[ntp:questions] Atom PPS with parallel port

Rob nomail at example.com
Sun Feb 23 23:23:35 UTC 2014

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.

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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