[Thumbgps-devel] USB handshake signals and Linux

[Eric S. Raymond]

Patrick Maupin <pmaupin at gmail.com>:
> The latency timer on the FTDI parts can be dropped to a millisecond.
> If we find a USB dongle that has a USB converter with similar
> capabilities and a GPS chipset that has rock-solid first-character
> timing, we might avoid needing the RS232 handshake altogether.

There's a reason I'm a bit wary of a non-1PPS design based on timing
the leading edge of the $GPZDA or whatever. That is this: unless the
device is self-identifying, there'll be no way to know when the time
information can be trusted.  Remember that the bufferbloat deployment
is planned to use GPSD and that we *cannot* assume that GPSD 
knows anything hand-configured about the USB devices it talks to.

By "self-identifying" that it ether (a) has a recognizable USB
vendor/ID pair, (b) ships a unique sentence type that GPSD can
recognize, or (c) there's a probe string GPSD can ship it that elicits
a unique response.  For reasons of history and vendor stupidity most
GPS-chip/adapter combos will fail all three tests.

One of the advantages of 1PPS is that it is self-identifying in this
way. If you're talking to a GPS, nothing but 1PPS produces DCD or RI
transitions, so when you see a 1PPS pulse you know what it is.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>