Hi 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. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] 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 feasible. 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? _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
