Garmin GPSMAP 62/78 Series Serial Interface
Information about the serial interface used on the Garmin GPSMAP 62 and 78 series GPS receivers.
The only I/O connector on these GPS devices is a mini USB socket, which is used for both USB mass storage access and serial (NMEA, RTCM, etc) data. Connecting the device to a computer using a standard USB cable causes it to enter mass storage mode. A special serial data/power cable, Garmin part https://buy.garmin.com/shop/shop.do?pID=26668 010-11131-00
, is used to access the serial interface.
Serial data formats
In the device's system setup menu, the serial interface can be set to one of 5 different modes.
This is the standard Natrional Marine Electronics Association 0183 protocol used by virtually all serial-connected GPS receivers. It can also be used with computer software such as
. This protocol always operates at 4800 baud. Update rate is 2 seconds.
sentences are always transmitted.
, and Garmin proprietary sentences can also be enabled.
The device can also accept NMEA input in this mode, including
to update waypoint information (not fully tested).
This is a proprietary Garmin protocol.
This protocol, also used by Garmin's USB-based OEM modules, is designed to be consumed by the Garmin Spanner program, which converts it to standard NMEA which it echoes to a virtual serial port that other applications can connect to (a less elegant version of
for the Windows universe).
This is a simplified text-based format containing only basic position information. It is presumably easier to parse than NMEA
The Radio Technical Commission for Maritime Services (RTCM) SC-104 protocol is used for input from a Differential GPS beacon receiver to improve GPS accuracy.
The electrical interface at the GPS receiver side is unknown.
At the other end, the data/power cable terminates in 5 bare wires, carrying 8-36V DC power, serial data in and out, and power and serial ground connections. The power supply wire includes an inline 750mA fuse. Serial output is sent with RS-232 compliant voltages (measured at approx. ±5V).
An adapter circuit board is located near the USB side of the cable. Its functionality is not yet known.
Argent Data Systems
sells a cable
for use with APRS tracking devices. It is said to work with the Garmin Colorado, but will probably support other models using the same connector. According to their specifications, it uses a resistor connected to the 5th pin on the Mini B connector (a custom connector is required since most cables do not connect this pin) to signal the GPS to send TTL-level serial data.