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

Hal Murray hmurray at megapathdsl.net
Sat Jan 5 03:25:02 EST 2008

> 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.)

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.

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 

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)

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 

These are my opinions, not necessarily my employer's.  I hate spam.

More information about the time-nuts mailing list