PRB: Run-Time Error 8015 If You Try to Open the Serial Port with Microsoft Comm Control

To work around this behavior, create a new virtual COM port and use it instead of COM1.

WARNING: If anything else prints on the printer attached to COM1, your program may not work as expected.

1. Add the following code to the General Declarations section of Form1:

           Private Declare Function DefineDosDevice Lib "kernel32" Alias _
               "DefineDosDeviceA" (ByVal dwFlags As Long, ByVal lpDeviceName _
               As String, ByVal lpTargetPath As String) As Long
           Const DDD_RAW_TARGET_PATH = &H1
   

2. Add another Command button to Form1.
3. Add the following code in the Click event of Command2:

   DefineDosDevice DDD_RAW_TARGET_PATH, "COM5", "\Device\Serial0"
   

4. Change the code on Command1 to open COM5:

   MSComm1.CommPort = 5
   

5. Run the program, click Command2, and then click Command1.

Steps to Reproduce the Behavior

1. Install a Generic Text printer, and then connect it to COM1 instead of LPT1.
2. Start a new Standard EXE project in Microsoft Visual Basic. By Default, Form1 is created.
3. Add the Microsoft Comm Control to the project.
4. Add a new MSComm1 control on Form1.
5. Add a new Command button on Form1, and then insert the following code in the Click event:

   MSComm1.CommPort = 1
   MSComm1.Settings = "9600,N,8,1"
   MSComm1.Handshaking = comNone
   MSComm1.InBufferSize = 1024
   MSComm1.OutBufferSize = 512
   MSComm1.RThreshold = 0
   MSComm1.SThreshold = 0
   MSComm1.InputLen = 0
   MSComm1.PortOpen = True
   

6. Run the program, and then click Command1.