[ntp:questions] ntp server specs

Chris Albertson albertson.chris at gmail.com
Tue Jun 14 23:26:37 UTC 2011


On Tue, Jun 14, 2011 at 2:58 PM, Chuck Swiger <cswiger at mac.com> wrote:
> On Jun 14, 2011, at 12:52 PM, Chris Albertson wrote:
> [ ... ]
>> The problem with both of those is that the Linux machines are all
>> going to want the PPS to be conected to the DTR pin of a serial port
>> and for that you need rs-232 levels with wides plus and minus volts.
>> TTL level or wose is not going to work.
>
> If you're distributing a serial signal to more than one receiver, you're already outside of the RS-232 spec.

A common type of distribution amp has many output ports to allow you
to connect a cable to each device you are driving.  I would not try
and drive sveral in parallel.

I think the device that was linked to had 12 BNC outputs

Yes most computers now days lack a real serial port but here we are
talking about computers used to timing and the specs are so tight that
NTP can't be used and we are resorting to PPS distribution.  In this
case, I assume we care about MICO seconds, or els why not simply use
NTP?   If you care about sub-milliseconds then USB serial converters
will not work.

I think this just goes to show that you can't design a system wthout
first writing up some clear requirements.

As the specs for maximum error get smaller then you really are forced
to use a distribution amp with multiple output ports.  A parallel
"bus" can not be equal distance to every device and speed of light
delay starts to matter to many uisers of PPS distribution systems.
Yes some people care about this, I'm able to count nanoseconds at home
with not very exotic equipment.


-- 

Chris Albertson
Redondo Beach, California



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