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Determining the COM Port of a USB to Serial Adapter

GPS Troubleshooting

Please Note:

This article is written for the customers of Trimble Forestry, providers of Terrain Navigator Pro and MyTopo Printed Maps. If you are not familiar with our products, please check them out at:  


Serial-to-USB adapters that do two things:
  1. They physically connect a serial cable to a USB port. 
  2. They have software drivers that create a Virtual COM Port.
A Virtual COM Port is a setting within the Windows Operating System that allows software products to connect to USB devices using traditional serial (COM Port) connections. While, the 'plug' on the back of the computer remains USB, Terrain Navigator connects to it as a traditional serial COM Port. It is the number of this Virtual COM Port that is needed for Terrain Navigator Pro to connect to the GPS device attached in this fashion.

Setting Up Your Serial-to-USB Adapter:

Your adapter includes a CD that contains drivers for your device. A driver is a piece of software that installs into the Windows Operating System that allows all applications to communicate with the new device. Without a driver, a device can not be used.

Insert the Driver CD that accompanied your Serial-to-USB adapter and follow the instructions to set it up. If possible, verify that it is working properly through whatever means the documentation for that adapter indicates. Unless the Serial-to-USB adapter is working properly, it will not be able to communicate with your GPS or any software application.

Determining the COM Port Number of a Virtual COM Port:

Once the driver is installed, you will need to find out the number of the Virtual COM Port that has been set up. In the majority of cases, you can determine the Virtual COM Port by following these steps:

1. Open Device Manager:
Click: Start > Control Panel > Hardware and Sound > Device Manager (under "Devices and Printers")

2. In the Device Manager list, look in Ports and find the Virtual COM Port, which was created by the USB driver.
i.e.___ USB to Serial Port___ (COM #).

In this example, it is COM 4:

Finishing the Job: Reading the GPS Stream in Terrain Navigator Pro

  1. From the 'GPS' menu, choose 'GPS Setup Wizard'. 
  2. Follow the on-screen instructions.
  3. If the GPS Wizard can not detect the GPS, select 'Setup' from the 'GPS' menu. Set the 'Manufacturer' to 'NMEA - generic' and specify the COM port determined during the above test. Also, ensure that the correct baud rate of the GPS is selected in Port Settings. Then press 'Close'.
  4. Once the signal is detected, choose 'GPS Tracking', 'Start Tracking' from the 'GPS' menu.

Related Knowledge Base Article:
Configuring Terrain Navigator Pro to the GPS - Basic Instructions.


The Virtual COM Port number may change on different computers, when you install additional devices, or when you switch the USB connection to a different USB port.

Before using navigation software, the COM Port numbers created by your computer and the GPS COM port entered in the settings in Terrain Navigator Pro must match. Otherwise, Terrain Navigator Pro won’t receive the satellite signal.

If you do not see a Virtual COM Port, try to uninstall the drivers that accompanied your Serial-to-USB adapter (or USB GPS) and reinstall them. All of the Serial-to-USB manufacturers include a CD or web download to uninstall and reinstall these drivers. In practice, it sometimes takes a few tries to create a working Virtual COM port.

Several customers have reported incompatibilities with the Serial-to-USB adapter sold by Radio Shack. It appears that this adapter is unable to handle the data rate of NMEA GPS units and, therefore, we do not recommend it for use in this manner.

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