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NMEA: What confusion is that?

In the recently discussions about my problem with the interface of my
autopilot with my GpsMap 276C, hapened discussions about NMEA, DSC and
AIS and to me this was very good to learn a litle bit about these
things, but now, I must confess that I am totally lost on this topic of
NMEA.

For example, I only knowed that NMEA 183 has versions 1.5, 2.0, 3.1
and was used to interface my simple gps with my old autopilot, the
speed was 4,800 bps and the messages were ASC II characters.

Now I am learning that this animal has a protocol type the old
start/stop, using CR/LF and "silence" bytes, each byte has 10 bits
instead of 8, it does do not alows more than 1 talker, can send
messages to no more than 3 listeners, the actual gps/ploters wich have
"Two NMEA Serial Ports" can not use more than one set as NMEA, some
says that NMEA 183 is only 4,800 bps, others says that DSC/DSCE are
NMEA 183, but uses 9,600 bps, but others says they DSC interface is
working well at 4,800, one says that AIS uses NMEA 2000, other says
that AIS is NMEA 183, but uses 38,400 bps, no one can show me what are
the format of the "secret" DSC, DSCE, AIVDM messages, no explain me how
NMEA can use only one port and can work at 4800, 9600 and 38400 bps?

Could the NMEA experts please, teach us here a litlle more, just about
NMEA 183 (please not NMEA 2000 yet) so that a simple sailor with an
autopilot, a VHF radio and dreaming of purchase an AIS receiver to plot
the ships in his Garmin 276C gps/ploter could understand?

Thanks

Pascal

"Pascal" wrote in message
oups.com...
In the recently discussions about my problem with the interface of my
autopilot with my GpsMap 276C, hapened discussions about NMEA, DSC and
AIS and to me this was very good to learn a litle bit about these
things, but now, I must confess that I am totally lost on this topic of
NMEA.

I'll answer some of the questions.

For example, I only knowed that NMEA 183 has versions 1.5, 2.0, 3.1
and was used to interface my simple gps with my old autopilot, the
speed was 4,800 bps and the messages were ASC II characters.

Correct.

Now I am learning that this animal has a protocol type the old
start/stop, using CR/LF and "silence" bytes, each byte has 10 bits
instead of 8,

That a byte has 10 bits on the serial line has nothing to do with NMEA, it's
just the way the serial port on your computer works. Every byte is lead by a
start bit and terminated by a stop bit, to be able to distinguish between
bytes. This has been the fact since the conception of the so called
"asychronous serial port". The number of stop bits can even vary between 1,
1.5 and 2 and there can even be an extra parity bit. Check the settings of
you com port for instance on your computer, there you can select these
parameters.

There are no silence bytes, there just has to be some silence in the data
stream very now and then, longer than one byte time, to reassure a receiver
can resync in the case it was lost, due to a spike for instance.

it does do not alows more than 1 talker, can send
messages to no more than 3 listeners

Only one talker can exist on one physical connection. And according to the
NMEA spec, it should be able to drive four listeners minimum.

the actual gps/ploters wich have
"Two NMEA Serial Ports" can not use more than one set as NMEA

That is of course, completely up to the manufacturer of the device.
some says that NMEA 183 is only 4,800 bps, others says that DSC/DSCE are
NMEA 183, but uses 9,600 bps

The official speed of NMEA 0183 is 4800 baud. Period. Then there is NMEA
0183-HS, which is 38400 baud. Period. Any NMEA device should support either
or both of these speeds. Period.
There is no way that normal NMEA is 4800 while DSC is 9600. DSC is also
4800.

but others says they DSC interface is
working well at 4,800, one says that AIS uses NMEA 2000, other says
that AIS is NMEA 183, but uses 38,400 bps

