[ntp:questions] Answer: PPS on a rack server

John Ioannidis ntp at tla.org
Tue Jun 3 01:20:39 UTC 2008

A few days ago I had asked this list for assistance with the following 
problem: my rack-mounted IBM x3650 servers only have one serial port, 
and the data I was feeding them from my GPS clock were interfering with 
the serial console feature, the BIOS in particular.  Thanks to several 
people who replied to my original message, I now have a working setup. 
Here it is:

1. A Spectracom 9283 netclock feeds *only* the PPS signal to the 
servers.  Pins 1 (DCD) and 5 (GND) of a DB-9F connector are connected to 
the core and shielding of the coax (through T connectors, and the cable 
is terminated with a 50 Ohm terminator from the old thinlan days!).  The 
signal seems to be strong enough to drive the DCD pin, but for longer 
runs I would use this: http://www.tapr.org/kits_tadd-3.html.

2. NTP is used to get the time reference from the Spectracom.

3. The harder part was figuring out what to put in /etc/ntp.conf.  Here 
is what I have (the address of the Spectracom is not the real one, of 

         server minpoll 5 maxpoll 5 prefer	# 9283

         server minpoll 5 maxpoll 5		# PPS
         fudge flag3 1

Of course, the kernel is configured with option PPS, and /dev/cuad0 is 
symlinked to /dev/pps0.

That's it!  The machines appear to be within a few microseconds of each 
other, so I'm happy.

4. I can still use the serial console, after a fashion -- the machines 
support IPMI, so I can use serial-over-lan with ipmitool to get console 
i/o (for the BIOS, which is all I care about) without actually needing 
to plug anything to the serial ports.  Of course, what I really should 
have done is paid the extra $1K or so and gotten the RSA-II cards for 
the boxes, but that's another story.

Thanks again to all who replied


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