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7b. Serial COM Port Pin Assignments for GPS and Autopilot
ConnectionsCreated:
12/10/2003Modified:
01/16/2011
TOPIC
Additional GPS Information: The proper way to connect a GPS
and/or Autopilot to any Navigation software running on a PC.
DISCUSSION
IntroductionSuccessful GPS and Autopilot hookup to any
navigation software product usually requiresthe use of on or more
serial "COM" ports. Making this connection tricky is the fact that
mostmodern laptop computers no longer include a serial COM port,
instead opting for USBconnections.
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
regardingSerial-to-USB adaptors.
9-pin Serial Port wiringA standard 9-pin serial (COM) port on
your computer (or USB-to-Serial adaptor) featuresthree pins for
connecting a GPS or Autopilot. Pin 5 is Ground (GRD) and is often
connectedto a Green wire. Pin 3 is Transmit (TXM) and is often
connected to a Red wire. Pin 2 isReceive (RCV) and is often
connected to a Brown wire.
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 theComputer; connect the Transmit from the
Computer to the Autopilot; and finally tie all threeGrounds
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 alsoindicate Positive (+) and
Negative (-) connections.
For the sake of the explanation below, we will use the
Transmit/Receive terms used in Serialcommunications. 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
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that device. A wire or connection on your GPS/AP that is labeled
"NMEA - (IN or OUT)" isthe "Ground" connection for that device.
Cabling - GPS Only, no AutopilotTo connect a standard serial GPS
to a 9-pin COM port, wire the Transmit connection fromthe 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 alsosupports transfer of
routes and waypoints to the GPS, you may also connect the Receive
wirefrom the GPS to the Transmit connection (pin 3) on the
computer.
GPS cable connection for stored coordinate transfer.
Cabling - GPS and Autopilot with a single COM portTo 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 thecomputer'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 Navigationsoftware to "NMEA Generic" mode.
Transfer of coordinate data to and from the GPS will not
be possible in this configuration.
Single-port connection for the GPS and Autopilot.
Cabling - GPS and Autopilot with two COM portsThis is the ideal
situation, as it allows the flow of waypoints to and from the GPS
while alsooffering a connection to the Autopilot. The drawback is
that this connection requires twoserial 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 theGround 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, TheCAPN can not be configured in this fashion
- only a single-port connection is allowed.
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