Resolving VBS Unit Access Inside a WinCC Screen Window

A WinCC Professional screen window is not the loaded screen itself. Access the embedded I/O field through the screen window’s Screen property, and do so only after the window is visible. Attempting access while it is invisible can raise Object required: 'screenWindow.Screen'.

Root cause: the embedded screen is unavailable

HMIRuntime.ActiveScreen.ScreenItems("test_settings_window") returns the Screen Window object. Calling ScreenItems directly on that object omits the embedded screen level. The required object path is screenWindow.Screen.ScreenItems("SP_field").

Object availability also depends on execution order. The reported failure occurred when the script tried to resolve screenWindow.Screen while the Screen Window was invisible. Visibility therefore must be established before retrieving the embedded field.

Correct the VBS object path and execution order

Dim data_udt
Dim face_plate
Dim sp_IO_field

data_udt = "changing_pressure_SP_test_DB"
Set face_plate = HMIRuntime.ActiveScreen.ScreenItems("test_settings_window")

If face_plate.Visible = True And face_plate.TagPrefix = data_udt Then
    face_plate.Visible = False
Else
    face_plate.Visible = False
    face_plate.TagPrefix = data_udt
    face_plate.CaptionText = data_udt
    face_plate.Visible = True

    Set sp_IO_field = face_plate.Screen.ScreenItems("SP_field")
    sp_IO_field.Unit = SmartTags("unit")
End If

The critical sequence is to assign the UDT prefix, make the Screen Window visible, resolve the loaded screen through .Screen, and then write the I/O field’s Unit property.

Apply and verify the change

1. Confirm that test_settings_window is the Screen Window name and SP_field is the I/O field name inside its loaded screen.
2. Save and recompile the project.
3. Restart the graphics runtime or change screens once so the updated script is loaded.
4. Open the faceplate with different UDT data and verify that both the process value and unit change.

Diagnose remaining VBS failures

Add the print job/script diagnose control to the picture and set its property to gsc-diagnose to display VBS errors. Add HMIRuntime.Trace statements before and after the visibility change and object lookup to identify the exact statement that stops execution.

HMIRuntime.Trace "Making screen window visible"
face_plate.Visible = True
HMIRuntime.Trace "Resolving SP_field"
Set sp_IO_field = face_plate.Screen.ScreenItems("SP_field")
HMIRuntime.Trace "SP_field resolved"

FAQ