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Subject: Re: TWL: redundant instruments
From: Arild Jensen (elnav@XXX.XXX)
Date: Fri Mar 01 2002 - 12:03:22 EST
At 11:09 AM 03/01/2002 -0500, David P. Ross wrote:
>I currently switch NMEA data from three GPS's and one LORAN into my
>chartplotter and the outputs of a handheld GPS, a fishfinder/GPS head, and
>the chartplotter to my autopilot. NMEA requires two wires, so a DP switch
>is required.
REPLY
NMEA 0183 is a communication protocol which has one talker ( source) and
several listeners ( displays ).
If you only have one display and two sources then you do need a selector
switch toavoid confusion when two sources mix their signals.
However, if you have one source and several displays then running th e
signal in paralle is useful,
NMEA has a limitation as to how many devices can be driven from one
source. It varies with brands.
NMEA bridges or repeaters solve this problem by providing an amplified
signal to drive more displays.
The optically isolated bridges such as Capt Jack provides is an
excellent solution.
The optical isolation blocks the DC path from one device to another.
In the event of a failure ( short circuit) in one it will not affect the
other devices.
Some of the more sophisticated bridges can even handle multiple inputs.
Autohelm ( now part of Raymarine) has a NMEA bridge which can mix Seatalk
and NMEA while providing bi-directional porting.
In additon ther eis an RS 232 port specifically for a laptop.
RS 232 is not quite the same as what the NMEA data stream uses and has in
the past caused problems so that a computer would not read data.
The better systems will have multiple data sources on line
simultaneously but will chose the preferred source until it fails; at
which time it reverts to the secondary source. However at least one
display would always be showing this secondary data as a means of
confirming it is valid and available. One of the navigator's duties would
be to compare multiple sources of positioning data and confirm that they
aggree.
Otherwise you might switch to the backup only to find it had failed
unnoticed sometime in the past.
Cheers
Arild
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