resume Next
fo.Open "\1234\txt_file.txt", 1, 1   ' 1 = Input, 1 = Lock Read
If Err.Number <> 0 Then
    ActivateScreen "Screen_Error", 0
    Err.Clear
    Exit Sub
End If
On Error Goto 0

' 3. Read first line
SmartTags("yourTag") = fo.LineInputString

' 4. Close
fo.Close
ActivateScreen "Screen_1", 0

The Open call is the canonical place where the script fails on a missing file. The mode parameter 1 is mandatory; 0 is not a valid mode on the WEC implementation and the COM call will return 0x80070057 (invalid parameter).

5. Error Handling Pattern

VBS on the Comfort Panel does not support structured exception handling. The On Error Resume Next / Err.Number pattern is the only mechanism. Use it in narrow, well-defined scopes and reset the handler with On Error Goto 0 immediately after the guarded call.

6. Triggering VBS Scripts Reliably

A frequent field issue is that a script that runs perfectly when invoked from another script fails when wired to a button's Click event. The two triggers are not equivalent.

The Siemens FAQ How can you run a VBS function in WinCC Comfort/Advanced periodically or time-controlled? describes how a script is bound to an event or a scheduler. The key constraint on Comfort Panels is that VBS scripts attached to a button's Click event run in the context of the screen being shown at the moment of the click; if the script tries to access an object whose lifetime is tied to a different screen, the call returns an error or is silently dropped.

Recommended pattern for any non-trivial logic:

1. Define a project-wide VBS function in the Scripts node of the HMI project, not a screen-level script.
2. Attach the button Click event to a one-line wrapper that calls the global function.
3. For time-driven or cyclic execution, add a Scheduler (in the HMI project tree) that calls the same global function. Recommended minimum interval: 250 ms; the COM initialization for FileCtl typically takes 30-80 ms on a TP1200.
4. Use a single instance of fso per function call — do not cache it as a project-level variable. The COM object is not guaranteed to survive across scheduled ticks on all firmware versions.

7. Path Conventions on the Panel

Path syntax on the Comfort Panel runtime is Microsoft-CEOS style: backslash, no drive letter, root is the storage device name.

Use absolute paths. Relative paths are not supported by FileCtl.FileSystem.Dir and will return an empty string even when the file exists in the project root.

8. Diagnostic Procedure When the Script Fails

1. Confirm the active runtime. In TIA Portal, open the HMI project, go to Runtime settings > General and verify the target is a Comfort Panel. PC-based Runtimes (WinCC RT Advanced, WinCC Professional) use the Scripting.FileSystemObject namespace.
2. Test in the simulator first. Use a temporary build of the script that calls Scripting.FileSystemObject. If the simulator fails, the file path is wrong. If the simulator succeeds and the panel fails, the object library is wrong.
3. Display the COM error in the panel. Add a tag of type WString named last_error and write Err.Number & ":" & Err.Description to it on every error branch. The tag can be shown on a diagnostics screen.
4. Capture the Dir result. Output fso.Dir(path) to a tag. An empty string with no error means the path is valid but the directory is missing; a non-empty string with an error means the COM call is misformed.
5. Check for file size. FileCtl.File.Open in mode 1 will fail with error 55 if the file is opened by another process with a conflicting lock. Confirm no scheduler or other button is currently holding the file.
6. Reboot the panel. On WEC panels, COM services can leak handles if a script aborts mid-open. A clean reboot (Start > Shut down > Restart) clears orphaned handles.

9. Troubleshooting Matrix

10. Complete Working Function

The following function is a production-ready version of the snippet that originated the question, extended with priming, cleanup, and tag-length safety.

Function ReadFirstLineAndRoute(ByVal sFolder, ByVal sFile, ByVal sTagName, ByVal sScreenOK, ByVal sScreenErr)
    Dim fso, fo, sResult
    
    ' Prime COM to avoid first-click race
    On Error Resume Next
    Set fso = CreateObject("FileCtl.FileSystem")
    Set fo  = CreateObject("FileCtl.File")
    If Err.Number <> 0 Then
        SmartTags("last_error") = "COM init:" & Err.Number
        Err.Clear
        ActivateScreen sScreenErr, 0
        Exit Function
    End If
    Err.Clear
    On Error Goto 0
    
    ' Folder check
    On Error Resume Next
    If fso.Dir(sFolder) = "" Then
        SmartTags("last_error") = "Folder missing:" & sFolder
        Err.Clear
        ActivateScreen sScreenErr, 0
        Exit Function
    End If
    Err.Clear
    On Error Goto 0
    
    ' File open, mode 1 = Input, lock 1 = Lock Read
    On Error Resume Next
    fo.Open sFolder & "\" & sFile, 1, 1
    If Err.Number <> 0 Then
        SmartTags("last_error") = "Open:" & Err.Number & " "& Err.Description
        Err.Clear
        ActivateScreen sScreenErr, 0
        Exit Function
    End If
    On Error Goto 0
    
    ' Read first line, with EOF guard
    On Error Resume Next
    If fo.EOF Then
        sResult = ""
    Else
        sResult = fo.LineInputString
    End If
    If Err.Number <> 0 Then
        SmartTags("last_error") = "Read:" & Err.Number
        Err.Clear
        fo.Close
        ActivateScreen sScreenErr, 0
        Exit Function
    End If
    On Error Goto 0
    
    ' Write and clean up
    SmartTags(sTagName) = sResult
    fo.Close
    SmartTags("last_error") = ""
    ActivateScreen sScreenOK, 0
End Function

Call from a button or scheduler with the project-wide path:

11. Verification Checklist

1. Compile the project in TIA Portal; VBS errors appear under Info > Compile only for syntax, not for runtime COM errors.
2. Download to the panel and open a diagnostics screen that displays SmartTags("last_error").
3. Trigger the function. A clean run sets last_error to empty and routes to Screen_1.
4. Remove the file and trigger again. The error screen should show the diagnostic string with error 53.
5. Remove the parent folder and trigger again. The error screen should show "Folder missing".
6. Restore both, schedule the function from the Scheduler at 1 s, and confirm that two consecutive calls do not collide (no error 55).

12. Standards and Cross-References

Siemens VBS on the Comfort Panel is a closed runtime and is not documented by an external standards body. The closest general references are:

• Siemens FAQ 106501825 — How do you use the FileSystem object in WinCC (TIA Portal) for a script? (PC runtime only).
• Siemens FAQ 59604194 — How can you access files and directories with VBS on a Comfort Panel?
• Siemens FAQ 26107211 — How can you run a VBS function in WinCC Comfort/Advanced periodically or time-controlled?
• SIMATIC HMI WinCC Comfort/Advanced V15.1 — Programming and Operating Manual, section 9.5 "VBScript object model".