[ntp:questions] Offset Average (Normal)?

Bruce Lilly bruce.lilly at gmail.com
Tue Mar 13 04:42:21 UTC 2012

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 

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 

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