[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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