Garmin FMI and NMEA Protocols
There is quite a bit of confusion surrounding Garmin's GPS products and their compatibility with APRS and other devices. This article will attempt first to answer the most common question of what cable is needed and what combinations will work together, and then to explain the technical details in more depth.
Choosing the Right Cable
NMEA 0183 is the standard used by most GPS receivers to send data, including position, velocity, time, and signal quality information. Most of Garmin's chart plotters, OEM sensors, and older handheld devices have an NMEA output, usually through a proprietary 4-pin connector. Many of their USB-capable devices, on the other hand, have no NMEA output at all and can interface only with a USB host - usually a PC.
To address the needs of commercial fleet operators, Garmin added the Fleet Management Interface (FMI) to some of their automotive products. FMI does not use NMEA, but it does provide a serial interface - usually through the USB connector - that can be used by some devices. Our Tracker2 and Tracker3 series have FMI support and as such only need an appropriate FMI cable. Garmin maintains a list of FMI compatible devices and which cable they require.
Devices expecting NMEA data, like the Kenwood TM-D710 and TH-D72, can not use FMI directly. However, our GTRANS product serves as a protocol translator and will convert between FMI and NMEA. Our regular GTRANS series takes the place of the Garmin 010-10813-00 FMI cable and connects directly to the Kenwood radios, Yaesu FTM-350, Uniden Home Patrol 1, and other NMEA devices. The GTRANS2 series takes the place of the Garmin 010-11232-10 (FMI 15) cable.
Additionally, some of Garmin's newer handheld GPS receivers have only a USB connector, but can put out NMEA data using the Garmin 010-11131-00 cable. This is not an FMI cable or a protocol translator. It's a simple voltage level converter and the NMEA functions are handled by the GPS receiver itself. With the proper connector, this cable can be connected to most APRS-capable radios and trackers.
FMI is built on top of Garmin's older proprietary serial interface protocol. Like the old protocol, it runs at 9600 baud. All FMI packets are encapsulated within the old protocol. However, some of the legacy protocol's functions are also supported by FMI-capable devices. This includes the PVT (position, velocity, and time) mesage. PVT can be enabled independently of the FMI functions. Also, some of the earlier FMI-capable devices also supported (possibly by accident) the old waypoint creation protocol. It's this protocol that the Tracker2 and Tracker3 devices use to plot stations on the map.
The physical interface is through a USB mini-B connector on most devices. The ID pin is used to identify the cable connected, using a resistor in the FMI cable's connector. For the 010-10813-00 cable this is an 8.2k ohm resistor to ground. In the FMI 15 cable it's 12k. The 010-10813-00 cable contains an inverter and RS-232 transceiver that adapts the low-voltage signals to RS-232 levels and polarity. The FMI 15 cable also includes a DC-DC converter to provide 5v power to the GPS unit.
The GTRANS cable, when used with FMI 2.0 and earlier devices, does not place the attached device into FMI mode. This avoids changing the menn layout of the device. For FMI 2.5 and above, however, the newer FMI-based waypoint creation protocol is used, and this requires placing the device into FMI mode.