Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

cooling fins on the back of the display, heat is a
concern, so as an installer I need to think about
getting air circulation around those fins.

Interfacing GPS with Other Equipment

Interfacing GPS with other equipment, such as
autopilots and repeater displays, can be one of
the biggest challenges in working with GPS,
although this is generally less of a problem if the
other units are supplied by the same manufac-
turer. The GPS unit calculates a lot of data as
your boat moves through the water, then it
updates the data in real time, and sends it over a
network to other pieces of electronic equip-
ment, such as a DSC-equipped VHF, auto-
pilot, and radar display (see illustration oppo-
site). We’ll look further at networking in Chapter
17, but specifically as it applies to GPS, there
are several things to point out here.
First is the network protocol. Much of this
has to do with the digital language the various
devices in the network are using. The two
mostcommon networking protocols for GPS
are NMEA 0183 and NMEA 2000 (see also
Chapter 17). There are several versions of
NMEA 0183, however, and even within these
standards, the “sentence structure” of the digital
messages may differ from one manufacturer to
another. Some manufacturers don’t use NMEA
at all, but go with proprietary networking
schemes based on Ethernet, a common office
network, or on controller area network (CAN),
a common automotive architecture.
The bottom line is that it can be difficult to
achieve complete compatibility between devices
from different suppliers. Some data may transfer
readily through the network and some may not.
The only way to ensure compatibility is to
locate the equipment’s network version number
and specific sentence structures in the specifica-
tion manual for any given piece of equipment.
Then compare this information to specifications
for the piece of equipment you are attempting
to link via the network. Obviously, if you do this

COMMONLY USED NMEA 0183

SENTENCE CODES

APB = autopilot, format B

BOD = bearing, origin to destination waypoint

BWC = bearing and distance to waypoint (great circle)

BWR = bearing and distance to waypoint (rhumb line)

DBT = depth below transducer

GGA = GPS fix data

GLL = position in latitude and longitude

GSA = GPS DOP (dilution of position) and active

satellites

GSV = satellites in view

HDM = magnetic bearing

HSC = command heading to steer

MTW = water temperature in degrees Celsius

RMB = recommended minimum navigation informa-

tion when a waypoint is active

RMC = recommended minimum specific GPS/transit

data

RTE = route information in active route

VHF = water speed and heading

VTG = track made good and ground speed

VWR = relative wind direction and speed

WCV = waypoint closure velocity

WDC = distance to waypoint

WDR = waypoint distance (rhumb line)

WPL = waypoint location

XTE = cross-track error, measured

XTR = cross-track error, dead reckoning

beforebuying a new piece of equipment, you’ll

have the luxury of working with equipment

that’s designed to work together.

Some examples of the specific codes found

in the sentences that talkers and listeners must

be able to transmit and receive are shown in

the sidebar.

It may be possible, however, to network

devices that utilize different protocols or lan-

guages with ancillary “black boxes” that are

installed between talkers and listeners. Basically

these boxes are translators, whose purpose is to

installing communicationsand position-finding systems 151