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

unruh unruh at invalid.ca
Wed Mar 21 20:02:52 UTC 2012

On 2012-03-21, David Lord <snews at lordynet.org> wrote:
> 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

What has ntpd got to do with it? You quoted some number of completely
unkown provinence. 

> 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.

I agree that the Sure PPS signal has about a 10ns rise time. But that is
before sending it through the rs232 level converter. After that it has
about a 1/2 to 1us rise time.
> Ntpd can be set to log stats including peerstats.

Yes, I do it all the time.

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

Yes, and you could write your own as well. I have written my own for

> The units are milliseconds so 0.001 represents 1000ns.

Thank you. It is always good to give units when you quote a number. 
(and yes, I know how many ns there are in a ms.)

> David
>>> David

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