The tripmate requires a special sequence to be sent from the PC to turn it on (/astral, if I recall). There is a mod to make it work properly: http://www.kd5om.com/tripmate.htm
I had a lot of problems when I was messing around with one a few years back. I wanted to use it with Tardis 2000 (a program that sets PC clocks), but had no luck making it work consistantly, even after working with the author.
I haven't used the Earthmate, but I recall it is only a binary only (let me check: yep. here's a link to an adaptor http://www.byonics.com/gst-1/
Good luck getting it to work. I like the Earthmate for some applications, but don't really need one now that the price of a basic GPS with display is not much more than the earthmate.