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

Stephen H. Smith wa8lmf2 at aol.com
Mon Aug 29 01:43:10 UTC 2011


On 8/28/2011 4:52 PM, Tom Russo wrote:
> There is no mention of a classic RS-232 serial port (required to connect to a 
> Kenwood), nor any mention of FMI support (that allows the USB interface to 
> "morph" into a serial port when a special cable is attached).
> FMI and "morph to serial" are separate things.

The FMI interface IS serial, but with a screwball proprietary binary data 
format rather than vanilla NMEA ASCII.

>
> Several of the handheld usb-only Garmin GPS units will indeed "morph to serial
> NMEA" with just the right cable, even though they don't talk FMI.  If you look
> at the Montana's spec  says its interface is "USB and NEMA 0183 compatible,"

One can have NMEA-0183-format ASCII data over USB rather than serial -- many 
USB faceless hockeypuck GPSs do exactly this .    This is what I assumed they 
meant in the Montana description.


> and on its "accessories" page there is a USB "Serial Data/Power" cable
> https://buy.garmin.com/shop/shop.do?pID=26668
> with bare wires to access the serial data.  This cable is compatible with
> several of the devices other than the Montana, including the Colorado, Oregon,
> and GPSMAP 62 series.
>
> 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.

This is how the USB-to-FMI-serial interface morphing works also.   It looks as 
through the complexity of the Garmin USB interfaces has just doubled,  with the 
magic USB-data-line pullup cable on some devices waking up FMI  while the 
pullup wakes up generic NMEA serial on others.

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