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

Magnus Danielson magnus at rubidium.dyndns.org
Thu Mar 20 20:57:05 UTC 2014

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.

> 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.

>> 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.

>>> 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.


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