[ntp:questions] ntp server specs

unruh unruh at wormhole.physics.ubc.ca
Wed Jun 15 09:07:59 UTC 2011

On 2011-06-14, Chris Albertson <albertson.chris at gmail.com> wrote:
>>> <http://www.ptsyst.com/TT-10188-B.pdf>
>> <http://www.tapr.org/kits_tadd-3.html>
> 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.

except that most serial ports mowadays will accept ttl level inputs. But
that second board can  deliver serial port output voltages anyway.

> So either your distribution amp needs to swinge from about +9V to -9V

-9 to 9? More like -5 to 5, but as I said, most serial ports will accept
0 to 3 as well. No guarentees, but...
> or you will need a level converter at each Linux machine.
> What to do depends on what other equipment you have.  Most electronics
> (other then a computer) will want the 0 to 5V TTL level pulse or maybe
> a 3 volt pulse.  With mixed equipment in a large lab you's want the
> local level converters but if you know that every user of the PPS is
> going to need rs-232 then build that into the amp.

The kit allow mixed outputs from thier description.

> For very long didtance runs you'd need the level converters because
> you'd have to send the PPS as a differential signal maybe using rs442
> drivers and receivers.
> I use an RS232 level signal on Cat-5 network cable and it will go 100
> feet with no problem.  Iuse the common  TTL->RS232 chip made by Max as
> the "amplifier"   I found a source on ebaywho sells this chip and the
> required capasitors built onto a DB9 connector for just a few bucks.

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