[ntp:questions] Very large jitter and offsets on GPS ref clock after upgrade to "p5"
snews at lordynet.org
Wed Feb 8 17:45:06 UTC 2012
David Lord wrote:
> David J Taylor wrote:
>>> My Oncore docs give a suggested circuit with a 74HC132:
>>> GPS-6_1PPS --+-----------------+ +--- MAX232_T2in
>>> | | |
>>> | 4 +-- |
>>> 1 +-- 3 & o--+
>>> | & o-- R=10k --+-- 6
>>> 2 +-- 5 |
>>> | +-- C=470p -- +5V
>>> 9 +-- 8
>>> | & o-- R=820 -- D=PPS LED -- +5V
>>> 10 +-- K A
>>> I think that the generated pulse is only about 5 usec and was
>>> doubtful it would work for me over a 15-20m cable run. I've had
>>> regular rs232 work without problem over the same distance in
>>> the past.
>>> I've not started on this yet but will test with a short cable
>>> run first and increase the pulse width as needed.
>> Difficult to read your diagram, even in a fixed spacing font.
>> My point was that the individual active lines should be paired with
>> ground lines for transmission.
>> If the pulse is only 5 microseconds, I would definitely want Chris to
>> see how mangled (or otherwise) it was at the end of the run he has.
>> It may well be pushing the limits, especially if it's not using the
>> cables as twisted pairs. Perhaps your circuit could be modified to
>> produce something nearer 100 ms by replacing the C with something
>> about 20K times bigger? A 1 uF or so?
> The circuit is from the "Oncore QuickStart Guide" and I
> guessed that the pulse shaper was to get the 5 usec pulse
> otherwise output from pin 3 of the 74HC132 could have been
> used for default pulse width.
> When I get around to assembly of my 2-to-6 rs232 expander
> I'll scope with both short and long cables and I've also
> rs422 MC3486/7 to try.
I've not put a scope on the pps but I added an extra section
of cable from my Sure GPS, 18 m of 4 x pair telephone cable.
I will need that length to reach from the GPS down to my PCs.
In the process the offset jumped from 0.004ms to about 12ms
but fairly quickly went back to a few usec.
PC = me6000 (VIA ME6000), NetBSD-5, ntpd 4.2.6p5
From my ntp-stat file:
18:24 0.006 extra 18 m cable added
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