On Mon, 12 Mar 2012 20:21:13 -0700, Alby VA wrote: > The RS232 is handing > the > Rx/Tx/PPS and such. Everything is coming in on /dev/cuau0.
But what is the internal implementation; it is a normal serial UART type device or an on-board USB conversion (consult the mother board hardware manual)? If you can't get a hardware manual, or if it is uninformative, check the boot-up logs to see what driver is used. Failing that, look at the physical board to see where the connector pins go. If you can't do that, capture the PPS timestamps with the PPS/NMEA drivers set to noselect and the system fully synchronized (loopstats time offset, jitter, and frequency offset all stable) to a reliable source and the system relatively otherwise idle. Collect timestamps for about an hour and plot the offset of the timestamps to the nearest second of the (externally synchronized) system clock. An approximately 1 million nanosecond peak-to-peak sawtooth offset is a tell-tale sign of 0.001 MHz USB-like polling. If you plot a histogram of offsets, USB or similar polling will produce a rectangular distribution of offsets (for a sufficiently large sample set); normally an interrupt-driven serial port PPS sample offset distribution is an asymmetrical long-tailed bell- shaped distribution with a distinct peak. N.B. it is important to capture the PPS edge which is the timing reference; consult your GPS device manual, taking into account any signal inversions due to line drivers. If you're not using a proper line driver (e.g. TI SN75155), do that first. The xmgrace program may be useful in plotting offsets and histograms. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions