[time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18

Bob Camp lists at rtty.us
Mon May 13 12:45:13 EDT 2013


Is anything else on the USB bus? If so, you may not have much luck. With
multiple devices, you can get pretty good throughput. If you look at
timestamps as sent / as arrived, it's pretty crazy who sends what and when.
Last time I looked, 10's of ms were not at all unusual. In a few cases 100's
of ms seemed to pop up. 

The worst cases were (no surprise) at low data rates. The USB seemed to like
to load up full packets of data, and was quite willing to wait a bit to see
if there was more coming. 

All of that was on a conventional Windows machine that had enough "stuff"
shut down that it would give low ms agreement on the same test when run with
conventional serial ports. The stamping was at the ms level, so it's not
really clear how much better than single digit ms it *might* have been. 

I basically gave up on the USB approach for low latency. I went back to hard
wired ports. Weather there was a multi-port USB solution that would have
worked - don't know. I certainly didn't find one. I still use the USB multi
port approach when I'm not super worried about what happened when. 

Throughput wise, the USB boxes I've been using are quite happy with 48 ports
running at 115K baud. You have to keep the data rates low enough you don't
run out of USB bandwidth. I try not to run them all on the same USB port,
but beyond that, no special care and feeding. Mostly this is under Win XP,
but I believe it would apply equally as well to the more modern versions. 


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Monday, May 13, 2013 10:43 AM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18

rummaged through archives and couldn't find anything..

I've got some GPS-18's with the RS232 and 1pps output.  BUT, I'm 
wondering if anyone has tried to get timing with the USB version (Linux 
or Windows..), and if so, is getting 1 millisecond absolute accuracy 

The underlying USB thing has 8kHz frame rate, but I suspect that the 
serial port emulation (which is probably what they use) might not hold 
it that well.  I can see there being some sort of fixed offset (going 
through hubs and such), but once the configuration is known, is that stable?

Any practical advice?
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