From GpsWiki

While the PN-20 has an RS232 output and outputs NMEA position information, the PN-40 lacks a serial output (USB only) and so cannot provide a serial NMEA stream. Although one cannot provide an NMEA stream with the PN-40 directly to a device expecting a serial NMEA stream, DeLorme provides a serial emulation driver that runs on a computer and can provide a serial NMEA stream in turn.

DeLorme describes the emulator as follows:

"The DeLorme Serial Emulator is a system tray utility that provides access to the settings and status of an Earthmate reciever. You can also use the Configure Virtual COM Ports utility to assign up to three different COM Ports for the Earthmate GPS reciever. The Earthmate GPS reciever can be configured to work with up to three different applications at the same time."

[DeLorme Serial Emulation Driver for Earthmate® GPS Receivers]