I believe www.byonics.com sells a small unit you place inline that converts the Rockwell binary stream to an NMEA stream.
I just bought it (NOT the USB one) and hyperterminal only sees the string "EARTHA".
I now read that performing the famous Tripmate hack (feeding the outgoing pin back into the incoming pin) won't work, since data is not even NMEA compliant.
Wish I read it before.
Does anybody own a serial Earthmate working in NMEA with third-party software (as opposed to provided Street Atlas that reads the Rockwell binary protocol) ?