[ntp:questions] garmin 18x and linux
david at ex.djwhome.demon.invalid
Tue Sep 6 21:13:31 UTC 2011
Greg Hennessy wrote:
> On 2011-09-06, unruh <unruh at wormhole.physics.ubc.ca> wrote:
>> Well, it is actually a $150 dollar unit. $30 for the unit, and $120 for
>> your time to solder a couple of wires across the board to get the pps
>> out to the serial port and make a box to house the receiver :-)
> Why don't the manufacturers provide the PPS signal to the serial port
> themselves? Is it meant to be NMEA only?
1) It will be intended for embedded system use;
2) TTL outputs are specified for rise times and propagation delays of
the order of 10ns, whereas RS 232 isn't really specified for much better
than 1 microsecond. RS232C is designed to driver medium length lines,
whereas TTL wasn't really intended to drive more than about 10 inches.
Both are mismatched to the transmission line. TTL aims to cope with
reflections by making the line too short, whereas RS232 attempts to do
so by controlling rise time.
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