[ntp:questions] Odd results with Oncore UT+ ref clock

unruh unruh at wormhole.physics.ubc.ca
Wed Feb 2 19:20:59 UTC 2011


On 2011-02-02, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> On 2/1/2011 10:57 PM, Chris Albertson wrote:
>> On Tue, Feb 1, 2011 at 5:50 PM, Mike S<mikes at flatsurface.com>  wrote:
>
> The "rule of thumb" for propagation in copper cable is one nanosecond 

Actually no, that is the  rule for vacuum. 1ft/ns=.3m/ns=3x10^8m/s which
is the speed of light in vacuum. In cable it is more like 2/3-1/2 of
that.  (Not sure what it is in cat5e)

> per foot.  Unless you are hoping for nanosecond accuracy, 75 feet of 
> cable should not be a problem.  If you feel it is a problem, ISTR that 
> you can tell NTPD to compensate for the delay in the cable.

The bigger problem may be bad termination. The signal bounces back and
forth along the cable, gradually attenuating, and exactly which one
triggers the interrupt can be a bit of a guessing game. You want to make
sure that the cat5 is terminated with something like a 100 ohm resistor
between the hot pulse line and ground. ( The itnernal impendance of the
RS232 input will be much higher than that and I thing the termination at
the driving end is probably also much higher than that).
Of course the question may also be whether or not the output driver is
capable of delivering to a 100 ohm load.
..




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