[ntp:questions] IEEE 1588 (PTP) at the nanosecond level?

Joe Gwinn joegwinn at comcast.net
Sun Mar 23 22:27:25 UTC 2014


In article <532E42C8.6080402 at rubidium.dyndns.org>, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:

> Joe,
> On 21/03/14 16:17, Joe Gwinn wrote:
> > Magnus,
> >>
> >> Thus, another fairly severe environment.
> >
> > I have a personal war story from 1992:  At a Air Traffic Control center
> > in Canada, one 19" cabinet had the green (safety ground) and white
> > (power neutral) cables transposed.  This caused 2.3 Vrms at 180 Hz to
> > appear between the VMEbus ground and the cabinet shell, with enough
> > oomph to cause a small spark when oscilloscope probe grounding clip was
> > connected to that VMEbus ground, this causing the system (and my heart)
> > to crash.  If left connected, the ground clip became warm.  And how can
> > ground generate a spark, even a small one?  Fixing the grounds dropped
> > the offset to around ten millivolts.  The 180 Hz arose because the
> > power supplies were single-phase capacitor-input, driven from the legs
> > of three phase prime power.
> Power neutral isn't really neutral when it takes a lot of beating.
> Similarly, a grounding wire isn't doing much grounding as frequency goes up.

Yes, but the issue was that by transposing the two grounds, they forced
the return current to take the long way around.  The fact that the
offset was at 180 Hz didn't help for sure, but I don't know that that's
high enough for frequency to be a major cause.  Plain old resistance in
the path from the antenna face back to the main power panel (where the
two grounds are intentionally connected together) probably suffices.

> >>> That fails economically - might as well stick to IRIG.
> >>
> >> Indeed. Doing 1 us level might be possible, going lower than that will
> >> cause you more and more grey hairs one way or another.
> >
> > Well, now, this could be an advantage -- my hair is already gray, and
> > more could be better.
> Well, you may have younger colleagues who fail to have this 
> advantage. I knew you would make the comment. :)

Not to worry -- time will make them gray.

> >>> There is a truism in the standards world, that it take three major
> >>> releases (versions) of a standard for it to achieve maturity.  PTP is
> >>> at version 2, so one more to go.
> >>
> >> I'd say it depends on for what application. The trouble is when the
> >> assumed applications increase at a quicker rate than the standard adapts
> >> to handle them.
> >
> > It does, but having the market grow faster than the standards cycle can
> > be the mark of success.
> To some degree. Being perceived to be a solution isn't the same as it 
> being a solution.

Yes, but it's a batter problem to have than the converse.

> > By the way, development of the third revision of 1588 started in 2013.
> > I joined what purported to be their reflector, but now that you mention
> > it I haven't gotten any traffic -- Something must be wrong.  I will
> > need to enquire.
> They formally had their first session at the ISPCS in Lemgo.

OK.  I'm sure that my not getting any traffic is a local problem.  I'll
query the WG chairman.

Joe Gwinn

More information about the questions mailing list