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

Fei Wu fxw115 at yahoo.com
Wed Nov 3 22:17:43 UTC 2004

Hi David:

Again thanks for help. So I understand I can not
bypass the problem of enlarging the pulse size by
setting the com port to higher bit rate.  I used a
converter to convert the TTL 1pps output to RS232
level, however, the converter do not have a function
of pulse shaping. Do you know where I can get this
pulse shaper?



--- David Schwartz <davids at webmaster.com> wrote:

> "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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