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

Chris Albertson albertson.chris at gmail.com
Thu Feb 16 02:55:02 EST 2017

On Wed, Feb 15, 2017 at 11:48 PM, Chris Albertson
<albertson.chris at gmail.com> wrote:
> On Wed, Feb 15, 2017 at 10:30 PM, Ruslan Nabioullin
> <rnabioullin at gmail.com> wrote:
>> On 02/15/2017 01:17 PM, Chris Albertson wrote:
>>> Why set up a dedicated NTP server if you only have two computers
>>> that will use it?    Your server will be accurate to a few
>>> microseconds but your two computers will only by good to a few
>>> milliseconds because ethernet is not nearly as good as PPS.
>> Well Ethernet can be *extremely* accurate if PTP is used (a whitepaper
>> specifies <= 100 ns accuracy if the LAN is optimized for it).

But PTP requires special hardware.   You may not have this.

A cheap why to distribute time is use PPS on a back channel.   You
build a little amplifier and rig it so all computers receive PPS from
the same GPS source.  NTP runs on all these computers and you add the
"Atom" ref clock.   The trouble is you need to run more wires,
Ethernet and PPS.    But notice that only 1/2  of the wires in a
standard Ethernet cable are in use.


Chris Albertson
Redondo Beach, California

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