Install
a DB-9 pigtail for interfacing a laptop or handheld GPS
When first installing
the fixed GPS ( Garmin 125) and interfacing it with the autopilot (
ST4000) I was using PC based charting software to develop
waypoints and routes. As the base gps couldn't easily be taken to the
PC. The solution was to load from the PC into a handheld gps and
then do a gps to gps transfer on the boat. A pigtail with a
female DB-9
connector was hard wired to the gps data bus which made it simple to
connect the handheld gps using its data wire which also has a DB-9
connector for the interface to the PC.
This was simple to do by Xing (out to in, in to out - null modem) the
data connections on the male to male adapter needed to mate the two
female pigtails. The adapter was made from two male DB-9
connectors screwed together with standoffs and wired of course in the X
configuration. Later when I bought a lap top and began running
onboard charting software, I simply connected my computer to the
DB-9 pigtail which provided data communications between the gps and the
computer.
The pigtail also will connect the handheld gps to to the autopilot
should the base gps fail.
While wiring your gps to the autopilot, simply add the pigtail to the
bus. The following pinouts should be of help.
GPS 125
- Red, 12-18v dc
- Black, dc ground, and [NMEA -] of autopilot
- Blue, NMEA out, this wire connects to the
autopilot NMEA + , and the data pigtails pin 2 on the DB-9
connector
- Brown, NMEA in, this wire connects to the data
pigtails pin 3 on the DB-9 connector
- White, no connection
- Green, no connection
- Yellow, alarm low, this is for connection of an
external alarm
GPS 45
- Red, 15-40 v dc
- Black, ground
- White, data in, connect to DB-9 pin 3
- Brown, data out, connect to DB-9 pin 2
Autopilot
NMEA IN + ,
connects to the gps blue , NMEA out of the gps 125
NMEA IN - , this pin (the most
confusing of the installation)
connects to the black wire of the gps 125, it is a data ground
wire and needs to go to the gps ground rather than grounding at the
autopilot
DB-9 Pig tail in boat connected to the NMEA bus (female)
- n/c
- data out, connects to blue NMEA out of gps,
Mates to DB-9 Pin 2 data
in on computer
- data in, connects to brown
NMEA in of gps, Mates to DB-9
Pin 3 data out on computer
- n/c
- ground
Mates to DB-9 Pin 5 ground on computer
- n/c
- n/c
- n/c
- n/c
DB-9 Adapter (null modem, two male Db-9s back to back)
Used when transferring data between the hand and base gps units.
2. crossed to pin 3 of the other
connector
3. crossed to pin 2 of the other
connector
5. ground to pin 5 ground
DB-9 Cable for handheld gps (female)
2. connected to brown data out of GPS 45
3. connected to white data in of GPS 45
5. connected to black ground of GPS 45
Serial
Cable (male - male)
Finally, one extension cable is used to connect the
handheld gps
to the computer or boats pig tail... it has male connectors at each
end. This
allows the handheld to reach into the cockpit and serve as a backup to
the fixed mounted gps.
Serial
Port Connections
| PIN |
PURPOSE |
SIGNAL
NAME |
| Pin 1 |
Data Carrier Detect |
DCD |
| Pin 2 |
Received
Data |
RxData |
| Pin 3 |
Transmitted
Data |
TxData |
| Pin 4 |
Data Terminal Ready |
DTR |
| Pin 5 |
Signal
Ground |
Gnd |
| Pin 6 |
Data Set Ready |
DSR |
| Pin 7 |
Request To Send |
RTS |
| Pin 8 |
Clear To Send |
CTS |
| Pin 9 |
Ring Indicator |
RI |
Note: The three yellow terminals are used when connecting
the computer to a gps.
Warning: Several inquires have been made because of
difficulties getting communication between the gps and autopilot.
Very often the problem is that the data ground of the autopilot is not
routed to the ground of the gps. This is confusing, as the gps
has no data ground, it's tempting to think that the wire from the
autopilot NMEA- should be taken to ground at the autopilot
installation. While I don't know the reason this doesn't work...
the NMEA- should be taken to the black ground wire of the gps, at the
gps. Doing so has proved to establish communication on numerous
installations that otherwise wouldn't work or worked erratically.
.