Modernizing your Garmin serial interface

by: Dale DePriest - all rights reserved.

Historically the Garmin interface was a standard serial port RS-232. This made sense and is still the standard for many users. However, modern PC's and PDA's may no longer even have the hardware to support the RS-232 serial port standard. As user's make new computer purchases they may find that there is no easy way to hook up their Garmin GPS receivers. Many new computers now support the latest interface standards including USB or Bluetooth for wireless connection. This article reviews two solutions to this problem, one for USB and one for Bluetooth. Each solution provides a direct interface to many Garmin models including the etrex family (except for the new C models), the emap, and the gecko family. These Garmin GPS receivers all share the same interface connector design. While not reviewed here there are also solutions for other Garmin models.

The solutions reviewed provides a wired connection to the etrex connector for serial cable connection and, as a bonus, they provide power such that the unit can be powered externally while the connection is made. This external power connection will not charge the internal batteries on the unit but can provide unlimited power for use in a vehicle or for home use.

The intent of these interface solutions is to provide real time tracking for portable use such as hooking your GPS to a PDA or to a laptop. In some cases these solutions can also be used to provide for map download as well as tracklog, waypoint, and route support.

The Bluetooth solution

The Bluetooth solution was provided by Semsons and Co. They also have solutions for other Garmin products but these were not tested.

The unit consists of a cigarette lighter adapter and a cable connetion that plugs directly into the GPS. The Bluetooth hardware is inside the cigarette lighter adapter as is a voltage regulator to drop the voltage to a suitable level to drive both the Bluetooth hardware and the attached etrex. The connector on the etrex side was made by pfranc.

The adapter has a power/bluetooth connection indicator that indicates power is applied (blinking) and the connection status (solid when paired). If secure pairing is required the unit supports a password of '0000'. The GPS side is hardwired for 4800 Baud, 8 bits, no parity as required for NMEA operation from the etrex unit. The interface is capable of bidirectional transfer (more about this later).

This is a standard class two version 1.1 Bluetooth device with a range of 32 feet or less. As such it does not require any software beyond the standard Bluetooth support on the computer. It will hook to any Computer or PDA using the Serial Port Profile (SPP). I found that it worked exactly like other dedicated Bluetooth GPS systems.

To do waypoint, tracklog, route upload/download or map download the Garmin receivers requires 9600 baud. This unit is shipped with only 4800 baud support so it will not support these features. It is possible to open the unit up and switch it to 9600 baud. If you want to do this then contact Semson for details. I do wish that there was a switch for this on the unit. This is the only fault I found with the unit.

The input Voltage can range from 9V to 24 V. I measured an unloaded 3.7 V on the output. This is a little higher than the Garmin spec but I suspect it drops a bit under load. I had no problem with either of my units. The cable itself is 5.5 feet long which is plenty long enough to set the GPS in a place where it will get maximum reception.

The USB solution

The USB solution was supplied by GPS Warehouse. The tested configuration included an emap and etrex Vista. They also have various data and data/power cables for other Garmin models on their web site.

The tested configuration included the cable shown to the left. It has a USB connector at one end and an etrex connector (pfranc) at the other. The box in the center contains the logic to convert the USB serial port to the serial port on the etrex. Power from the USB side is also routed through the convertor to power the etrex. A CDROM containing the USB driver was included with the adapter.

There are many solutions for USB support on legacy serial devices but what makes this one different is the custom connector on the GPS end of the cable. This avoids the cumbersome adapters and cables plugged into cables that often are needed with other solutions.

You must install the CDROM before plugging in this cable. There is only Windows support on the CDROM. There is a manual on the CDROM that describes this installation and setup instructions. Installation is straight forward but setup is a little more difficult.

There is no user interface provided so you will need to configure the port from the Control pannel, hardware settings. The easiest way to reach the panel is usually to right click the "my computer" icon and select properties. On the hardware page you will need to find the entry under Serial Devices. Take note of the com port that was assigned to the USB to serial device and check the Baud rate. For me the default seemed to be 9600 Baud which will not work with standard NMEA data which required 4800. Unlike a real serial port on my other PCs this port will not negotiate the speed so you have to set it right to get it to work. You will need to return to this screen any time you need to change the Baud rate such as when you wish to work in Garmin mode. Garmin mode setting to 9600 Baud is required to work with maps, tracklogs, wayppoints, and/or routes. The advanced settings permits changing the com port.

Once I had the com port set and the baud rate set I was able to run my GPS with PC based navigation software. You are likely to have to set the com port in the navigation software settings although some will search the port. It worked fine.

The power output from the interface measured 3.3 V which is exactly what Garmin specifies. The cable length is a little over 5 feet. If you need something longer then you might use a USB cable extender.

If you have problems make sure some other device hasn't grabbed the serial port. Pocket PC ActiveSync and Palm Hotsync are both known to do that depending on how they are set up. Also be sure the Garmin GPS is set to NMEA mode.

2005/1/29 original release