Using USB serial introduces amusing amounts of jitter. This is usually not a problem for the NMEA sentences, but I wouldn't want to be detecting the PPS signal via USB1 or USB2 serial devices. I've heard that USB3 should be much better. I haven't touched any of these yet. -- Harlan Stenn <stenn at ntp.org> http://ntpforum.isc.org - be a member!