[time-nuts] Syncing Computer to Datum 9700 Programmable Time System

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 5 04:24:23 EST 2008

Hal Murray wrote:
>> I noticed all the wonderful outputs on the back, and I'd like to take
>> the 1PPS signal and use it as a timing standard for my IBM workstation
>> running XP Pro SP2, thereby making it the home time server for the
>> rest of our computer network.  I'm assuming that it would have to go
>> into one of the available serial ports, but is there an off-the-shelf
>> cable w/software to enable this, or am I heading into uncharted
>> territory? 
> On the hardware...
> The usual approach is to feed the PPS signal into one of the modem control 
> signals.  (I think it's usually DCD, but be sure to double check that.)
DCD is the usual choice.
> Some people claim you need a TTL to RS-232 level shifter.  A raw TTL signal 
> works just fine with almost all RS-232 input chips, including whatever is 
> used on all the systems I've worked with.
If you want good noise margins and to avoid the ringing that can occur
when driving
high capacitance cables of indeterminate length with TTL, a TTL to RS232
level translator is desirable.
> The normal level shifter includes an inverter so the two approaches end up 
> with the opposite polarity.  (Most software has an option to select 
> rising/falling edge.)
> I don't know of any off-the-shelf cables, but it's pretty simple to make your 
> own.  (at least if you use the no-level-shifter approach and have a soldering 
> iron)
> On the software...
> I don't know much about Windows.
> There is a version of the reference implementation of ntpd compiled for 
> Windows and packaged with an installer.  I don't know if that includes any of 
> the refclock drivers which you probably need to use the PPS signal.  
> (probably not)
>   http://www.meinberg.de/english/sw/ntp.htm
> To get great time, you need some extra code in the kernel.  The key idea is 
> that the interrupt routine grabs the time when DCD changes.  User code can 
> ask for that time and use it for updating drift and offset.  The net result 
> is much more accurate (less jitter) than if the user code asked for the time 
> after it got woken up via an interrupt and such.
> A PPS signal doesn't tell you which second the pulse corresponds to.  So you 
> need something else for that.  Most boxes that generate PPS signals also 
> generate an ASCII string with the date/time in it.  If not, you can probably 
> get it from NTP servers on the net.
> So I wouldn't call it "uncharted", but it doesn't sound like mature 
> technology.
Performance with windows is so poor that its hardly worth the effort
except as a learning exercise.


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