[ntp:questions] problem with ntpd refclock and pps via parallel port

David Lord snews at lordynet.org
Mon Feb 1 11:48:23 UTC 2010

Hal Murray wrote:
> In article <7smiguFph8U1 at mid.individual.net>,
>  David Lord <snews at lordynet.org> writes:
>> on system A I had type 22 pps working ok from pps at ttl
>> level to DCD of serial port. This was a bit erratic and
>> temperature sensitive possibly due to mismatch of ttl/rs232
>> levels.
> Anything close to a TTL level should work OK into a RS232
> reveiver port.
> I wonder if you are on a wild goose chase.
> What sort of toys do you have available?
> The simplest solution would be a scope.
> You can probably rig up something with a comparator chip
> and a pot.  Feed your PPS signal into one side of the comparator
> and use the pot to adjust the other side. Put a LED on the output.
> Twiddle the pot until it stutters.  Then measure the voltage
> on the pot.  If you have a narrow PPS, you will have to rig the
> LED up differently to measure the other voltage.  Mumble.
> I'm pretty sure something like that will work.

Some rs232 receivers I've checked datasheets for are not ttl
compatible. Different versions have different thresholds, some
versions can have threshold changed by external resistor.

Either using parallel port or serial with proper rs232 levels
is probably easier than experimenting with levels vs
temperature on all my different hardware.

Ntpd statsfiles will eventually show if there has been any
gain from parallel port connection. At moment it looks
worse but there's been a > 1C drop in temperature overnight
and frequency offset hasn't yet caught up. PPS(0) is still
in line with best of remote peers although all with offset
about -2ms.

Next two nights are forecast to have even lower early
morning temperatures. ie variation of  server system clock
with temperature is the biggest factor just at moment. The
frequency offset of this pool server is around -50ppm which
ntpd still has great difficulty keeping under control vs
other pool server with frequency around -2.5ppm which shows
less temperature sensitivity.



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