Commit d453c3f3 authored by Eric S. Raymond's avatar Eric S. Raymond

Documentation polishing.

parent cd620896
......@@ -123,24 +123,26 @@ in a cycle will be used. Using more than one sentence per cycle is
impossible, because
* there is only the link:#fudgetime2[time2] option available to compensate
for transmission delays but every sentence would need a different one
for transmission delays but every sentence would need a different one
and
* using more than one sentence per cycle overstuffs the internal data
filters.
filters.
The driver uses 9600 bits per second by default, unless {projecct-shortname}
was built in which case the old default of 4800bps is used. Faster bitrates can
be selected using the +baud+ option.
The driver uses 9600 bits per second by default, unless
{project-shortname} was built in Classic strict-compatiblity in which
case the old default of 4800bps is used. Faster bitrates can be
selected using the +baud+ option.
*Caveat:* Using higher line speeds does not necessarily increase the
precision of the timing device. Higher line speeds are not necessarily
helpful for the NMEA driver, either. They can be used to accommodate
an amount of data that does not fit into a 1-second cycle at 4800 bps,
but high-speed high-volume NMEA data is likely to cause trouble with the
serial line driver since NMEA supports no protocol handshake. Any
device that is exclusively used for time synchronisation purposes should
be configured to transmit the relevant data only, e.g. one +$GPRMC+ or
+$GPZDA+ per second, at a linespeed of 4800 bps or 9600 bps.
precision of the timing device. 9600 baud is useful to accommodate an
amount of data that does not fit into a 1-second cycle at 4800 bps;
some GPSes (especially when emitting skyview sentences) emit more than
this. Any device that is exclusively used for time synchronisation
purposes should be configured to transmit the relevant data only,
e.g. one +$GPRMC+ or +$GPZDA+ per second; this will slightly reduce
jitter by reducing variation in transmission times.
== Monitor Data ==
......@@ -185,7 +187,9 @@ the total but otherwise ignored.
http://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks[Configuring
NMEA Refclocks] might give further useful hints for specific hardware
devices that exhibit strange or curious behaviour.
devices that exhibit strange or curious behaviour. Not that it
describes NTP Classic and the old refclock confihuration syntax, and
assumes the old default speed of 4800 baud.
To make a specific setting, select the corresponding decimal values from
the mode byte table, add them all together and enter the resulting
......@@ -251,7 +255,7 @@ switched on by sending the following string.
------------------------------------------------------------------------------
refclock nmea mode baud 19200 # All sentences from /dev/gps0 at 19200 baud
refclock nmea path /dev/ttUSB0 # All sentences from /dev/ttyUSB0 at 9600 baud
refclock nmea path /dev/ttyUSB0 # All sentences from /dev/ttyUSB0 at 9600 baud
------------------------------------------------------------------------------
== Additional Information ==
......
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