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[aprssig] advice on purchesing GPS for use with D-710

Scott Miller scott at opentrac.org
Wed Nov 2 22:36:25 UTC 2011

> I believe that the way these devices work (Scott Miller can correct me here
> if I'm mistaken) is that they detect a certain resistance in the cable between
> certain pins, and switch from USB to plain RS232 serial protocol on the data
> pins.
> FMI, on the other hand, is a specific fleet management protocol that enables
> appropriately written software to control (and be controlled)  by the GPS,
> and it is usually present only in devices intended for vehicle use.  The
> Montana is targeted for handheld use, particularly for hiking.  It is not,
> as I understand it, specific to whether the actual communication is done
> over RS232 or USB.  I believe the FMI cables generaly *do* also kick the
> device into serial mode, though.  Again, Scott Miller can pipe in and
> correct me if I've misunderstood him or the various FMI specs I've read.

Sorry, I'm a couple of months behind on APRSSIG traffic.  Yes, various 
Garmin cables identify themselves using a resistor between ground and 
the 'X' pin on the mini-B connector.  Some models (Colorado 300, for 
example) will put out NMEA with the right resistor present.  Probably at 
LVTTL levels - there's a level shifter in the cable.

FMI runs at 9600 baud, and it's built on the old Garmin binary protocol. 
  The GTRANS cable we sell speaks FMI and does translation of the 
important stuff (but not things like signal strength) to NMEA, and 
translation of waypoints (NMEA and Kenwood) back to FMI.  It'll also 
translate KPC-3 style APRS packets (and also packets from the FTM-350's 
TNC in the latest firmware) to waypoints as well, which makes it usable 
with some dumb TNCs.

As far as I know, no FMI capable units have NMEA output on their own.


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