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'[PIC]: Old GPS thread...'
2002\12\14@142517 by Chris Loiacono

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I dug up this old posting because I just read in the NMEA 0138 standard that
4800 baud is Max. Is the document I'm reading out of date? any experts out
there today?

Chris

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2002\12\14@160223 by fred jones

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To this original thread, I had given several links that gave me enough info
to write my code.  The standard NMEA 0183 baud rate is 4800 baud however
most GPS units will have a setup menu that allows you to change it to a
higher rate.  I have found that it would be more appropriate to call it a
recommendation rather than a standard as there is so much vagueness that has
lead to many variations of its implementation.  Maybe version 2 will nail it
down tighter.
Goodluck,
Fred






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2002\12\14@212448 by Russell McMahon

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AFAIK all "normal" GPS's output at 4800 baud.
No doubt you could get manufacturers to use another data rate if you were
buying a moderate amount.

At 4800 baud a typical collection of output "sentences occupies about half a
second (around 250 bytes). If you were wanting a slower baud rate (unlikely)
you'd need to select what sentences were sent each time.

       RM


> I dug up this old posting because I just read in the NMEA 0138 standard
that
> 4800 baud is Max. Is the document I'm reading out of date? any experts out
> there today?

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2002\12\15@071437 by Olden A (SApS)

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All GPS (0183 standard) units should output at 4800 - that's specified in
the standard, (and you could get a manufacture to up but, without wanting to
should pedantic - then it would not be the 0183 standard).

There is a revision of the 0183 (well, it's more of a complete rewrite)
called the NMEA 2000, which rather than using a serial transmission method
is based around what appears to be the OSI 7 Layer Model (or TCP/IP haven't
worked it out yet). And than can do up 1.5Mbit a second (AFAIK!) with a
range of transmission speeds up to that. But the 'old' 183 is still there,
And will be for years.




{Original Message removed}

2002\12\15@124956 by Olin Lathrop

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> There is a revision of the 0183 (well, it's more of a complete rewrite)
> called the NMEA 2000, which rather than using a serial transmission method
> is based around what appears to be the OSI 7 Layer Model (or TCP/IP
haven't
> worked it out yet). And than can do up 1.5Mbit a second (AFAIK!) with a
> range of transmission speeds up to that.

NMEA is layered on the CAN bus, and is specified at 250Kbits/second if I
remember right.  It does not implement TCP or IP.


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(978) 742-9014, http://www.embedinc.com

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2002\12\16@040927 by Olden A (SApS)

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Yep, that's the one, couldn't remember the exact details, I could just
remember that it was a multilayered system (Network/Application Layers etc).

I thought it could do a lot faster than 250kbit a Second though?

Andrew



{Original Message removed}

2002\12\16@153151 by Olin Lathrop

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> Yep, that's the one, couldn't remember the exact details, I could just
> remember that it was a multilayered system (Network/Application Layers
etc).
>
> I thought it could do a lot faster than 250kbit a Second though?

CAN can do up to 1Mbit/second, but the NMEA 2000 implementation specifies
a slower speed.  This is because NMEA 2000 has to work with a maximum
length CAN bus the same length as an oil tanker or aircraft carrier.  The
low level CAN protocol requires that the maximum round trip delay be a
fraction of a bit time.


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(978) 742-9014, http://www.embedinc.com

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