Use the P4 property to configure a driver. Check driver's documentation to correctly set its parameters. This property cannot be modified after communication starts. Example:

Sub Driver1_BeforeStart()
  ' Driver1 is an object of type I/O Driver
  DriverLocation = "c:\driver\plc.dll"
  P1 = 2
  P2 = 1
  P3 = 9600
  P4 = 500
End Sub