[ntp:questions] Setting the COM port for the NMEA driver

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Thu May 14 07:27:43 UTC 2009

Augustine wrote:
> On May 9, 2:17 am, "David J Taylor" <david-tay... at blueyonder.not-this-
> part.nor-this.co.uk.invalid> wrote:
>> Background: I have a GPS 18 and USB
>> serial converter, and I wanted to see how well that actually
>> performs.
> Very interesting experiment.  May I suggest using USB directly?


You may indeed suggest that, but the key point here is (I think) that the 
USB/serial preserves the PPS (pulse per second) signal from the RS-232 
connection, and it is the PPS signal transition which is used provide the 
precision part of the timing.

I understand that USB alone will typically not have any PPS information, 
so the timekeeping accuracy would be just that of the serial port 
messages, degraded by whatever USB latency adds, but improved by the fact 
that the messages are much shorter on USB than on serial.

It would indeed be interesting to see how well it performed, either with 
FreeBSD or with Windows.  I don't know if a USB/GPS driver is available 
with the present version of NTP.


More information about the questions mailing list