[time-nuts] Chooses for a desktop/server NTP external 1PPS reference

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Dec 9 00:33:45 UTC 2009


Alexander Sack wrote:
> On Tue, Dec 8, 2009 at 3:43 PM, Bruce Griffiths
> <bruce.griffiths at xtra.co.nz>  wrote:
>    
>> Alexander Sack wrote:
>>      
>>> On Tue, Dec 8, 2009 at 3:01 AM, Robert Atkinson<robert8rpi at yahoo.co.uk>
>>>   wrote:
>>>
>>>        
>>>> Hi Alexander,
>>>> Welcome to the list.
>>>> Another professional option is the Trimble Accutime Gold.
>>>> http://www.trimble.com/timing/  The older Accutime 2000 and Palisade turn up
>>>> on ebay. A desk top box is the CNS Clock II
>>>> http://www.cnssys.com/cnsclock/CNSClockII.php cost about $3000 complete.
>>>> Some idea on if you want new / professional or used / amateur and if you
>>>> are a keen constructor would allow more targeted advice.
>>>>
>>>>
>>>>
>>>>          
>>> Thanks Robert!
>>>
>>> Yeah I didn't really specify price or performance metrics in my
>>> request.  I think I should document this stuff and write up a
>>> beginner's FAQ (I didn't see anything on LeapSecond.com).
>>>
>>> Alright, the reason WHY I didn't actually specify much is because I
>>> really don't know WHAT to look for.  I found out today that the
>>> rinky-dinky Endrun Technologies Cf/Ct receivers are 1k each (GPS and
>>> CDMA variants)!  I was kinda shocked because there ain't much to these
>>> units AFAIK.  So I want to step back for a second (pun intended?) and
>>> ask everyone, WHAT SHOULD I BE LOOKING FOR?  Say I want nanosecond
>>> accuracy with respect to the PPS rising edge to absolute UTC....I know
>>> that for sure.  My target system is FreeBSD though I would like to use
>>> something with Snow Leopard as well (even if that's just ntpd synced
>>> locally to the FreeBSD box).  I am interested in running NanoBSD on a
>>> stand alone system.
>>>
>>> As for as assembly, I'm open, though I haven't soldered anything since
>>> undergraduate engineering school in a lab.  I am a software guy by
>>> trade.
>>>
>>> Another issue is what governs the price of these units?  I'd like to
>>> think its not going to be like the audio industry.
>>>
>>> Hope that helps,
>>>
>>> -aps
>>>
>>>
>>>        
>> Alexander
>>
>> If you want the best performance possible from nanobsd then using an M12M or
>> M12+T is advisable.
>> In this case hardware timestamping (with a resolution of about 100ns) of the
>> PPS signal is used together with a customised version of the driver created
>> by Poul-Henning Kamp.
>>
>> The cheapest option would be to obtain used  M12+T timing receiver boards.
>> however using these would entail constructing a board with a TTL/CMOS to
>> RS232 transceiver and adding a 50 ohm driver for the PPS signal (required
>> for highest time stamping precision as it avoids the delay of an RS232
>> driver and receiver). The only complication being that the receiver board
>> uses a mating connector with 1.27mm pitch rather than the more common 2.54mm
>> pitch connectors used by older receivers.
>>      
> Ahh thanks for this info.  I was gravitating toward trying to setup
> something similar using phk's ntpd driver?  I need to read more.
>
>    
The Soekris 4501 also needs a little mechanical work to mount the second 
serial port connector on the front panel.
Whilst the Greenlee punch as used by JRA: 
http://www.febo.com/pages/soekris/ can be made to work there are better 
(and more expensive) punches out there that incorporate alignment 
features, even so the lip on the Soekris case means that the punch guide 
cannot be used (without grinding it to fit). With careful alignment this 
guide isnt necesaary.
Failing that its not too difficult if one has a drill press and and 
suitable files to make a neat hole for the DE9 connector shell.
If one uses the standard PC 2nd serial port cable the metal plate on the 
end can be removed (not much use in a Soekris 4501) and used as a template.
>> Of course network congestion and queueing delays add to the timing noise
>> seen by client machines on the network.
>>      
> Right.
>
>    
>> If one used PTP (IEEE 1588) instead of ntp then the performance can be
>> substantially improved if one can avoid standard network switches with their
>> queueing delays.
>> However PTP is relatively new and as yet IEEE 1588gear hasn't yet hit the
>> surplus market.
>> Usually PTP uses hardware timestamping of the relevant network packets,
>> however there have been some efforts at software timestamping.
>>      
> Yes but in fact I would like to play with IEEE 1588 as well for work reasons!
>
> -aps
>
>    
Bruce




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