[ntp:questions] Performance?
David J Taylor
david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk
Tue Dec 30 17:36:56 UTC 2008
Unruh wrote:
[]
> Garmin defines the leading edge as the transition from 0V to 5V on
> the PPS line. Now serial has two levels -12V and +12V. with
> capacitive coupling, the garmin signal would be something like -2.5V
> to 2.5 V which really is way out of spec for the serial port. (Or
> with the typicallysmall duty cycle, more like -1V to 4V)
You do /not/ capacitor-couple RSR232 lines! Directly couple. Serial has
more than two important levels - read up on hysteresis levels of typical
receiver chips, particularly those used in PCs. I.e. the levels where a
logical "0" changes to a logical "1" is not +12V or -12V, and it may not
be the same level where a logical "1" changes to a logical "0", if you see
what I mean.
Cheers,
David
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