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Re: [gpsd-dev] Very basic PPS question:

From: Greg Troxel
Subject: Re: [gpsd-dev] Very basic PPS question:
Date: Sat, 19 Oct 2013 13:44:35 -0400
User-agent: Gnus/5.130006 (Ma Gnus v0.6) Emacs/23.4 (berkeley-unix)

"Eric S. Raymond" <address@hidden> writes:

> When a GPS asserts PPS, is it top of second for the in-stream 
> data *preceding* or *following*?  
> I presume following, but I want an expert to confirm before I
> put it in the HOWTO.

There can be no standard answer, and the question is really just about
how in-stream is sent, not about the PPS pulse.

PPS is supposed to be the on-time tick of every second.  That can exist
without any timecode data.  It is possible in the NTP world to get PPS
From one device and timecode from the net or another device.

For every device, it will (should) have a specification that explains
how the timecode (serial, usb, whatever) data relates to some point in
the transmission of the timecode.  This is the hard part, and it really
has little to nothing to do with the PPS pulse.

I would say that typically devices that have PPS have a relatively sane
relationship of timecode tranmission to the on-time seconds tick, like
the start bit of the CR after the time code is aligned, or something.
So for sane devices, this should not be super hard in practice.  But
I've mostly paid attention to the details of a TrueTime XL-DC, which is
a 20-year-old fairly high-end timing receiver.

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