[ntp:questions] Shared PPS source/Multiple PPS sources

Rob nomail at example.com
Sat Feb 7 09:29:13 UTC 2015


William Unruh <unruh at invalid.ca> wrote:
> On 2015-02-07, Rob <nomail at example.com> wrote:
>> walter.preuninger at gmail.com <walter.preuninger at gmail.com> wrote:
>>> Ok, so these questions might be off the wall.
>>>
>>> Is there any reason why I could not share the PPS output of say, my u-blox 7 GPS module on multiple computers? Would it be good or bad to peer these 2 systems with each other?
>>
>> We do that in our project to synchronize several systems to the same
>> GPSDO accurately.   We use surplus "video distribution amplifiers" because
>> they neatly fit into this (BNC In/Out, correct levels).  There also exist
>> dedicated devices for this.
>
> Actually the biggest problem with this is that it takes time to service
> a gps interrupt. Thus the first pps to come in, or be serviced delays
> the servicing of the second interrupt. Thus the accuracy of the two GPS
> is very ( 10-20usec) different. (I call that "very" because a gps pps is
> capable of 1usec accuracy, so a factor of 10 worse is  alot)

No, the single PPS is fed to multiple systems that each service the
interrupt at the same time.

I presume you meant this followup to the "multi PPS sources to a single
system" and then it is not true either, of course our systems have
at least 4 cores and they can service multiple interrupts at the same
time.



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