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7b. Serial COM Port Pin Assignments for GPS and Autopilot Connections

Created: 12/10/2003
Modified: 04/10/2007

TOPIC

Additional GPS Information: The proper way to connect a GPS and/or Autopilot to any Navigation software running on a PC.


DISCUSSION

Introduction
Successful GPS and Autopilot hookup to any navigation software product usually requires the use of on or more serial "COM" ports. Making this connection tricky is the fact that most modern laptop computers no longer include a serial COM port, instead opting for USB connections.

For the purposes of this discussion, we are assuming that a serial COM port is in-use. If your computer lacks a serial COM port, see this technical document for information regarding Serial-to-USB adaptors.

9-pin Serial Port wiring
A standard 9-pin serial (COM) port on your computer (or USB-to-Serial adaptor) features three pins for connecting a GPS or Autopilot. Pin 5 is Ground (GRD) and is often connected to a Green wire. Pin 3 is Transmit (TXM) and is often connected to a Red wire. Pin 2 is Receive (RCV) and is often connected to a Brown wire.

9PIN.jpg
A 9-pin connection. Pin 5 = Ground (GRD), Pin 3 = Transmit (TXM), Pin 2 = Receive (RCV).

The basic gist of wiring is to connect the Transmit from the GPS to the Receive of the Computer; connect the Transmit from the Computer to the Autopilot; and finally tie all three Grounds together.

Terminology
NMEA connections (or the connections from your GPS) may not indicate "Transmit" or "Receive." They may be labeled NMEA IN or NMEA OUT. Moreover, they may also indicate Positive (+) and Negative (-) connections.

For the sake of the explanation below, we will use the Transmit/Receive terms used in Serial communications. Thus, a wire or connection on your GPS/AP that is labeled "NMEA IN" or "NMEA IN +" is the "Receive" connection for that device. A wire or connection on your GPS/AP that is labeled "NMEA OUT" or "NMEA OUT +" is the "Transmit" connection for that device. A wire or connection on your GPS/AP that is labeled "NMEA - (IN or OUT)" is the "Ground" connection for that device.

Cabling - GPS Only, no Autopilot
To connect a standard serial GPS to a 9-pin COM port, wire the Transmit connection from the GPS to pin 2 (Receive) on the computer's COM port. Next, wire the Ground connection from the GPS to the ground connection (pin 5) on the computer. This is all that is required for real-time positioning with a "NMEA Generic" GPS. If your Navigation software also supports transfer of routes and waypoints to the GPS, you may also connect the Receive wire from the GPS to the Transmit connection (pin 3) on the computer.

Gpsonly.jpg
GPS cable connection for stored coordinate transfer.

Cabling - GPS and Autopilot with a single COM port
To connect a standard serial GPS and NMEA-compatible Autopilot to a single 9-pin COM port, wire the Transmit connection from the GPS to pin 2 (Receive) on the computer's COM port. Next, wire the Receive connection from the Autopilot to pin 3 (Transmit) on the computer's COM port. Finally, connect the Ground (pin 5) on the computer to the Ground on both the GPS and the Autopilot. In this configuration, be sure to set your GPS and Navigation software to "NMEA Generic" mode. Transfer of coordinate data to and from the GPS will not be possible in this configuration.

ONEPORT.jpg
Single-port connection for the GPS and Autopilot.

Cabling - GPS and Autopilot with two COM ports
This is the ideal situation, as it allows the flow of waypoints to and from the GPS while also offering a connection to the Autopilot. The drawback is that this connection requires two serial COM ports.

To connect the GPS, follow the instructions for a "GPS Only" connection as stated above. Next, on the second serial COM port connect the Receive connection from the Autopilot to pin 3 (Transmit) on the computer. Finally, connect the Ground (pin 5) on the computer to the Ground on the Autopilot. There is no need to make a connection to pin 3 (Receive) - in fact, such a connection may cause a "NMEA Loop" which can cause a multitude of problems.

Please Note: Not all Maptech applications support multiple COM ports. Specifically, The CAPN can not be configured in this fashion - only a single-port connection is allowed.

Related Technical Documents
Connecting a GPS to a USB port
GPS and Computer Communications - HyperTerminal Test
How do I "share" a GPS between two (or more) different applications?





 


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