[ntp:questions] almost decided which new PPS GPS to buy

Dave Hart davehart at gmail.com
Mon Mar 5 02:24:12 UTC 2012

On Mon, Mar 5, 2012 at 01:26, David Lord <snews at lordynet.org> wrote:
> You need to use PPS to get lower offsets. I've no idea if
> PPS will work via a serial to usb adapter but I'd guess
> that it will and you will get an improvement.

In general, it won't.  The problem is the PPSAPI DCD timestamping
needs to be part of the driver handling the serial port.  On Linux, I
think you'll find the 16550 UART driver knows how to timestamp DCD for
PPSAPI, but the pl2003 driver doesn't.  Same story on Windows for real
PPSAPI support -- it's available for 16550 using serialpps.sys, but
not using the pl2003 driver.

On Windows, if the NMEA driver is configured to use the first sentence
emitted after each PPS, the user-mode PPS hack will get you sloppy PPS
timestamping, delayed by a few hundred microseconds compared to
interrupt-time timestamping under the best of circumstances, and
subject to much higher delays under load.

Dave Hart

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