You can try to synchronize several PCs with a standard switch.
I measured the delay with one master and one slave and then slightly modified
ptp4l with the value obtained (hardcoded). I disabled the delay req messages on
the slaves (to avoid messages accumulation on the master side). There is no
other traffic on the links.
I could measure a delta of a few hundred ns between master and slaves with the
oscilloscope and i210 with PPS generation enabled.
I use the following schema :
|------ netgear GS724T
------ slaves
master ---- netgear GS108E |------ netgear GS724T ------ slaves
|------ netgear
GS724T ------ slaves
The result is even better when I use less slaves and a single GS108E (a few
tens of ns). But on three GS108E, two give very good results and one is very
bad... The best is to test.
Julien.
________________________________
De : Richard Cochran <richardcoch...@gmail.com>
À : Koehrer Mathias (ETAS/ESW5) <mathias.koeh...@etas.com>
Cc : "linuxptp-users@lists.sourceforge.net"
<linuxptp-users@lists.sourceforge.net>
Envoyé le : Vendredi 21 février 2014 14h33
Objet : Re: [Linuxptp-users] One PCs with multiple NICs acting as multiple PTP
master / PTP chains
On Fri, Feb 21, 2014 at 01:16:09PM +0000, Koehrer Mathias (ETAS/ESW5) wrote:
> Hi all,
>
> to synchronize a couple (6-8) PCs I want to use one PCs PTP master and the
> others as PTP slaves.
> To interconnect the PCs I can use a PTP capable switch. However these
> switches are fairly expensive.
> The first question I have: How does a "standard" Ethernet switch perform?
> Which accuracy will be possible if no other traffic is running over the
> switch but PTP packets?
The answer is, it depends.
It depends on the switch itself and the amount of traffic through the
switch. Usually switches add something like 10 microseconds delay and
a few microseconds jitter.
I would definitely try this first and just see what kind of RMS
numbers come out.
> As PC 1 ... PC 7 have two PTP clocks running, these clocks have to be
> synchronized.
> Is this done via phc2sys? If yes, do I have to synchronize directly from PTPx
> to PTPy or does it work via the PCs System clock?
You can use phc2sys to synchronize the PHC clock pairs. However, this
will be a PITA to set up, and remember, unless you feed signals from
one card to the next, you will be using a kind of software time
stamping. Chaining those servos together accumulates the errors.
I would guess that the normal switch would perform much better. Try
the switch idea first.
HTH,
Richard
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