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

Joe Gwinn joegwinn at comcast.net
Fri Mar 21 15:17:28 UTC 2014


Magnus,

In article <532B5621.1040001 at rubidium.dyndns.org>, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:

> Hi Joe,
> 
> On 20/03/14 01:53, Joe Gwinn wrote:
> > In article <5328AD2B.9 at rubidium.dyndns.org>, Magnus Danielson
> > <magnus at rubidium.dyndns.org> wrote:
> >
> >> On 18/03/14 01:24, Joe Gwinn wrote:
> >>> I've used IRIG-B004 DCLS before, for cables two meters long within a
> >>> cabinet.  Worked well.  How well do they handle 100 meter cables, in
> >>> areas where the concept of "ground" can be elusive?
> >>
> >> The rising edge of the 100 Hz is your time reference, the falling edges
> >> is your information. Proper signal conditioning and cabling should not
> >> be a problem given proper drivers and receivers.
> >>
> >> IRIG-B004 DCLS also travels nicely over optical connections, and
> >> grounding issues will be less of a problem. Known to work well in power
> >> sub-stations, so there can be off the shelf products if you look for them.
> >
> > That's a pretty severe environment.
> 
> I thought it would get your attention.

It did at that.  

> > I should give more context:  On ships at full steam, there can be a
> > steady seven volts rms or so at power frequency (and harmonics) between
> > bow and stern, which will cause large currents to flow in the shield.
> > This is well below the frequency at which inside and outside shield
> > currents become decoupled due to skin effect, so the full voltage drop
> > in the shield may be seen on the center conductor.
> >
> > We use optical links a lot, and triax some.
> >
> > One can also make RF boxes largely immune with a DC-block capacitor in
> > series with the center conductor.
> 
> 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.


> >> Maybe, depends on your needs. Consider doing a separate network for PTP.
> >> That approach have been used in systems where you want to make sure it
> >> works.
> >
> > 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.


> >>> This is my fear and instinct.  But people read the adverts and will
> >>> continue to ask.  And some customers will demand.  So, I'm digging
> >>> deeper.
> >>>
> >>> Are there any good places to start?
> >>
> >> You asked here, it's not the worst place to start. :)
> >
> > To be sure.
> >
> > 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.

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.


Joe Gwinn



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