[ntp:questions] OT: GPS18x LVC failure

Unruh unruh-spam at physics.ubc.ca
Mon Oct 12 08:07:12 UTC 2009


"David J Taylor" <david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid> writes:


>"Unruh" <unruh-spam at physics.ubc.ca> wrote in message 
>news:TtqAm.47651$Db2.44785 at edtnps83...
>[]
>> USB only has one data line. It cannot "do" a DCD line except as another
>> bit of serial data, interleaved with all the other serial data.
>> Ie, you are not going to get any good signals for an "interrupt".
>>
>> You definitely need either a real serial port with a true hardware line,
>> or a parallel port to get true interrupts.

>Be that as it may, a GPS/PPS system working over USB provided better 
>results (45 microseconds jitter) on my test PC than a 1Gb/s Ethernet card 
>(110-140 microseconds jitter) on a 1Gb/s lightly loaded LAN talking to a 
>stratum-1 server, so I would not say that USB is useless.

Actually those Gb ethernet switches seems to have some really bad and variable
latencies. My system, before they installed Gb switches, had a consistant
roundtrip of 140 usec and gave better than 20usec jitter (with 100Mb switches). Since those new switches
were installed things have gone to hell-- variable roundtrips ( 140usec, 300usec
and up to 1 sec roundtrip) and much much worse jitter. NOt sure where the problem
is, but it sure is terrible. 
Doing a plot of roundtrip vs offset it is definitely one way delay.


>David 




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