[time-nuts] GPS NTP update on Mac OS-X 10.7.4

Russell Rezaian r.rezaian at earthlink.net
Sat Jun 16 22:03:26 UTC 2012


I've used a number of different time signal 
receiver devices as accurate clock sources for 
NTP running on Mac systems over the last few 
years.  I have been doing this sort of thing from 
Mac OS X 10.3 onwards, though I have not tried 
using a clock receiver on the Mac OS 10.7 
machines yet.

I've used a few different WWVB receivers, as well 
as a few GPS receivers.  I've been using both 
Spectracom and TrueTime receivers.

The NTP software works exactly as one would 
expect.   Use a recent version of the main line 
NTP code and it should compile and work very 
smoothly on the Mac.  I have submitted a few bugs 
to the NTP team when there have been compile 
issues, and these have been addressed very 
quickly.

The big difficulty in using a Mac as a primary 
NTP source, as has already been mentioned, is 
that all the recent (e.g. in the last decade) Mac 
systems do not have native serial ports.

The most common approach most people use to get 
Seial on the Mac is to use USB serial ports.

These work, but due to the architecture of USB as 
a polled buss, tend to have some fairly 
noticeable issues with latency and jitter for any 
medium to high accuracy application.  USB is not 
great for time sensitive applications in general. 
It can be OK, but is not great.

In personal experience Ethernet, despite some 
somewhat non-deterministic latency issues, 
actually seems to work much better for precision 
time sensitive applications in my experience.

This experience, which has been shared with 
another time nut on the list, suggests that 
Ethernet to serial interfaces actually usually 
work at least as well as, and usually much better 
than, USB to serial interfaces.

If you don't mind 100 MS related limits to 
precision, then you can probably be quite happy 
with a USB to serial interface or a USB GPS 
device.  I personally have used USB serial 
interfaces on a Mac as a backup NTP source for 
many years quite happily.  I also have a small 
herd of higher accuracy (and higher precision) 
dedicated NTP servers that are actually the first 
line of time sources in my network.  These are 
sufficiently better that I can see the USB 
limitations pretty clearly.

If USB is OK for your applications, then you 
should be able to get things working on a Mac 
without too much trouble.

The next trick to remember is that US devices are 
dynamically allocated, and NTP normally expects 
to find a clock source under a specific file name 
which is usually made available by making a 
symbolic link.

Since the Mac tries to manage USB device node 
creation dynamically this means that one usually 
needs to create or update that link every time 
the machine is rebooted (or has it's device tree 
updated for any other reason).
--
Russell

At 6:04 PM -0700 2012/06/12, Chris Albertson wrote:
>On Tue, Jun 12, 2012 at 8:13 AM, mike cook <michael.cook at sfr.fr> wrote:
>>
>>  Le 12 juin 2012 à 16:37, Ross T61AA a écrit :
>>
>>>  Hi all,
>>>
>>>  I recently connected up a USB GPS to my Linux 
>>>box and found it quite easy to get NTP to up 
>>>from the GPS. I was hoping to get the same GPS 
>>>to update the Mac running Lion 10.7.4, 
>>>however, it has been a very frustrating uphill 
>>>battle.
>>>
>>>  Has anybody managed to get GPS NTP update working on OS-X?
>
>I'd expect it not to run as well on a Mac simply because these are no
>serial ports on any current Mac.
>
>But getting it running should be easy because the Mac ships with NTP
>already set up and running.  All you should have to do edit the config
>file.
>
>If you already have a Linux system running at a server with a direct
>connect GPS there is little need to put a GPS on the Mac because
>without a good PPS signal on a serial oct you may as well use a
>networked GPS server for time
>
>
>Chris Albertson
>Redondo Beach, California
>
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