Telnet to the Digi device/terminal server and  login as root.  Change the "dev" setting to "prn"

#>  set port range= dev=prn

A reboot of the Digi unit should be done when changing this setting:

#>  boot action=reset

Also, another process may be on the port in question

Type 'who' command at the command line of the Digi terminal server
If output shows process on a port you are troubleshooting type

#> kill tty_number (look for tty number in the first column