Re: Whats the point of 2 COM ports? (276)

On Tue, 31 May 2005 12:18:46 GMT, "Pieter Litchfield"
<pvcl@*nospam*plitch.com> wrote:

>
>"Pascal" <pascalg@xxxxxxxxxx> wrote in message
>news:1117502575.148427.40090@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> If you read the specifications on Garmin site, what is said is: "two
>> serial NMEA ports", not "two serial ports". So, this implies that the
>> unit has 2 serial NMEA ports and could use these TWO serial ports as
>> NMEA ports...
>
>You are exactly right. I did have a look at the Garmin site (before I
>ressponded), and I can't explain their description. I noted that the GPS
>also has a USB porrt, which I assume would be the preferred path for
>uploading/downloading files like routes, waypoints, or detail maps.
>
>A quick discussion about the terms being used. A "serial port" is any
>arrangement that allows serial communicatins between two devices. This can
>be as simple as 2 wires since all the bits comprising the characters in the
>message are transmitted one after the other, and a simple binary arrangement
>will suffice. While not a requirement of serial communications, the DB-25
>or DB-9 style connectors are often used to carry the data, ground and other
>signals that are part of the RS-232 serial standard
>
>NMEA is not the same as serial. NMEA is a series of data "sentences" that
>allow for the transmission of location and naviagtion information between
>two intelligent devices. It has nothing to do with wires.

Jumping into this late, so maybe I missed something, but this is not
so. NMEA0183 specifies both sentence structure and electrical
characteristics. Specifically, RS-232 is a ground referenced signal
with extra control lines that are often unused, as you seem to know.
NMEA is RS-422, which has no control lines and is a differential
signal. It is not ground referenced, which is why you can sometimes
have trouble communicating betwenn RS-422 and RS-232. However, it is
a fact that the NMEA requirement for RS-422 is not always honored in
the execution. Many GPSs do seem to be nothing more than a 3 wire
RS-232 implementation. If the wiring is ground, + power, NMEA in, and
NMEA out, you obviously aren't looking at RS-422. The RS-422
equivalent would be ground (power only) , + power, NMEA in +, NMEA in
-, NMEA out +, NMEA out -.
>
>To say this GPS has " serial NMEA ports" is, at the least, syntactically
>incorrect. You really can't have a "NMEA port" since there is no such thing
>as a physical presence of NMEA. You can have a serial port cpable of
>sending and receiving NMEA messages. When I saw the description, I took
>this to mean that like most GPSs, this unit could send or receive NMEA
>sentences through a serial port. Perhaps like most ofther GPSs, that port
>could be used to send and receive other data like files and/or DGPS
>corrections intheir own format. I have 4 handheld GPSs - 3 have serial
>ports (1 has USB only). In all cases, that 1 port can send and recieve
>waypoints, tracks, detail maps, DGPS corrections, and NMEA data. The
>disadvantgae is that they can only "talk" to one device at a time.

Again, an NMEA port should be RS-422, but would be capable of handling
anything you can send over RS-232, as long as control functions like
RTS/CTS are not required by the othe port. However, you cannot
necessarily mix signals. For instance, Garmin NMEA signals and PVT
signals (I think that is the term?) can both be sent over a given
port, but not at the same time in any implemmoving waypoints around
would be nice too.entation I've seen. Probably as much a software
limitation as a hardware limitation, but I've never looked into it.
>
>Having 2 ports could be a real advantage since one could be permanently
>connected to a navigation device of display while the other could be used
>for real-time corrrections. By using the USB port for moving data like
>waypoints and detail maps, it would be possible to keep everything hooked up
>all the time to the different ports. I suppose there is no need to have all
>the ports NMEA capable. But clearly this is what is implied by the
>manufacturer's claims.
>
Yep. I would like my GPS to supply NMEA data to my PC, autopilot, and
VHF without rewiring on the fly. I do this by running the GPS to a
RS232-RS422 convertor to isolate the ground, then paralleling the
RS422 output to all three devices. However, the GPS can only receive
data from one of the three without rewiring. Having 2 NMEA ports
would solve that problem. Having a third USB port for moving
waypoints around would be sweet.

____________________________________________________________
Glen "Wiley" Wilson <usenet1 SPAMNIX at world wide wiley dot com>
To reply, lose the capitals and do the obvious.

Take a look at cpRepeater, my NMEA data integrator, repeater, and
logger at http://www.worldwidewiley.com/
.

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