[ntp:questions] site with lots of GPS's and accessories

David Lord snews at lordynet.org
Wed Mar 21 19:32:46 UTC 2012

unruh wrote:
>>>> The MAX232 datasheet gives maximum for output slew as 30V/us.
>>>> The MC1488 circuit assumes a capacitor at the output and has
>>>> a graph of slew rate vs capacitance. I've used values between
>>>> 100pF and 330pF.
>>> No, it gives that as the maximum over all samples. Unless your
>>> particular one was cherry picked, the typical is more like 5. 5V/us slew
>>> rate means a transition time of 2us for -5V to 5V (even 30V/us is a
>>> transition time of .3us) That then goes along a long (or short) wire,
>>> which introduces noise) into an rs232 chip on the computer motherboard.
>>> Computer motherboards are horribly noisy places. That on board chip then
>>> choses some place along that slow ramp to trigger and interrupt. That
>>> place can vary as the temperature varies and as the noise from the
>>> motherboard adds or subtracts from the ramp voltage. 
>>> Furthermore, that positive feedback is also positive feedback for noise,
>>> including noise reflected back to the output of the chip by the wires
>>> connecting to the computer. 
>> Hi
>> seems that my datasheet must be wrong then.
>> The lack of noise must be due to my scope being faulty.
>> I'll continue using the MAX232 feeding the pc via about
>> 60ft of cable with no problems so far.
>> Last dozen summary lines
>> 	mean=0.000 all same
>>           rms=0.004 avg of 12 between 0.003 and 0.006
>>           max= between 0.027 to 0.039
> Not at all sure what those lines mean. The mean of what? What are the
> units of those (ntp usiually works either in seconds of ms). 
> mean of what? rms of what? 
>> This Sure has been online since about April last year
>> and the Garmin 18x-LVC used through 2010 gave similar
>> offsets.
> offsets from what?
> Recall that I was talking about ns resolution, not ms. 

I forgot that you know nothing about ntpd

My oscilloscope is supposedly 30Mhz bandwidth so can just
about display 10s of nanosecond variations. My diy scope
from around 1970 could do much better.

Ntpd can be set to log stats including peerstats.

Ntpd distribution comes with a set of scripts that can
process the stats files and output loop_summary,
peer_summary etc.

The units are milliseconds so 0.001 represents 1000ns.


>> David

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