[ntp:questions] Atom PPS with parallel port

David Lord snews at lordynet.org
Sun Feb 23 19:43:39 UTC 2014


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.


David



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