Working with the Garmin Interface

Table of Contents


All Garmin gps receivers support a computer interface. This can be used to backup the waypoints and routes on a computer and to provide real time display information on a computer screen. In addition Garmin supports differential gps input signals. The modes supported by Garmin receivers are given in the table below.

All of the Interface Modes
Interface Mode 38/40/45XL 12/12XL/48 III/III+ E-map E-trex Notes
None Y Y Y Y Y .
None/NMEA Y Y Y Y Y NMEA out 0183 2.0
Garmin DGPS Y Y Y Y Y Has Garmin Interface control
RTCM/None Y Y Y Y Y DGPS input only
Text Out N N III+, 12Map Y Y 1200 2400 4800 9600
RTCM/Text N N III+, 12Map Y Y .
NMEA 0180, 0182, 0183 1.5 Y Y N N N .

Be sure that you have selected the correct mode and baud rate for the program or unit you are trying to interface with. This is the main problem with interface failures. For all moving map programs you will likely need NMEA mode with the baud rate set to 4800. For programs that upload and download data you should probably be in Garmin mode with the baud rate set to 9600. Some digital cameras will need text mode with a baud rate of 9600. In all cases set the data width to 8, no parity, and 1 stop bit. Make sure the interface mode is selected in the program and the correct com port is selected.

Hardware Connection

The hardware interface for Garmin units meets the NMEA requirements and is sufficient to drive 3 NMEA loads. It is also compatible with most computer serial ports using RS232 protocols. The interface speed can be adjusted as needed by the particular interface but it usually set automatically to the appropriate setting when the interface selection is made. There is only a data in and data out line with ground. There are are no handshake lines nor should you attempt to set up a software handshake using xon/xoff as the unit does not recognize this and may interfere with binary data uploads and downloads.

In order to use the hardware interface you will need a cable. See the accessories chapter for the available cables. In some dedicated applications you may need to wire your own or perhaps you would just prefer to do that. The Garmin cable connector shown below will work for all of the handheld gps units except the etrex and emap.

Garmin gps receivers may be used to interface with other NMEA devices such as autopilots, fishfinders, or even another gps receivers. They can also listen to Differential Beacon Receivers that can send data using the RTCM SC-104 standard.

Some of the latest computers no longer include a serial port but only a USB port. Garmin receivers are known to work with Serial to USB adapters and serial ports attached via the pcmcia (pc card) adapter.


The National Marine Electronics Association has developed a specification that defines the interface between various marine electronic equipment. The standard permits marine electronics to send information to computers and to other marine equipment. GPS receiver communication is defined within this specification. Most computer programs that provide real time position information understand and expect data to be in NMEA format. This data includes the complete PVT (position, velocity, time) solution computed by the GPS receiver. The idea of NMEA is to send a line of data called a sentence that is totally self contained and independent from other sentences. There are standard sentences for each device category and there is also the ability to define proprietary sentences for use by the individual company. All of the standard sentences have a two letter prefix that defines the device that uses that sentence type. For gps receivers the prefix is GP. This is followed by a three letter sequence that defines the sentence contents. In addition NMEA permits hardware manufactures to define their own proprietary sentences for whatever purpose they see fit. All proprietary sentences begin with the letter P and are followed with a letter that identifies the manufacturer controlling that sentence. For Garmin this would be a G.

Each sentence begins with a '$' and ends with a carriage return/line feed sequence. The data is contained within this single line with data items separated by a comma. The data itself is just ascii text and may extend over multiple sentences in certain specialized instances but is normally fully contained in one variable length sentence. An example sentence might look like:


With an interpretation as follows:

