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[aprssig] RE: Stupid D7 tricks

Alex Carver agcme2002 at yahoo.com
Fri Feb 16 18:36:30 UTC 2007


Oh, I completely understand what I did.  I did it on
purpose and knew what the outcome would be.  "Stupid
D7 tricks" is a little twist on the phrase "Stupid pet
tricks" which is a segment of the David Letterman show
here in the US.

I wasn't trying to imply that I didn't know what was
going on, just that I decided to implement a built-in
feature in a rather amusing but useless way -- much
like stupid pet tricks are amusing but useless.

Anyway, doing this does not work well because most
consumer GPS receivers send out four or five NEMA
sentences every second.  Since the internal storage
buffer is only queried at one second intervals (the
maximum rate available through the LTMON command) the
end result is either the same sentence type making it
through or, as was the case of my GPS receiver, two
sentences alternating back and forth at times.  The
actual results are unpredictable and completely depend
on the timing between enabling GPS receiver data
collection, enabling the LTMON command, and any
latencies in the TNC or GPS receiver.

It would be more useful if there was a way to pass
data in the other direction (PC port to GPS port) but
I don't think there's a chance of that unless hidden
deep within the firmware of the TNC there exists a
command (even a low level bit-banging command) to do
so.

> ------------------------------
> 
> Message: 7
> Date: Fri, 16 Feb 2007 09:18:23 -0000
> From: "Dave Baxter" <dave at emv.co.uk>
> Subject: RE: [aprssig] Stupid D7 tricks
> To: "TAPR APRS Mailing List"
> As in all things...
> 
> Computers only do what they are told.  (Even the
> tiny embedded ones!)
> Just that what you tell them to do, is not always
> what you want, expect
> or sometimes need them to do...
> 
> In this case, I suspect that any string sent from
> the attached GPS with
> a $ at the start (just about all of them!) will get
> passed through to
> the attached device on the PC port.   Not stupid,
> just doing what you
> told it to do...   :-)
> 
> It's actually useful to know it can do that sort of
> thing.
> 
> If you have a PC with two working COM ports, connect
> one to the GPS port
> (4800N81) and one to the PC port (9600N81 or E71,
> depending) and
> experiment by sending from the terminal program on
> the GPS port, to the
> other via the D7, and I expect you'll find that
> anything with a $ at the
> start will get passed through.
> 
> You can also do that trick (connect a PC to the GPS
> port) to verify that
> the D7 (or D700) sends posits in NMEA format "to"
> any attached GPS if
> you need that functionality.  The radio's PC and GPS
> port are physically
> wired the same at the radio.
> 
> Have Fun!..
> 
> Dave G0WBX.



 
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