"Dave Hart" <daveh...@gmail.com> wrote in message
news:CAMbSiYBZw1wMb0cn1t5u=l5f+aduomywhyx6ayznbatrbzj...@mail.gmail.co[]
No, the PPSAPI support is in serialpps.sys, a slightly-modified 16550
UART driver. When using serial-over-USB, a different driver (for the
pl2003 or similar chip) presents the serial port and it will not have
the PPSAPI ioctl to timestamp DCD in the driver.
Thanks, Dave. Yes, the kernel-mode PPSAPI support will only be true for a
real serial port. I appreciate that, but thanks for the clarification.
On Windows only, ntpd has a hack user-mode PPS timestamping
implemented in the serial I/O layer which substitutes the last PPS
timestamp taken by ntpd after Windows indicates the DCD transition for
the end-of-line serial timestamp of the first line received after each
PPS event. That requires configuring ntpd to use only the first
sentence after the PPS, and it won't work on Linux, so I doubt Ron
will be interested in that approach.
The Linux PPSAPI has the same type of driver issue -- the code to
timestamp DCD is in the UART driver, but not the pl2003 driver, so
PPSAPI will work with a real serial port but not with a USB/serial
adapter.
Cheers,
Dave Hart
Dave, perhaps you could clarify one thing on Windows about the user-mode
PPS timestamping. When you say "serial I/O layer", my understanding is
that this /would/ include virtual COM port devices such as RS-232/USB
adapters. So when a DCD transition occurs (and providing the USB drivers
handle the DCD line properly), NTP's code would be called to say a DCD
transition had happened, and that time would be noted, albeit in user
mode. Later, when the first sentence has been received over that same
port - real or virtual port - the noted time would be substituted for the
end of line serial timestamp.
So the user-mode timestamping works for both real physical and emulated
virtual COM ports.
From the test I made with Windows XP, this allows reported NTP offsets to
stay within about 400 microseconds with a GPS 18 LVC connected over USB,
which is much better than what I would get using Internet servers alone.
Perhaps the Linux port could also benefit from a user-mode DCD timestamp,
or is there not the justification for the effort?
Cheers,
David
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