If you wish to wire your own cable or need to wire the far end of the cable you will need to know the wiring scheme. Look at the connector in the unit and find the locating guide. Proceeding clockwise the first pin is Data in, usually the white wire, the second is Ground, black, and is used as a return for both power and data. The third pin is data out, brown and the fourth is power, red. Garmin sells a data cable with the cpu end unwired so that you can use the connector of your choice for the computer connection.
The etrex and the emap need an entirely different connector that has all of the pins in a single line. The power is physically separated from the data connectors and the ground wire is on the opposite end. The data in connection is next to power while data out is next to ground. The power for these units is only 3 Volts so it also requires an external power adapter. Here is a diagram of the wiring for this connector. It shows the PFRANC version.
There are many times when external power can be a useful accessory for your gps. All of the Garmin handheld units have an external port available that can be used for power and for data, however some of the cables you purchase do not support both. The information in the cable discussion above provides the pinouts for the power connection to make your own power connection if you wish. The ground connection is shared between power and data use.
Be careful! Wiring power to the unit can destroy the gps if you input voltages that are not in the correct range for the unit. Also, if you get the polarity wrong you will destroy the unit. Power input is the positive (+) side of the power supply while ground is the negative (-) side.
When you buy a cable that includes a connector for a cigarette lighter some folks assume the unit itself uses 12 Volts. This is often not the case as there are plenty of 12 Volt adapters that will fit inside the cigarette lighter housing so do not cut off a cable and assume you can just wire it direct to 12 Volts. Check my Technical specs chapter for details on the power requirements for various models or look in the manual that came with the unit. However, often you may also want to run the unit from an AC outlet. If you already have a 12 volts adapter cable then I would suggest that you buy an AC to 12 Volt power supply with a cigarette lighter receptacle and then plug in the 12 Volt adapter you already have. Radio Shack sells these kind of adapters.
External power is isolated from the internal batteries such that you cannot charge the batteries from the external power source. This is by design since Garmin cannot know what kind of batteries may be present in the unit and trying to charge the wrong kind can damage the unit. However some units, notably the G-12, are not isolated to the extent that you cannot continue to use the batteries even when external power is applied. The isolation is done with a diode, so on these units you need to supply at least 6.7 Volts to avoid partial discharge of the internal batteries. Garmin's own external power supply provides close to 8 Volts to these units. If you really want to charge a set of batteries which they are inside the unit then I would suggest that you snake a couple of wires inside the battery compartment itself and connect them to the unit.
One very useful hardware attachment for NMEA use is a datalogger. The idea of a data logger is to collect data in real time from the gps and then save it for later processing. One such datalogger is available here. It will log up to 270,000 points from the NMEA $GPRMC sentence as fast as the sentence is generated or every 10 seconds as chosen by the user. Other more specialized units are used by glider pilots to verify their flights in contests. One such unit is made by EW Avionics Another hardware attachment is ERIC. This is a hardware attachment that reads route (RMB) sentences available through the NMEA interface and report your progress verbally. It hooks directly to the data port and can share the port with other devices. You load a route into the gps and activate it. ERIC will then warn you about the waypoints in the route as you near them. This is especially useful if your gps doesn't have audible alarms. (The web page for this site seems to have vanished.)
There are plenty of other devices that work with NMEA output from a Garmin gps. These include flight computers, boat autopilots, depth finders, pda's, and more. The GPS can be used to steer a boat from the NMEA data when set up in Navigation mode.
Some digital cameras have been designed to accept gps data for imprinting as part of the image. Generally most of these cannot accept NMEA data but require a simple text output. Some Garmin receivers have the ability to output this simple text format. These include the III+, the 12Map and the etrex units. The newest versions can now accept NMEA data as well.
Some Garmin units can accept data input from NMEA compatible devices
as well. For example the 76 marine units will accept input from a
depth finder unit and will then display and record depth data in the
unit. Internal alarms can also be set based on this data.
Many Garmin gps receivers can accept a remote antenna. Generally
these are active antennas meaning that they have pre-amplifiers inside
and Garmin supplies power through the connector to run the antenna.
