it's all pretty fubar, that's why it's hard to understand.

luc wrote:
I am an NMEA neophyte. I am trying to connect my Raymarine speedo,
depth, and wind indicator. They use SeaTalk. Apparently, SeaTalk is
not NMEA language. I thought the point of NMEA was to make multiple
manufactures equipment communicate, much like MIDI (musical instrument
digital interface) works for electric guitars, keyboarsd, and
synthesizers.

here's kind of the basics ...

NMEA ... depends on which standard you are talking about. there are
two, the NMEA 0183 standard which is a one way (one talker, many
listener) protocol for communicating GPS information from a GPS
receiver to one or more GPS data clients and a seperate NMEA 2000
protocol that is used for something more sophisticated. the NMEA 0183
protocol is sort of a language protocol, a series of text based
sentences, each one being a GPS fix, and the GPS receiver sends them
out on a periodic basis over 4800 baud. i don't think NMEA 0183 is a
hardware specification, however ... it's just a protocol, and it runs
over an rs-232 interface USUALLY, but it doesn't have to. it can also
be over USB, bluetooth, or whatever, the underlying hardware interface
as far as i know isn't specified. GPS receivers can put their GPS
information out on a serial cable in all kinds of formats, for example
my garmin i think has at least 10 different formats it can use. one of
those formats will be NMEA, usually, but some GPS receivers don't
support the NMEA format. that's all NMEA 0183, more about NMEA 2000
further down.

seatalk is a proprietary protocol and hardware specification created by
and for a manufacturer of marine equipment. it's not an open standard,
that is, as far as i know it's not published at all. it's kind of like
the ICOM CI-V protocol, it's just made by the manufacturer and they do
whatever they want with it to make their stuff work together. and they
charge a pretty penny to hook it all together, of course. seatalk is a
protocol that is used to transfer mostly sensor information such as
depth, wind speed, GPS information, etc, between instruments and
control equipment that talk it. at a hardware level it's also a
hardware spec which is basically rs-232 except that it's inverted ...
for god only knows what reason. a cynic might think they did it just
so it would be different than rs-232 and they could call it seatalk and
charge more, but i'm willing to give them the benefit of the doubt and
think they had a better reason.

NMEA 2000 is basically a network type of protocol like seatalk, except
it's an open standard that anyone can use. i haven't actually seen the
standard, but it's a protocol and a hardware specification, and i would
assume that it's very similar to rs-232 if not the exact same thing.
sort of the world's answer to a successful proprietary protocol when
the manufacturer is secretive and won't put out a specification for how
their stuff is hooked together. this is a bidirectional "networked"
type of protocol for transferring information like seatalk does around
different instruments and sensors.

I have a Garmin GPS, which does have some NMEA capabilities. I would
like to connect it to my ICom VHF, for the DSC feature. I would also
like to connect my sounder to the GPS to show depth on the Garmin. Is
this possible? Does anyone know where to get more information about
NMEA and connecting these various devices? I've already learned a bit,
but have a long way to go.

well, at a hardware level it's all pretty much rs-232, even seatalk
isn't that much different and can be converted, so you can learn a lot
about it just by reading about interfacing rs-232 devices together.
it's mostly just TX, RX, and ground wire(s), with sufficient attention
being paid to RF interference. at a software level i.e. the
communications protocol, that's a different animal altogether and
depends totally on what you are doing.

Also, from what I hear, the issue of "daisy chaining" is somewhat
resolved by the new NMEA 2000 protocol. Easy connectors, simple
wiring. Still, NMEA reminds me of DOS based computing. (it amazes me
that windows has DOS underlying it!)

if you mean it's very basic, it is, it's certainly not ethernet or
something like that. but you'd want it to be pretty basic. the
instruments aren't transferring very complicated data, it's all just
sensor readings and things. rs-232 is great for a lot of reasons, one
is that it's understood by a great many people, and another is that
it's fairly simple. it's also strong over longer cable lengths, easy
to interface to and make devices for, etc, and it's not that hard to
keep it from picking up distracting amounts of RF noise. so though it
isn't the most sophisticated way of doing things, maybe, it is pretty
hard to destroy and relatively easy to fix with available wiring.

in terms of development, rs-232 is much better than something
sophisticated like ethernet for these low bandwidth applications.
rs-232 is very simple and interfaces for it are even included in things
like PIC processors, very simple devices that are inexpensive to buy
and use. but as soon as you start wanting to talk ethernet then you're
talking about a TCP/IP stack, much higher bus speeds, some kind of a
basic operating system (typically), etc, you're going up a level in
sophistication and price. and you're also adding in a lot of potential
for failure as those devices get more sophisticated. there's something
to be said for being able to just grab some dumb terminal off the shelf
from like 1960 and hook it to your serial cable and see data coming
across it.

I am sorry I bought my Ray Marine instruments that are not NMEA
compatible, if that's the word. Apparently, if I get a multiplexer, I
can make Sea Talk convert to NMEA. I'm sure all this junk is in it's
infancy, and one day will be resolved by some very cool technology.
It's already pretty cool, especially the Blue Tooth multiplexer made by
a company I don't know but recommended by someone on a google group.

anyone that cares to discuss this, I'm listening.

thanks

Luc