viVPrintf/viVScanf Example using String

Sub Test_String()
' Using viVScanf and viVPrintf in Visual Basic 6 is tricky because
' of the way VB6 passes arguments on the stack. The 'vararg'
' argument to the data needs to be a 'byRef' pointer -- which means
' it needs to be a pointer to a pointer.
'
' VB treats string arrays passed to external DLL's as a special case
' so when a string is passed, it ends up being passed correctly as a
' pointer to a pointer.
'
' Numeric arrays, on the other hand, cannot be passed directly. To
' pass a pointer to the numeric array pointer, we create another
' variable which we set to the address of the start of the array
' and pass that variable 'byRef' to the function.
'
' Declare variables used by VISA
'
Dim err As Long
Dim drm As Long
Dim vi As Long
Dim buf As String
'
' Open a default resource manager session and open the device
'
err = viOpenDefaultRM(drm)
err = viOpen(drm, "GPIB2::2::INSTR", 0, 0, vi)
'
' ==================================================
' Read a string array using viVScanf
' ==================================================
'
Dim buf256 As String * 256 ' pre-allocated, string
Dim posn As Long
'
' Tell the instrument to send its IDN string
'
err = viVPrintf(vi, "*IDN?" + vbLf, 0)
'
' Read the string from the instrument
'
err = viVScanf(vi, "%t", buf256)
'
' Truncate the returned string at the first null
' (A null terminated string is returned)
'
posn = InStr(buf256, String(1, 0))
If (posn > 0) Then
buf = Left(buf256, posn - 1)
Else
buf = buf256
End If
'
' display the string that was read from the instrument
'
MsgBox buf, 0, "viVScanf of a String"
'
' Close the VISA sessions
'
err = viClose(vi)
err = viClose(drm)
End Sub

See Also

viVPrintf, viVScanf Using the VISA C API in Microsoft Visual Basic 6