BUG: eVB:MSCEComm:Unable to Get a Dialtone Using Built-in Modem (266215)

CAUSE

The reason is that the MSCECOMM control waits to receive a EV_TXEMPTY signal when the output buffer reaches 0. The control uses this signal to ensure that it does not cause an outbuffer overflow, which would cause data loss. Some of the drivers used with the built-in modems do not send this signal.

RESOLUTION

To workaround this problem, add the following code in the code window of Form1, for the sample created under Steps to Reproduce Behavior:

Declare Function WriteFile Lib "Coredll" _
    (ByVal hFile As Long, ByVal lpBuffer As String, _
    ByVal nNumberOfBytesToWrite As Long, _
    lpNumberOfBytesWritten As Long, _
    ByVal lpOverlapped As Long) As Long

Public Sub SendData(ByVal hCommID As Long, sData As String)
    Dim lRet, i, iWrite
    For i = 1 To Len(sData)
        lRet = WriteFile(hCommID, ChrB(Asc(Mid(sData, i, 1))), 1, iWrite, 0)
    Next
End Sub

Next replace the Command1_Click event code with the following:

Private Sub Command1_Click()
    SendData Comm1.CommID, "ATDT" & vbCrLf
End Sub

Now run the project on the device and click on the CommandButton. You should now hear a dialtone.

MORE INFORMATION

Steps to Reproduce Behavior

Start a new Windows CE HPC PRO Project. Form1 is created by default.
On the Project menu, click to select Components. Select Microsoft CE Comm Control 3.0 and click OK.
Add a CommandButton and a Comm control to Form1.

Paste the following code into Form1:

Option Explicit

Private Sub Command1_Click()
    Comm1.Output = "ATDT" & vbCrLf
End Sub

Private Sub Form_Load()
    Comm1.Handshaking = comRTS
    Comm1.RThreshold = 1
    Comm1.CommPort = 6
    Comm1.PortOpen = True
End Sub

Private Sub Form_Unload(Cancel As Integer)
    If Comm1.PortOpen Then Comm1.PortOpen = False
End Sub

To determine which Comm port the modem is using use the Remote Registry Editor to look at the HKEY_LOCAL_MACHINE\ExtModems key. You will want to make sure that you use the "built-in" modem.

Run the application on the device and click on the CommandButton. You would expect to hear a dialtone.