HOWTO: Identifying which process is using the serial port. Print ZTN1838 ID: ZTN1838 This Zetafax technical note applies to: Zetafax ( all versions ) Summary Zetafax with serial attached modems requires exclusive access to the serial port. Due to the nature of serial port communications, only one application can control a serial device at once. If another process is attached to the serial port, this will cause Zetafax to fail due to a port locking conflict. More information The Zetafax server will fail to open the serial port if another process is attached to the serial port. This will cause the Zetafax server not to start correctly. The Sysinternals Process Explorer application from Microsoft can be used to identify the process which is locking the serial port. It is available from Microsoft here: http://technet.microsoft.com/en-us/sysinternals/bb896653 Simply open the Process Explorer application and search for the term " serial" by using ctrl-f. In the above example the user had left open putty on COM1. This would stop Zetafax from being able to open COM1. We now know to kill putty.exe (PID:3024)using Task Manager to end the process and unlock the port. References PRB: Modem trouble shooting guide for Zetafax Last updated: 05 April 2011 (AF/MW)