[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
Sat Jun 13 05:32:38 UTC 2009


David Woolley wrote:
> David J Taylor wrote:
[]
>> The GPS 18x LVC only claims an accuracy of one microsecond for PPS in
>> the first place.  The RS-232 receivers may well have some filtering
>> limiting the risetime to a couple of microseconds (i.e. good enough
>> for
>
> Note, if you actually do this you are deliberately grossly
> mis-terminating the line, not that that is necessarily a bad thing to
> do.
> Incidentally, it is the transmitter's responsibility to control rise
> time, not the receiver's in RS232C.
>
>> the maximum baud rate, typically 115,200 baud), so even if the final
>
> Baud rate should be irrelevant for modem control signals, but even for
> asynch data, the actual sampling resolution is 16 times better.

David,

In the RS-232 implementations I have seen, the line is /never/ correctly 
terminated, as I use the term.  Correct termination implies, if the cable 
impedance 50 ohms (such as the often used Belden 8777), that:

- the driver output impedance is 50-ohms
- the cable impedance is 50-ohms
- the receiver input impedance is 50-ohms

This would mean that there were no reflections either at the receiver 
input, or the transmitter output.  RS-232 typically has a much higher 
value of input impedance, in the region of 3-7K-ohms, according to:.
  http://www.arcelect.com/rs232.htm

so the line is not "correctly terminated".  That page also includes a 
maximum slew rate for the driver of 30V/us, so the driver controlling the 
risetime as you said.

Checking today's ICs I see that they suggest direct connection to the 
RS-232 socket, so the static protection must be better than it was some 
years back, and there is no need for external components.

Point taken about baud rate and the control signals.

Cheers,
David 




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