[ntp:questions] NMEA ref.clock better than my ISP's timeserver?
David J Taylor
david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Thu Jun 11 13:41:50 UTC 2009
David Woolley wrote:
> When used with proper balanced drivers and receivers, twisted pair is
> often better than coax.
> For any type of cable, group delay characteristics may be more
> important than characteristic impedance match.
Whilst I agree with both points, for carrying what are TTL-level signals
with rise-times probably greater than 100ns where you are using an RS-232
receiver to detect pulses 0.2s wide, and over a distance or 30m (i.e. a
time of around 100-150ns), almost any cable will do. The only exception I
might make is where the environment is electrically noisy - i.e. an
industrial setting or running behind a photocopier, air conditioner, heavy
motor or similar, where I would want at least shielded cables.
I say this from years - too many years! - experience with RS-232.
More information about the questions