If you are using the RealPort driver on your host server. Make sure the device setting on the Digi One/PortServer unit is set to prn. This can be checked by telnetting to the unit and logging in as root:
#> set ports range=*
To set the port device setting to prn for a 16 port unit:
#> set port dev=prn range=1-16
The unit should then be rebooted:
#> boot action=reset