     GGA - Global Positioning System Fix Data
     123519       Fix taken at 12:35:19 UTC
     4807.038,N   Latitude 48 deg 07.038' N
     01131.000,E  Longitude 11 deg 31.000' E
     1            Fix quality: 0 = invalid
                               1 = GPS fix
                               2 = DGPS fix
     08           Number of satellites being tracked
     0.9          Horizontal dilution of position
     545.4,M      Altitude, Meters, above mean sea level
     46.9,M       Height of geoid (mean sea level) above WGS84
     (empty field) time in seconds since last DGPS update
     (empty field) DGPS station ID number
     *42          the checksum data, always begins with *
Each Data type would have its own interpretation that is defined in the NMEA standard. This particular sentence provides essential fix data. Other sentences may repeat some of the same information but will also supply new data. Whatever is reading the data can watch for the data sentence that it is interested in and simply ignore whatever sentences that is doesn't care about. In the NMEA standard there are no commands to indicate that the gps should do something different. Instead each receiver just sends all of the data and expects much of it to be ignored. On NMEA input the receiver stores information based on interpreting the sentence itself. While some Garmin receivers accept NMEA input this can only be used to update a waypoint or similar task and not to send a command to the unit. There is no way to indicate whether the sentence is being read correctly or to re-send some data you didn't get. Instead the receiving unit just checks the checksum and ignores the data if the checksum isn't correct figuring it will be sent again sometime later. No error can be generated to the remote system.

The NMEA standard has been around for many years and has undergone several revisions. The protocol has changed and the number and types of sentences may be different depending on the revision. All Garmin receivers understand the latest standard which is called: 0183 version 2.0. This standard dictates a transfer rate of 4800 baud. Some Garmin receivers also understand older standards. The oldest standard was 0180 followed by 0182 which transferred data at 1200 baud. An earlier version of 0183 called version 1.5 is also understood by some Garmin receivers. Some Garmin units can be set to 9600 for NMEA output but this is only recommended if you have determined that 4800 works ok and then you can try to set it faster.

If you are interfacing a Garmin unit to another device, including a computer program, you need to insure that the receiving unit is given all of the sentences that it needs. If it needs a sentence that Garmin does not send then the interface to that unit is likely to fail. The sentences sent by Garmin receivers include:

NMEA 2.0

NMEA 1.5 - some units do not support version 1.5

In addition Garmin receivers send the following Proprietary Sentences: The new etrex summit sends a $HCHDG sentence for its compass output.

This list is specific to the handheld units. Other Garmin units may send other sentences and some use proprietary sentences to send control commands to the units themselves. Note that Garmin converts lat/lon coordinates to the datum chosen by the user when sending this data. This is indicated in the proprietary sentence PGRMM. This can help programs that use maps with other datums but is not a NMEA standard. Be sure and set your datum to WGS84 when commuicating to other NMEA devices.

It is possible to just log view the information presented on the NMEA interface using a simple terminal program. If the terminal program can log the session then you can build a history of the entire session into a file. More sophisticated logging programs can filter the messages to only certain sentences or only collect sentences at prescribed intervals. Some computer programs that provide real time display and logging actually save the log in an ascii format that can be viewed with a text editor or used independently from the program that generated it.

NMEA has its own version of essential gps pvt (position, velocity, time) data. It is called RMC, The Recommended Minimum, which might look like:


With an interpretation as follows:

     RMC          Recommended Minimum sentence C
     123519       Fix taken at 12:35:19 UTC
     A            Status A=active
     4807.038,N   Latitude 48 deg 07.038' N
     01131.000,E  Longitude 11 deg 31.000' E
     022.4        Speed over the ground in knots
     084.4        Track angle in degrees True
     230394       Date - 23rd of March 1994
     003.1,W      Magnetic Variation
     *43          The checksum data, always begins with *

NMEA input

Many of the Garmin units also support an NMEA input mode. While not too many programs support this mode it does provide a standardized way to update or add waypoint and route data. Note that there is no handshaking or commands in NMEA mode so you just send the data in the correct sentence and the unit will accept the data and add or overwrite the information in memory. If the waypoint name is the same you will overwrite existing data but no warning will be issued. The sentence construction is identical to what the unit downloads so you can, for example, capture a WPL sentence from one unit and then send that same sentence to another unit but be careful if the two units support waypoint names of different lengths since the receiving unit might truncate the name and overwrite a waypoint accidently. If you create a sentence from scratch you should create a correct checksum. Be sure you know and have set you unit to the correct datum. A WPL sentence looks like:

