[time-nuts] NTP/1-PPS/RS232 question

Hal Murray hmurray at megapathdsl.net
Tue Aug 20 01:35:06 EDT 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.


-- 
These are my opinions.  I hate spam.





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