Garmin GPSMAP 62/78 Series Serial Interface

Information about the serial interface used on the Garmin GPSMAP 62 and 78 series GPS receivers.

Physical connections

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.

NMEA In/Out

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 gpsd. This protocol always operates at 4800 baud. Update rate is 2 seconds.

The basic $GPRMC, $GPGGA, and $GPGLL sentences are always transmitted. $GPGSA, $GPGSV, $GPRTE, $GPWPL, and Garmin proprietary sentences can also be enabled.

The device can also accept NMEA input in this mode, including $GPWPL and $PGRMW to update waypoint information (not fully tested).

Garmin Serial

This is a proprietary Garmin protocol.

Garmin Spanner

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 gpsd for the Windows universe).

Text Out

This is a simplified text-based format containing only basic position information. It is presumably easier to parse than NMEA

RTCM

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.

Electrical interface

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.

References

• Garmin - Device Interface SDK
• Argent Data Systems - USB Mini-B R/A 5-Conductor Pigtail

-- StephenCavilia - 2010-09-20