The RealPort driver controls communication between the RealPort driver module and your Digi devices across the network. One driver must remain in the background for each Digi device, maintaining the communication link. You should not need to start the driver by hand. The dgrp_cfg_node program does this automatically. See Managing Digi Devices with Digi RealPort Manager or Managing Digi Devices with dgrp_cfg_node.
To determine whether any RealPort drivers are currently running on your system, as well as to determine which Digi device each driver is communicating with, you can execute the following command:
ps ax | grep drpd
For more information about the RealPort driver, see the drpd man page which is installed with the driver package.