"David Woolley" <[email protected]> wrote in message
news:[email protected]...
[]
To the extent those constraints don't apply and both ends are terminated
well above the characteristic impedance, the output voltage actually
goes up in a staircase, with the steps being the round trip time.
Some other mistermination conditions cause a ringing approach to the
final value, which is why it is better to operate with a high impedance
load, and therefore capacitive characteristics.
I wish Chris would just look at the remote signal with a 'scope! A wide
pulse over the length of line he talks about should be no problem at all
(but a microsecond-wide pulse might). The timing PPS I've seen are in the
tens of milliseconds wide. If there is overshoot, perhaps a capacitor to
slow things down might help, or on the other hand, if the edge is too slow
providing a better match might help.
Of course, "clean" transmission does rely as I said before on each signal
being provided with its own ground in the twisted pair, a point also made
by Bill Unruh. For PPS a screened 50-ohm coax cable is not needed except
in extra ordinary circumstances (exceptional noise or sub-microsecond
accuracy). Even level converters may not be needed - even though the
specification says so. Many RS-232 ports work just fine with TTL levels.
Cheers,
David
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