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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Interesting stratum 1 NTP clock
From: Tim Pozar <[email protected]> > Would a GPS hooked to the serial port of a Linux box do the same? No. You need to dectect the event of the second. Some OEM GPS boxes will have another pin that will go low/high when this event occurs. In places where I've seen this done, that pin usually ended up tied to DCD. The Cisco appears to permit this to be on { RI, DSR, DTR, CTS, RTS, DCD } and also support an "inverted" signal, which I take to mean high->low transition is PPS, not low->high. It is noteworthy that certain manufacturers (Rockwell and possibly Trimble come immediately to mind) do not synchronize the PPS pulse to the top of the second as standardized by USNO (and indirectly BIH) but rather just give you one pulse per second starting at an arbitrary (but accurately expressed in the NMEA sentence!) point in the second. Also, beware that currently GPS time is 12 seconds fast from UTC. Not if you have a "good" GPS that picks up that data in the ephemeris and corrects automatically. Almost all of them will do this now. Your average end-user has no interest in the fact that GPS has no concept of "leap seconds" to bring itself into compliance with international standard time, and thus should not have to compensate for it. ---Rob
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