NMEA2000 is a completely different specification. It is CAN based, RS-485
"like" and runs at 250 kbaud. And the messages are binary data instead of
ASCII.
AIS is just plain NMEA and ASCII, but, the messages are so complex that they
can not be conveyed within 80 characters and in plain ASCII. Therefore, AIS
messages are binary data, which in encoded 6 bits at a time, where those 6
bits are transormed into an ASCII character. Normal hexadecimal encoding
requires two characters per byte encoded (4 bit per character), now 6 bits
can be encoded with one single character.
Because an AIS receiver can spit out many sentences in a busy area, all run
at 38400 baud to accomodate the amount of data better than at 4800 baud.

no one can show me what are
the format of the "secret" DSC, DSCE, AIVDM messages, no explain me how
NMEA can use only one port and can work at 4800, 9600 and 38400 bps?

Well, ALL NMEA sentences are perfectly documented in the NMEA 0183 manual,
which can be purchased by anyone at www.nmea.org. The problem is, that this
is copyrighted material, so those who have that manual, cannot simply copy
it and/or publish that information to anyone who does not want to pay for
that. It is sad, but that is just the way it is.

Could the NMEA experts please, teach us here a litlle more, just about
NMEA 183 (please not NMEA 2000 yet) so that a simple sailor with an
autopilot, a VHF radio and dreaming of purchase an AIS receiver to plot
the ships in his Garmin 276C gps/ploter could understand?

Well, the website of Peter Bennet shows much good information about the
basic set of NMEA sentences but like many other sites showing this info, it
is not complete and it cannot be since one is not allowed to publish the
standard. I've been there and got bitten by the NMEA organisation. The only
reliable source is the manual from NMEA itself, which is frequently updated
with new sentences and additions to old ones. But it costs around $250 for
non NMEA members :-(

Meindert

Meindert,

Thank You very much for your answers. The things now are much more
clear to me.

And now I could understand wy the folowing batch of AIS messages seams
me like a encripted message.

!AIVDM,1,1,,A,33M@HUP00OlOi@LD3O=?gvb0000,0*18
!AIVDM,2,1,4,A,53M@HP1s`ThLa22218tpEb22222222222 2221?6@:7374QN9QmADST,0*76
!AIVDM,2,2,4,A,iFH888888888880,2*7F
!AIVDM,1,1,,A,33M@HUP00OlOi@LD3O=?gvd0000,0*1E
!AIVDM,2,1,5,B,53M@HP1s`ThLa22218tpEb22222222222 2221?6@:7374QN9QmADST,0*74
!AIVDM,2,2,5,B,iFH888888888880,2*7D
!AIVDM,1,1,,B,33M@HUP00OlOi@LD3O=?gvd0000,0*1D
!AIVDM,1,1,,B,139OT10036OkBhLVT=aoGpf0L13,0*53
!AIVDM,1,1,,B,139OT10037OkAe0LVSKan7q608Fk,0*58
!AIVDM,1,1,,B,34QB:@002TwjB3lLQD1C:3AN20TQ,0*07
!AIVDM,1,1,,B,34QB:@002WwjBlLQD8S:3=V20S@,0*60

Best Regards

Pascal

Thank You Jacquelin,

Very informative sites, I will look at its more in depth later.

Regards

pascal

Pascal wrote:

Could the NMEA experts please, teach us here a litlle more, just about
NMEA 183 (please not NMEA 2000 yet) so that a simple sailor with an
autopilot, a VHF radio and dreaming of purchase an AIS receiver to plot
the ships in his Garmin 276C gps/ploter could understand?

Read my article on NMEA posted on my web site and at gpsinformation.net.

dale

Thanks

Pascal

--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

Nmea Plus Is the way to go. I downloaded it and Wow!! Fantastic.

It decodes all AIS strings and you can even use it to transmit to other
AIS units too!

I see that these guys are using it to display AIS targets on there website
in real time on real maps!

I just registered my copy and I was blown away by the new features that
opened up. Give it a try.

You can now run one copy of NMEA Plus on one computer (server mode)and run
client versions on other pcs in the same network and display the data on
those pc's too!

http://www.electrotech.net.au/ea_downloads.php