Actisense NMEA 0183 Bedienungsanleitung PDF herunterladen (Seite 6)

Connecting devices together

The Basics

NMEA data is transmitted from an information source such

as GPS, depth sounder, gyro compass etc. These data

sending devices are called “Talkers”.

Equipment receiving this information such as a chart-

plotter, radar or NMEA display is called a “Listener”.

Unfortunately, onlyoneTalkercan beconnected ontoa

single NMEA 0183 system at any one time. Two or more

Talkers are simply not possible because they are not

synchronised to each other, and will attempt to ‘talk’ at the

same time (over each other), resulting in corruption of the

NMEA data, and potentially in disaster if valuable data

such as navigation information is lost or corrupted so that

it is incorrect and/or misleading.

Actisense produces a full range of products to solve all

NMEA interfacing requirements.

Please visit the Actisense website (www.actisense.

com) for full details on these and other Actisense

interfacing, Depth sounding and NMEA 2000 products.

The NMEA Signals

TheNMEA0183systemv2.0andlaterusesa”differential”

signallingscheme,wherebytwowiresareusedtotransmit

the NMEA data. These connections will be labelled as

either NMEA ‘A’ and ‘B’ or NMEA ’+’ and ‘-’ respectively,

depending on the instrument and manufacturer.

Whenconnectingbetweendifferentmanufacturers,there

can be some confusion, but it is simple and easy to

remember:NMEA‘A’ connects to NMEA ‘+’ and NMEA ‘B’

connects to NMEA ‘-’.

The Different NMEA Standards

TheNMEA0183specicationhasslowlyevolvedoverthe

years, so connecting one device to another is not always a

straightforward matter. The earlier versions of NMEA 0183

(before v2.0, as detailed above), used slightly different

connection methods and signal levels: the instruments

hadjustone“NMEA”dataline(‘Tx’or‘Out’),andusedthe

ground as the other line - similar to the way a computer

serial port works. This connection method is referred to as

“singleended”insteadofthe“differential”methodusedby

NMEA 0183 v2.0 devices.

Thedataformatislargelythesamebetweenbothsystems,

with v2.0 adding some extra sentence strings, and

removing older (redundant) sentence strings from the

specication.Thesituationisfurthercomplicated,asmany

manufacturersstillusetheold(“singleended”)methodof

connectionbecauseitischeapertoimplement.

SohowcananoldertypeNMEAdevicebeconnectedtoa

newertypedevice?

Careisneeded–itispossibletodamageoroverloadthe

output of a newer differential device if it is incorrectly

connected to an olderdevice.This is because the older

devices used ground as the return, whereas the newer

devices actually drive the NMEA ‘-/B’linebetween5vand

0v. Thus, connecting this output to ground will result in

high currents being drawn by the driver instrument,

resulting in potential overheating and damage to the driver

circuits.

However, the new Actisense ISO-Drive technology allows

the user not to worry about this potentially damaging

incompatibility. Instead, an ISO-Drive output can be

connected to an old type single-ended system, by

connecting the NMEA ‘+/A’ output from the ISO-Drive to

the single-ended NMEA ‘Rx’ or “In” input of the device.

Connect the NMEA ‘-/B’ output of the ISO-Drive to the

ground of the single-ended device. This provides the

required data signal return current path.

To connect an old type single-ended device to an OPTO

IN input, connect the NMEA ‘Tx’ or ‘Out’ output from the

single-ended driver to the OPTO IN ‘+/A’ input of the NDC-

4. Connect the ground line of the single-ended output

device to the OPTO IN ‘-/B’ input of the NDC-4. This

provides the data signal return current path. If the NMEA

‘-/B’inputisleftoating,thendatacorruption/errorsmay

occur.

Please refer to “Connecting to NMEA devices” on page

8 for examples of these connection methods.

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