[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Chris Albertson albertson.chris at gmail.com
Thu Feb 16 15:56:57 EST 2017

sub Millisecond is EASY.   my Apple 27" iMac is doing that right now
using just Internet pool servers.   Yes I have a very good Internet
connection.  100 Mbit fiber and then the last meter is 1000BaseT

But still, milliseconds are really EASY.  It is sub microseconds that
requires things like PTP and special hardware.   It's at the uS level
where you have to work hard

If anyone needs to break a millisecond, just run the PPS to the
machine that needs it edit /etc/ntp.conf and you are now in the "few
uS" level.  It costs nothing (if PPS is available.)  But breaking that
uS barrier is not easy

On Thu, Feb 16, 2017 at 9:59 AM, shouldbe q931 <shouldbeq931 at gmail.com> wrote:
> On Thu, Feb 16, 2017 at 7:55 AM, Chris Albertson
> <albertson.chris at gmail.com> wrote:
>> But PTP requires special hardware.   You may not have this.
> I have to disagree.
> I run PTP on a Raspberry Pi using its onboard USB connected NIC, and
> onboard NICs on HP and Dell servers, I see +- 5 microsoconds jitter in
> the one way delay across 4 fanless HP switches, and when PTP is
> running on a Pi with a GPS hat with PPS, I see +50 -30 microseconds
> jitter in the reported offset from master across the slaves.
> If the requirement is for sub millisecond then PTP on commodity
> hardware is a _workable_ solution.
> If sub microsecond accuracy is required, then the NIC, Ethernet switch
> and time source hardware requirements change.
> Cheers
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Chris Albertson
Redondo Beach, California

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