If you wish to hook an unpowered antenna to the external antenna
connector you will need to block the dc power using a capacitor to
avoid damaging the unit. Garmin supports two kinds of remote
antennas. One uses a very small mcx connector and when plugged into
the unit it removes power to the internal patch antenna. To do this
Garmin has a circuit inside that sense the power drawn by the external
unit and switches the antenna source. The external antenna needs to
draw at least 5 ma for this switch to take place. The second kind of
remote antenna is used with the Garmin II and III families. These
unit already have an external antenna attached to the backside. You
simply remove the supplied antenna and attach a replacement. The
connector in this case is a BNC connector. In this configuration there
is no need to sense and switch the antenna since only on is plugged
in at a time.
Since the helix antenna is simply plugged into the back of the unit, it can also serve as a remote antenna. In this case you only need a cable to remote mount the antenna. Garmin even sells a short cable with a suction cup mount at one end that can be used to place the supplied antenna a short distance from the unit. You could also easily make your own. Note that the helix antenna is not amplified and thus cable losses will limit the maximum length of this cable to about 6 feet.
Most Garmin receivers that support remote antennas supply 5VDC in the cable to operate the antenna. The newest units, the 12Map and the e-map, only supply about 2.5VDC to the antenna connector. This must be considered when choosing an antenna. Since power is used by the remote antenna you can expect a shorter battery life whenever using a remote antenna.
The following handhelds do not have a provision for a remote antenna: The G-38, the G-12, and the etrex. For these units or for any unit where you don't want to take advantage of the built in antenna extension you can purchase a re-radiating antenna. A re-radiating antenna is really two antennas. One amplified receiving antenna collects the gps signals and sends it to a second antenna that re-transmits the signals. The gps receiver is placed close to this second antenna to pick up the signals using its own built in antenna. An external power source is needed to power the re-radiating antenna independent from the gps receiver. On such antenna is available from Tri-M Systems.
One question that usually arises is: "Do I need an external antenna?" You can answer this question yourself by observing the behavior of your unit on the satellite status screen. The screen displays all of the satellites your unit could be expected to observe at any point in time. If you are receiving all of them with reasonable strength or are missing only those that are obviously behind buildings or hills then you will not benefit from an external antenna. However, if reception is spotty or weak and you observe this behavior while moving under tree cover or other light cover conditions then an external antenna may help your reception considerably.
The bicycle brackets from Garmin come in two pieces. One is attached to the unit and the second is attached to the bike. The bike piece is a small ring that is intended to be left permanently on the handlebars. The piece that attaches to the gps varies from one unit to another. The most interesting is the etrex which replaces the battery cover with a different cover that contains a plastic clip that is designed to slide into a slot on the bicycle ring piece. The cover also has a rubber casket which improves the water resistance of the battery compartment which may make it useful even if you don't want to use it with a bike.
The G-12 family, the 12 map and the older multiplex units have a similar case use a plastic clip that permanently replaces the rubber backing on these units. This clip mates with a slot in the ring like the etrex unit. Since this clip is permanent it does make the unit slightly more awkward to lay on its back and prevents its use in one of the car/boat mounts available. The emap uses a similar mounting idea that screws into the back of the unit.
The III series uses a more complicated clamp that clips onto edges of the unit and can be held in place with a screw. This clamp, in turn, slides into the slot in the ring. Some have reported that the clamp can break under vibration so this mounting should include a safety hook up.
Memory cartridges are available with maps already installed in 4 Meg, 8 Meg, and 16 Meg sizes. You can also purchase blank cartridges in 8, 16, and 32 Meg sizes. Larger cartridges, 64 and 128, are available in the US as well. Any of the cartridges can be programmed from the available cdroms. To install a cartridge you will need to open the battery compartment and remove the batteries. You will then see a small opening in the upper right corner of the compartment. Insert the cartridge into this opening with the label facing up. To remove you need to use a fingernail to pull out the extraction handle and then you can remove the cartridge.
The cartridge extractor is attached with a small screw. Be sure that this screw is tight and flush with the side of the unit or you may have problems attempting to extract the cartridge.
Garmin has also released a USB programming module for the cartridges. In the larger sizes the ability to download maps as USB speeds can significantly speed up the process. Once the cartridge is programmed it can be transferred to the emap unit.
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