| Re: [chrony-users] Possible bug in PPS support |
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On Mon, 23 Oct 2017, Rob Janssen wrote:
Bill Unruh wrote:
If you really need 20usec, then relying on one gps is certainly a bad
decision. You should have two or three machines all with independent gps
sources so you could catch one of them going rogue, or quitting.
The GPSDOs we are using are 2-3 orders of magnitude better than that.
These are not your typical $50 modules, but professional GPSDO with OCXO
or better oscillator.
It is not the accuracy of the individual gps but the the fallback in case one
of them goes mad (as happened to you). You do not want them on the same
machine unless they have hardware timestamping, since the interrupt latency is
far larger than 1us for servicing each interrupt.
Monitoring of their accuracy is done by their owners, we only get the signal
via distribution amplifiers. That is why we would prefer to have some
additional
validation, like the PPS signal completely missing.
(which could also be caused by a mistakenly unplugged or cut cable, which
would never be detected by the GPSDO monitoring)
As I said, you could do that with a cron job every 5 min cheching.
You seem to be saying that having no time source whatsoever is better than
having one which may be off by 20us? I think you need to set out the real
conditions that you need in detail ("We need accuracy to 20us" could be
because it was a number that some administrator with absolutely no idea of
time came up with, or it could be a legal requirement, or it could be "we
should be able to do that" kind of requirement)
The time is used for a single-channel simulcast transmitter system. That is,
the same signal is transmitted from multiple locations on the same frequency
at the same time. When this is not done within 20us at the same time, it
will cause
severe distortion of the signal. When we don't know we are within 20us, we
prefer to not transmit at all, so disable that particular transmitter.
OK then you should have redundancy on each transmitter, and monitoring eg via
that cron job.
I think I know better what is involved and what the limitations are than you
do.
Of course. But that is not what is at issue here.
Also, I prefer to discuss with Miroslav, who concentrates on the problem
under
discussion rather than casting doubt on everything. Thank you for you input
until now.
???
You are making claims. I ask for what your evidence is for those claims, and
you have never given the evidence. Operating on false evidence is a sure way
of making bad decision.
I am not casting doubt on everything. I am trying to explain how chrony works
and why it does what it does.
Rob
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