With an interpretation of:

     WPL         Waypoint Location
     4807.038,N  Latitude
     01131.000,E Longitude
     WPTNME      Waypoint Name
     *31         The checksum data, always begins with *

Garmin mode

Garmin mode is a bi-directional binary proprietary interface protocol that is used by Garmin and many third party vendors do communicate directly with a Garmin receiver. All of the handheld units understand Garmin protocol but may not understand or respond to a specific command in that protocol. For example, if you were to try and store altitude information in a waypoint on a unit that cannot store altitude information then this command would fail. Garmin mode includes a set of published API (Application Programming Interface) specifications and other commands that are not published or made public in any fashion except as used by a Garmin product. There are other commands that are not even used by a Garmin product and are probably used by internal test groups and custom test equipment at the Garmin factory. It is beyond the scope of this manual to describe the detailed interface specification. A manual describing the published interface specifications is available for downloading at the Garmin site. In addition some of the undocumented commands and features are available from web sources.

Some of the things that you might be able to do using Garmin protocol include:

  1. getting the version number of the software.
  2. Finding out the capabilities of the unit.
  3. Uploading or downloading waypoint data.
  4. Uploading or downloading Track data.
  5. Uploading or downloading Route data.
  6. Uploading or downloading Almanac data.
  7. Downloading the current gps time.
  8. Downloading the current gps position.
  9. Uploading a new release of the firmware.
  10. Uploading a set of maps.
  11. Downloading a screen snapshot.
  12. Receive a complete PVT solution in real time.
Older multiplexing units cannot do many of the items in the list. Specifically they cannot do item 2, or any item beyond 8 in the above list. The G-12 family cannot do items above 9 but this could change with new firmware releases.

User data backup

The most often used capabilities include the backup of critical user data such as waypoints and routes. To do this you would need to secure a Garmin capable program. These are available from Garmin or several third party sources. There are programs for pc's running dos, or windows, for macs, for unix, and for palm pilots. Once you have the correct program you can place your unit in Garmin mode and set the baud rate to 9600. Generally all programs accept this baud rate but some may support other rates and even change the rate. Cable the unit to the computer and make sure the computer program is set to the correct serial port and the baud rate to 9600. The standard serial port settings are 8N1, 8 bit data, no parity, 1 stop bit. Do not use xon/xoff since this may interfere with proper transfer of binary data. If the program cannot access the unit then check to ensure some other program is not using the port (the palm sync program is notorious for this) and that the port is configured correctly. This protocol is binary and requires handshaking so all three wires need to be hooked up correctly. Perhaps a null model adapter may be required to get the receive and transmit signals hooked up properly.

You cannot modify individual waypoints or routes using this interface. Instead you load the full set of routes or the full set of waypoints. If you wish to revise certain data you should download the full set and the revise the data you wish on your computer, clear all the waypoint data, and then reload the full set back. Otherwise you may get unpredictable results on some units. Most units will simply overwrite waypoints with the same name but the emap will create a new waypoint if the location is different. This can make updating a waypoint a bit frustrating. Similarly you load the full tracklog. On units with multiple tracklogs you may find them all concatenated together on download. Some programs may be able to upload saved logs directly but some may not. On units that support the uploading of maps the rule is similar. You must assemble all of the maps you wish to upload and then send them all at once replacing the previous upload.

Of course, the data in the computer program need not originate in your gps. It is quite possible, in some programs, to import external data to the program for later uploading or to edit the data files directly to provide this new information. It is also useful to modify and add comment data to waypoints using a computer keyboard instead of toggling it in with the unit keypad. Be aware that waypoints are always interpreted as using the WGS-84 datum for this interface.

