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Why do I get a GPS Error about 'two modes of operation' and how do I correct it? (Also "USB Open Failed")

GPS Troubleshooting: Spanner Mode vs. Mass Storage/Garmin Serial Mode


The newest generation of GPS units (most notably Garmin's Oregon, Colorado, Montana, and GPSMap62/64/78 series) work significantly different than their older counterparts made before about 2008. In short, GPSs made after 2008 connect to the computer using a "Mass Storage Mode" like a digital camera or USB "thumb drive", allowing the PC to have direct access to the files on the GPS. Prior to these GPSs, only applications that communicated directly with the GPS through a special protocol could request contents.

While direct file access has been a good design decision, it prohibits software products from ascertaining a position from the GPS satellites. For example, in its GPS Tracking menu, Terrain Navigator Pro can be used to plot a real-time position; those desiring to do so must configure the GPS and Terrain Navigator Pro in a different manner.

This technical document specifically describes this connection process for Garmin GPSs. Other GPSs that employ this technology will have different configuration tricks, but the concepts remain the same.

What is "Spanner" and why do I need it?

In the event that you would like to receive a live GPS connection from a Garmin Oregon, Colorado, or GPSMap62/64, (or virtually any Garmin GPS made since 2008) it must be configured to have a "Spanner" Interface Mode. To set the Interface Mode, select Setup, System, Interface on the GPS. Set the Interface Mode to "Garmin Spanner" if you want to allow the GPS to send live position (tracking) information to Terrain Navigator Pro. If you are only looking to share markers, routes, tracks, and maps, set the Interface Mode to "Garmin Serial." (Garmin Serial is also known as Mass Storage, see below.)

How do I switch between the different modes of the GPS?

If the Interface Mode of the GPS is set to Garmin Spanner, the you will get the following message (or one similar) when you plug the GPS into the PC:
USB Cable Detected.
Would you like to go to Mass Storage?
(Yes or No)

Press Yes to enter Mass Storage Mode, which will allow you to share markers, routes, tracks, and maps between Terrain Navigator Pro and the GPS. (Do not "connect" to the GPS using the Connect item at the bottom of the TNP's GPS menu. Instead, just open the appropriate menu items to send or receive markers, tracks, etc.)

Press No to enter Spanner Mode, which will allow you to track your position on Terrain Navigator Pro's maps and photos. (In this case, open TNP's GPS menu and choose Connect to engage communication from the GPS satellites.)

To switch between modes, disconnect and reconnect the USB cable. You will be asked which mode you wish to enter. Press Yes for Mass Storage (data sharing) or No for Spanner (position tracking.)

If you always want to be in Mass Storage mode, and not be bothered with this question, set the Interface Mode (on the GPS in the Setup menu) to Garmin Serial.

While you are in Mass Storage mode, the GPS screen will show a computer connected to a GPS, a USB symbol, or similar graphic. No options on the GPS screen will be available (and no position/tracking information will be available to Terrain Navigator Pro; the GPS light on TNPs status bar will remain dimmed.) In Spanner mode, the GPS will work exactly as it would if it were not connected to the computer - and Terrain Navigator Pro can use the GPS's position for live tracking. The GPS light on TNPs status bar will reflect the connection status. (However, the GPS will not be able to share markers, routes, tracks, or maps until the plug is pulled, reconnected, and Mass Storage mode selected.)

When plugging the GPS into the computer, be sure to wait a few seconds for Windows to recognize that the GPS is connected. You may hear a "chime" when this occurs, and when Mass Storage is used, the GPS will appear in the list of drives in File Explorer. Once Windows recognizes that the GPS is connected, then the appropriate functions in Terrain Navigator Pro will work correctly.

What if my GPS does not have a "Spanner" Interface Mode?

Some Garmin GPSs (such as the GPSMap78) may not include a Spanner Interface Mode. Please contact Garmin for a system software update to add this feature to the GPS. (If available.) Garmin's Dakota series of GPSs do not have a Spanner Interface Mode and can not be used for position tracking in conjunction with Terrain Navigator.

I don't want to do GPS position tracking on my TNP window; I want only to share markers, routes, tracks, and maps. How do I accomplish this?

If you do not wish to track your position on Terrain Navigator Pro's screen, set the Interface Mode of the GPS to Garmin Serial. This will automatically set the GPS into Mass Storage Mode whenever it is attached to the computer.

Once the connection is made (and the GPS screen shows a GPS connected to a computer) use the Send and Receive functions in Terrain Navigator Pro's GPS menu to share markers, routes, tracks, and maps. Note: Do not select the "Connect" option at the bottom of Terrain Navigator Pro's GPS menu. This Connect option is only needed when Terrain Navigator Pro is to receive a live GPS position (via the Garmin Spanner interface.) Note also that the GPS light on TNP's status line will remained dimmed when a GPS is attached in Mass Storage Mode.

When I attempt to send or receive waypoints (or connect to the GPS) I receive an error: "USB open failed". What does that mean and how do I fix it?

An USB open failed error indicates that the driver used by the Garmin GPS to connect to the Windows operating system is either damaged or missing. You can correct this by downloading and installing the latest USB Drivers software for use with all Garmin GPSs. This ensures that all the drivers are installed and that the connection is working properly. You can find the Garmin USB Drivers software at:

That said, Garmin GPS units manufactured after 2008 do not require a USB driver for sharing markers, routes, tracks, and maps between the GPS unit and Terrain Navigator Pro. Instead these GPS units use the Mass Storage Mode described above.

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