[time-nuts] Trimble Thunderbolt 1pps

Brian Lloyd brian at lloyd.com
Sun Jan 5 00:48:36 EST 2014


On Sat, Jan 4, 2014 at 3:51 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

> > Pulse quality of single-ended RS232 over unbalanced twisted pair is going
> > to be pretty bad beyond a few feet. If you want to transport the 1pps
> over
> > twisted pair there are a couple of options:
>
> Hi Brian,
>
> I suspect this is true at one level, but what would be helpful to to
> *quantify* it. What is "pretty bad"? What is "few" feet? You are implying
> that 1PPS timing is dependent in cable quality and cable length. I would
> agree. But please provide some numbers, even rough numbers, because what is
> important for modern T&F applications (picoseconds and nanoseconds) can be
> irrelevant for NTP, which still lives in the millisecond and microsecond
> world.
>

Oh, called on the carpet I am! ;-) Very good point. More to the point would
probably be protection, isolation, noise pick-up, ground noise, etc., for a
long run. RS-232 is pretty susceptible to noise, hence my recommendations.
And what might work for one person might not for another.

But here is the point -- there is the concept of Best Engineering Practice.
There are lots of things you can get away with most of the time but they
are a bad idea to do as a general thing. Using an RS-232 signal over any
distance at all is one of those bad things. It has poor noise immunity and
when you are trying to catch the leading edge of a pulse it just isn't
going to do very well. Or maybe it will be just fine ... until you are
counting on it. Or maybe you have a transmitter around.

What I'd like to see, and what would be educational for the group, is if
> you could take some 'scope traces at a few inches, at a "few feet", and at
> a few meters or tens of feet to graphically demonstrate your point.
>

I suppose I could. And what would be the advantage of that? Just because it
can be made to appear relatively good in controlled environment still
doesn't make its use valid. My situation is not yours. I am not operating
in the same EM field you are. I have no idea what kind of ground-loop you
are going to create and I am not (or vice versa).

So, the key point is, and remains valid: RS-232 is a poor means to transmit
signals over more than a relatively few feet. Sometimes it works great.
Other times it won't work at all. If you want to transport a pulse over any
distance the right answer is coax or a differential signal, e.g. RS-422.


>
> My gut tells me 1 ns or 10 ns or 100 ns or 1 us or 10 us makes no
> measureable difference to the quality of NTP/PC timekeeping.
>

And my gut, based on designing data communications equipment, is that it is
poor engineering practice.

-- 
Brian Lloyd, WB6RQN/J79BPL
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.com
+1.916.877.5067


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