[ntp:questions] PPS via USB-to-serial adapter

Per Hedeland per at hedeland.org
Sun Aug 11 13:44:06 UTC 2019


Since the idea of using a USB-to-serial adapter for PPS is often
dismissed here as more or less pointless/useless, (due to the inherent
delays in the USB communication AFAIU), I found this recent post to a
couple of FreeBSD mailing lists quite interesting:


TL;DR^2 The author carried out a pretty sophisticated (IMHO) test with
two different USB-to-serial adapters feeding PPS to ntpd, and found an
offset of some 200 usec with 20-30 usec jitter. You can of course tell
ntpd to correct for the offset (once you know how large it is...), and
the jitter doesn't seem too bad to me, although it is of course higher
than for more "direct" connections.

In subsequent discussion he pointed out that it is significant that
the test was done on FreeBSD - while he would expect similar results
on Linux, performance on Windows would be "all bets off" due to
varying driver quality.

--Per Hedeland

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