Unit to Unit transfer

One interesting use for this protocol is to transfer information between units. To do this you need a Garmin to Garmin cable (available from Garmin and other sources) to hook the two units together. One of the units is placed in Host mode and the other unit sends commands to upload and download data. The commands are shown on your units menu. They may include:

  2. RQST/SEND CFG - configuration
  3. RQST/SEND PRX - Proximity alarm data
  4. RQST/SEND RTE - Routes
  5. RQST/SEND TRK - Track Log
  6. RQST/SEND WPT - Waypoints
  7. RQST/SEND ALM - Almanac
The emap and etrex do not support this mode. Other units may not work correctly in this mode or may not support some commands. For example a G-III does not have proximity alarms. The G-38 and the G-12 can talk to each other but the G-38 will not be able to support waypoint icons and you can easily overflow the track log on a 38 with the tracklog on a 12. It is also possible to use a computer as an intermediary for this transfer by moving the data from the unit to the computer and then uploading it to the target platform. Some Palm pilot programs even support the host mode so that they can serve as a temporary storage point for gps user data.

Firmware Upgrade

Garmin releases firmware upgrades for all of their twelve channel units to fix bugs and to add functionality. These upgrades are available from the Garmin web site and are free so long as you agree with the terms and conditions. They come with the appropriate program for pc platforms and are only supported by Garmin. There is no third party source and users on Macintosh units will have to find a friend to do the upgrade or use a pc emulator. Be careful that you only try and use the firmware for you specific unit, or you could break your unit completely and have to send it back to Garmin for repair. The older multiplex units cannot be upgraded in this fashion and if needed they must be returned to Garmin for any upgrades. Be sure and read the instructions that accompany the upgrade at the Garmin site.

To ensure success make sure you download the upgrade using a binary mode. It comes as a zip file so if it unzips correctly you can be sure you downloaded it correctly. Make sure you have a good connection to the gps. Try one of the Garmin interface programs to backup your data. Generally an upgrade does not lose user data but this is not always the case so it is a good idea to back it up. Leave the unit in Garmin mode for the upgrade. Your pc baud rate should be set to "maximum rate" possible so that the program can increase the baud rate to minimize the download time. Expect to lose any customization that you may have performed on your unit. Be sure you have fresh batteries in the unit. Writing the firmware into the prom can use significant battery power and if the batteries are weak you may not get a good load or you may start out with a seemingly good load that will fail later. Do not abort the process once it has begun. It can take several minutes to do the upgrade so be patient. If the upgrade fails, try it again. You must get an good upgrade before your unit will be operational again.

If you lose power or connection during the upgrade you may have a unit without any code at all. If you were to attempt to power up the unit it will tell you the firmware is missing. Some have reported that this happened some time later. This is usually caused by weak batteries that were not able to burn the new code in the machine. To recover perform these steps:

  1. Connect your cable to the computer.
  2. Get the computer ready to load the firmware but don't press ENTER.
  3. Turn on the GPS and then press the ENTER key on your computer.
  4. Watch - The program should begin to load the new firmware

If you are unsuccessful then call Garmin and arrange to return your unit for them to upgrade. If you feel that the upgrade has a bug in it and the older release is better you can generally use an earlier upgrade to downgrade your unit. Garmin generally does not keep older versions available but they are often available on the net from other users.

PVT data

Some of the Garmin receivers support a PVT mode as part of the Garmin mode. If you are using a computer program that supports this then you can remain in Garmin mode even while running your real time mapping application. You set your unit to Garmin mode and then select this solution from the menus in the application. Delorme mapping products support this mode. This is an advantage in that you don't need to switch modes and you can leave your interface at 9600 baud which makes the real time response a bit faster. The update interval is 1 second and this mode does not require handshaking nor does it support retransmission of data. The following data is typically included as part of the pvt structure in the D800 message:

Undocumented modes

The Garmin Interface specification defines much of the exact interface requirements for Garmin mode. However, there are many things that are in the interface that are not described in this manual. Garmin has indicated that these are for test purposes and are not to be used by customers. They may also be changed from release to release and may only work with a particular test setup. However, many of these modes have been discovered and decoded by third party programmers. Such additional features such as screen captures fall into this catagory. One vendor has actually managed to get the pseudo range data out of the Garmin 12 family and provides a post processing capability with these units by collecting data on a pc in real time for later processing thereby opening the possiblility of using this unit for surveying applications.

