"purple_stars" wrote in message
oups.com...
it's all pretty fubar, that's why it's hard to understand.

That is obvous...

With all respect Luc, but you're mixing things up very badly in your attempt
to explain things.
I'll comment between the lines:

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

No. NMEA 0183 is a protocol AND a hardware specification, which exactly
specifies the voltage levels and the number of interface wires too. The
hardware side is the same as RS-422.
Any device with an RS232 port is not compliant with the NMEA-0183 standard
and is as such even forbidden on commercial vessels sailing under SOLAS
rules.

A GPS with an RS-232 interface will output NMEa formatted sentences but it
is stricktly speaking not an NMEA interface. The problem is that many
manufacturers choose to deviate from the standard in order to make a cheap
computer interface.

Seatalk is a proprietary protocol and hardware specification created by
and for a manufacturer of marine equipment.

Seatalk is a hardware AND a protocol (messages) specification which is
completely different from NMEA and RS-232 or RS-422. It is a one-wire
multiple-access collision detect protocol and indeed proprietary.

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 ...

Not at all. The only resemblence with "RS-232" is that it is serial data.
RS-232 is no protocol spec either, it just specifies the signals and their
levels on a connector. And an RS-232 link is meant to be a point-to-point
connection between two devices only.
Seatalk works on different levels AND it has 9 bits per byte while your
ordinary serial computer port can only handle 8 bits per byte (there are
some unreliable workarounds...).

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.

There is a better reason in the fact that it is a multiple-access collision
detect protocol which simply means that you can connect more than one device
on a line and they can all talk and listen.

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.

NMEA 2000 is far far away from RS-232. Like I said earlier, RS-232 is only
for point-to-point connections while NMEA 2000 (CAN based) is a network for
multiple devices. The standard is open but you have to pay around $10,500
for the documents, test suites and manufacturer/product codes.

Meindert