Actisense NMEA 0183 Bedienungsanleitung PDF herunterladen (Seite 8)

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 for full details on

these and other Actisense interfacing, Depth sounding

and Sonar 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.

Connect the ground line of the single-ended output device

to the OPTO IN “-/B” input of the NDC. 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 the Output Connections section for

example of these connection methods.

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