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[aprssig] APRS and Garmin Nuvi

Alex Carver kf4lvz at yahoo.com
Fri Feb 1 21:48:45 UTC 2008


> Date: Fri, 01 Feb 2008 07:45:55 -0800
> From: Scott Miller 
> Subject: Re: [aprssig] APRS and Garmin Nuvi
> 
> Looks like a standard mini-B to me.  That's not to
> say they couldn't 
> overload the pin functions - you can already signal
> things like bus 
> resets with single-ended conditions on the
> differential lines.  Could be 
> as simple as a pull-down on the + line that switches
> D- and D+ to RS-232 
> operation.
> 
> The blob on the cable isn't large enough for
> anything bigger than an 
> LQFP32 at most.  I really don't expect to find
> anything that complicated 
> in there.

There's probably nothing more than a resistor or two
in the cable.  This is a pretty common thing to do
with dual protocol devices.  Think back a bit to
standard keyboards.  A bunch of the USB keyboards also
came with the PS/2 adapters (and some mice, too).  The
USB client chip on board could look at the signals and
loads present on the pins and determine which type of
port it was connected to and then speak that protocol.
 The pins there were overloaded as well.  Same thing
here.

With USB-To-Go still a bit of a ways out, RS232 is
still the nearly universal, easy way of getting things
to talk together.  So this is the nice, cheap way of
getting the benefits of both into one package with one
chip.  You can talk to it by USB or you can still talk
to it by RS232 (probably with some limitations) and
satisfy multiple customer markets with one reference
design and a few extra lines of code.


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