[time-nuts] NTP/1-PPS/RS232 question
Hal Murray
hmurray at megapathdsl.net
Tue Aug 20 05:35:06 UTC 2013
david-taylor at blueyonder.co.uk said:
> Although, IIRC from discussions with Dave Hart, the Windows port can only
> accept the normal positive-going signal on the DCD line. The devices I've
> tested all have positive going signals. Pulse width is typically 100 or 200
> milliseconds, with a few receivers giving a much shorter pulse.
Low cost GPS receivers usually have long PPS pulses. "Fancy" GPS receivers,
like the TBolt or Z3801A have short pulses, ballpark of 10 microseconds. The
short pulses sometimes don't get captured by the standard PPS software.
The standard TTL to RS-232 chips include an inverter.
Most RS-232 receivers will work correctly with "TTL" levels on the inputs.
That includes 5V CMOS or maybe even 3.3V CMOS. That's assuming short
cables, ballpark of 5 ft.
If you want to go a long distance (ballpark of 100 ft), it's probably
worthwhile to use some sort of differential signaling.
--
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