Knowledge Base FAQ Item #3100055
Q3100055: When I open a port, I get the following error: 30103 ("Unable to perform operation because the device is already opened"). What does it mean?
This means that the port you are attempting to open is already in use by another application.
Check if there's another application that is already using the COM port. In some situations, you may want to benefit from a thing called 'port sharing', where one port can be shared by multiple applications. To benefit from 'port sharing', you must use Windows Telephony devices (like 'Standard 9600 bps Modem').(viewed: 1777)