DGPS mode

All Garmin receivers support DGPS. DGPS is a method of improving accuracy your receiver by adding a local reference station to augment the information available from the satellites. This station transmits correction data in real time that is received by a separate box, called a beacon receiver, to send correction information to the gps receiver. Most folks fabricate a custom cable to work with the beacon receiver. Here is a diagram for a fairly complicated version, but you may not need a setup that is this complicated depending on what else you may be doing.
For simple dgps connections you can just wire a beacon receiver output signal along with its ground to the data input terminals of the gps. If you need to be able to control the beacon receiver from the gps receiver then you will also need to send the ouput for the gps receiver to the beacon receiver. A standard computer interface cable can usually be used for this connection. If you also need to talk to a pc at the same time things start to get a little more complicated. To talk to a pc in NMEA mode you can simply send the output of the gps to both units. Wire the output signal to the input on the computer and the input on the beacon receiver. There is sufficient power in the signal from the Garmin to drive both units and even a third item. Note if the beacon receiver doesn't need to receive commands from the gps then there is no reason to send the signal both places but the ground wire is still needed. Finally if the gps needs to talk to the pc in Garmin mode and also the the beacon receiver you will need a switch to permit the beacon receiver to transmit difference signals or the pc to interact with the gps. You won't be able to do both at the same time. This should not present any real problems since the bi-directional Garmin mode is used to upload and download waypoint, route, and track data which does not need the beacon receiver to be operational.

*** More To be supplied

Text Mode

Text mode is a simple output mode that supplies velocity and position information in realtime. Currently this is primarily used by certain digital cameras to include this data on the picture. In the future many other uses will be found for this mode which requires very little processing on the part of the device receiving the data. An example is shown below.

@yymmddhhmmss Latitude Longitude error Altitude EWSpd NSSpd VSpd

Each item is of fixed length making parsing by just counting the number of characters an easy task. It is grouped by use permitting a digital camera, for example, to just read the first 30 characters and report the time and position. Some of the data will require some programming to make meaningful for most users, such has the speed which is devided into the X, Y, and Z components. This is the only format that provides vertical speed which should be a great boon for ballonists.

A more formal description of the fields are:
 Sentence start1Always '@'
Year2Last two digits of UTC year
Month2UTC month, "01".."12"
Day2UTC day of month, "01".."31"
Hour2UTC hour, "00".."23"
Minute2UTC minute, "00".."59"
Second2UTC second, "00".."59"
Latitude hemisphere1'N' or 'S'
Latitude position7WGS84 ddmmmmm, with an implied decimal after the 4th digit
Longitude hemishpere1'E' or 'W'
Longitude position8WGS84 dddmmmmm with an implied decimal after the 5th digit
Position status1'd' if current 2D differential GPS position
'D' if current 3D differential GPS position
'g' if current 2D GPS position
'G' if current 3D GPS position
'S' if simulated position
'_' if invalid position
Horizontal posn error3EPH in meters
Altitude sign1'+' or '-'
Altitude5Height above or below mean sea level in meters
East/West velocity
1'E' or 'W'
East/West velocity
4 Meters per second in tenths, ("1234" = 123.4 m/s)
North/South velocity
1'N' or 'S'
North/South velocity
4Meters per second in tenths, ("1234" = 123.4 m/s)
Vertical velocity
1'U' (up) or 'D' (down)
Vertical velocity
4Meters per second in hundredths, ("1234" = 12.34 m/s)
  Sentence end2Carriage return, '0x0D', and line feed, '0x0A'

Notes on the table:

00/6/4 initial release
00/6/5 revised NMEA section based partially on user feedback
00/6/12 added text data and pvt data.
00/7/14 added some dgps stuff and error recovery for softare download
00/7/29 added cable drawing and NMEA input mode.
01/1/8 added NMEA 1.5 data.

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