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Re: Mac and NetBSD compatible usb to serial rs-232 (was: which serial-to-usb devices do you use?)
On Jan 15, 2011, at 10:42 AM, River Tarnell wrote:
> Chuck Swiger:
>> The FTDI USB-to-serial devices work well with both platforms (I was using
>> them for a GPS receiver for NTP time-sync'ing purposes)
> How well did this work for you? I've always heard that for time sync use,
> serial adapters are to be avoided because the latency/jitter is much higher.
> Unfortunately, many new systems no longer ship with real serial ports...
The Mac version of a Garmin utility for the GPS receiver let me switch the unit
to fixed location mode rather than mobile mode, which lets it average results
over time for better positional accuracy and/or wanting fewer visible sats to
keep a fix. It seemed to figure out it's position and height above ground to
the right building and floor level, both from work and at home.
I was able to get NMEA and PPS signals out of it via gpsd + SHM driver, so ntpd
decided it was stratum-1. The timing precision seemed to be comparable with
the timestamps I was getting from nearby NTP timesources-- MIT and Columbia U.
were both less than 20 ms network latency away from me, in NYC-- and all seemed
to agree with offsets of less than 2-3 ms, and jitter generally under 1 ms.
I seem to recall trying to run at 9600 rather than 4800 baud, and playing
around with different NMEA sentences, and while I could see reasonable results
in tip or kermit (ie, # of visible sats, accuracy estimates), gpsd didn't seem
to understand configs other than the default too well and would drop synch
after a while.
I didn't have an oscilloscope and rubidium oscillator handy, which I'd need to
try and judge timekeeping accuracy to better than millisecond level, but it
seemed to be good enough to keep timestamps to the right scheduler tick at
kern.hz=1000, maybe give-or-take 1. On the other hand, running ntpd as a
stratum-2 or -3, if you've chosen decent nearby timesources, seems to also
provide that level of accuracy. Folks who care about microsecond or nanosecond
level timing would likely notice differences between network timesources, USB
connectivity, and true serial or parallel connections for the PPS signal.
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