[ntp:questions] Atom PPS with parallel port

William Unruh unruh at invalid.ca
Mon Feb 24 06:06:55 UTC 2014

On 2014-02-23, Rob <nomail at example.com> wrote:
> David Lord <snews at lordynet.org> wrote:
>> Rob wrote:
>>> David Lord <snews at lordynet.org> wrote:
>>>> Rob wrote:
>>>>> I would like to use the Atom driver (22) on a Linux system with a
>>>>> parallel port.  It is not clear to me from the scattered info I have
>>>>> found on internet if this is going to work.
>>>>> Using a modern Linux kernel with the PPS module, is it possible to
>>>>> symlink /dev/pps0 to a parallel port device and then connect the PPS
>>>>> signal to the ACK input (pin 10)?
>>>>> If not, what else is required to get this working?
>>>>> Examples always refer to the use of a serial port DCD input, but for
>>>>> best accuracy (in the microsecond range) I think the parallel port
>>>>> is better.
>>>>> (no RS232 drivers/receivers, no funny UART that may delay interrupts)
>>>>> Any other suggestions for an accurate PPS input?
>>>> On NetBSD with stock ntpd, pre 2010, I did comparisons of
>>>> pps from Sure GPS with output from dcd at ttl level vs the
>>>> "serial" dcd but didn't really see any consistent difference.
>>> Did you try the parallel port?
>>> I am interested not only in jitter but also in any constant offset
>>> between the PPS pulses and system time on different systems (possibly
>>> using different makes of serial card).  4-6 us would be good enough for
>>> my purpose, but it would not be good when one system had a 10us offset
>>> because of a propagation delay in a linedriver/receiver.
>> Yes, and from archived ntp.conf at that time /dev/pps* at ppbus
>> but I also tried without ppbus and had a symlink to /dev/lpt*
>> For past couple of years I have pps2 -> /dev/tty00 and there
>> is no ppbus in my recent kernels so I guess it was from a
>> custom kernel.
> I am not familiar with BSD speak and we are not considering use of BSD.
> It appears that with the current Linux PPS and the modules that are
> included after 2.6.38 it should not be symlinked but the module directly
> creates the /dev/pps* devices via udev.  That is fine with me as well.
> I only wonder why a simple cookbook is not readily available on the
> linuxpps site or even as a comment in the sourcefile.

/usr/src/linux*/Documentation/pps/pps.txt ?
cookbook  for what? ntp use?

> Now that I have it, the next step is to test things.
> In fact I can do that using the Datum 9360 as it also has a PPS input
> and a measurement mode where it displays the offset between the pulses
> on that input and its internal PPS.

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