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[aprssig] Really CHEAP Puck-type USB GPS Receiver

Stephen H. Smith wa8lmf2 at aol.com
Sat Jan 21 16:28:57 UTC 2012

On 1/21/2012 10:57 AM, Stephen H. Smith wrote:
> Actually, if you wanted to provide 5 VDC from an external source, it should 
> be possible to tap in ahead of the serial-->USB converter chip to get at the 
> serial data stream.    However, the serial signal will probably be at TTL 
> levels and would have to be inverted, possibly with an added MAX-232    
> TTL-->RS-232   level-converter chip.
> I'll crack mine open as soon as I receive it and investigate this possibility.
> -------------------------------------------------------------------------------

1)      Actually, you probably only need a single NPN transistor and a couple 
of resistors to invert the TTL signal, rather than convert to true +/- voltage 
swing RS-232.    The majority of serial ports will accept "half-232" signals 
that swing from 0 to 5 volts rather than the full RS-232 swing of -8 to +8 
volts.   Indeed the TinyTrack (and many other smal-box serial-connected 
gadgets) has this this kind of "inverted TTL" I/O to avoid the need for a 
negative voltage supply or MAX-232 converter chip.

Getting at the serial data before the USB conversion is not new.

During the transition period from serial to USB on PCs a decade or so ago, 
there WERE some universal USB+serial GPS units that brought the TTL/serial 
data, before the USB conversion, out of the unit on additional conductors in 
the cable.  You typically had some odd connector on the end of the cable and 
then two pigtails adapters; one with a USB "A" plug,    and one that forked 
into DB-9 plug (for data) and  round PS/2 connector (to steal 5VDC from a 
mouse/keyboard port on a laptop).  Unfortunately these versatile units, that 
were the perfect universal solution for APRS hams, disappeared from the market 
before the excellent SiRF-III GPS chipset appeared.

2)     My intent in the original posting "Really CHEAP Puck-type GPS" was to 
mention this device as a promising candidate for *PC-based* APRS setups because:

o     It was really cheap.

o    USB-interfaced devices are far less hassle to hook up to today's 
"legacy-free" no-serial-port PCs.

o    The USB hookup neatly avoids the cable mess with serial-interfaced units 
connected to today's PCs,  where you have to provide some external power source 
AND a serial<-->USB converter dongle.

Obviously, for use with non-PC devices with classic serial ports (like 
TinyTracks, Tracker2s, Kenwood radios, etc) it would make far more sense to use 
a serial GPS like the ones available from Argent or Byonics in the first place, 
rather than "undo" and hack a USB unit.



Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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