[ntp:questions] Re: 1pps signal,20us width

David Schwartz davids at webmaster.com
Wed Nov 3 19:28:03 UTC 2004


"Fei Wu" <fxw115 at yahoo.com> wrote in message 
news:mailman.8.1099501654.537.questions at lists.ntp.isc.org...

> I want to use this 1pps signal to directly connect to
> the serial ports of several hosts at the same time. In
> this way, I can have several strtum 1 time servers
> with only one external clock. That is my plan. The
> hosts run FreeBSD4.9 or FreeBSD5.1 and NTP 4.1.1 or
> 4.2.0.

    Well then the same hardware that boosts and distributes the signal can 
stretch the pulse a little bit. Check the specs on whatever is generating 
the PPS signal and see if it's TTL or RS232.

    FreeBSD has true PPS support, so you shouldn't need the input to match 
any particular size. It will just latch the time of the edge. However, too 
short a pulse may get lost or distorted.

    I would do it in three steps, first an input circuit to convert the PPS 
input into a clean logic level. Then a pulse shaper to stretch the pulse and 
clean it up a bit (could be a one shot). Last, as many RS232 output chips as 
you need to put out an RS232 level PPS pulse.

    DS





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