"Larry" wrote in message
...
"Meindert Sprang" wrote in
:

You need to get your facts straight! A Raymarine C/E series
chartplotter
runs at 38400 baud when in AIS mode. Most likely others do that too....

Oh and besides that, there is also something called NMEA0183-HS which
IS
38400 at RS422.

Meindert



hMM....We're buying new equipment, again....sorry. What about wonderful
NMEA2000, or whatever NMEA is calling their latest and greatest version
to sell new equipment this month? Does it run 38,800 baud?

No, NMEA2000 is CAN based and runs at 250,000 bps.

I'm still wondering why we don't just make the AIS receivers run 4800
baud. On a boat with a 12 mile range, there aren't enough AIS ships to
jam a 4800 baud port up, at least that couldn't be buffered cheaply for a
few milliseconds as it spits it out.

First of all, the AIS data transmission speed over VHF already 9600 baud so
it would be strange to pack this received data in an NMEA string (more bytes
added) and then squeeze it down a 4800 baud channel.
Further, a moving ship transmits his info at least once every 10s (0-14kn)
to every 2 seconds when between 14-23kn and changing course. So in worst
case your receiver gets one AIS message/2s. Each message contains 168
databits, which are packed into 6 bit charachters for NMEA, that is at least
21 chars of AIS data in a VDM sentence. The minimum VDM sentence length is
then 43 characters. At 4800 baud, this takes up 89ms so at 4800 baud your
AIS receiver would be able to transfer the bare minimum info of 11 ships at
a time. Many AIS messages are however longer and can take up to appr. 1000
bits of info, which would result in at least 3 longer VDM sentences per
ship. Hence the need for 38400.


Do you know why they insist on 38,800 baud RS-232C, not RS-422 levels??
Seems really stupid to me.

Yes and no. One would expect a true NMEA0183-HS interface (38,400 at RS422)
but most, if not all AIS receivers will be connected to either a computer or
a computer based ECDIS. And an simple galvanically isolated intput
consisting of an opto-coupler doesn't really care wether the offered signal
is RS422 or RS232. Between the wires, there is always the same differential
voltage swing of 5-10V in either direction.

Meindert