[LinuxPPS] (OT) A question about DCD.

[clemens at dwf.com]

This is somewhat off topic.
I have one machine, well two that linuxPPS and NTP run on just fine.
However, on my 'main-machine' I have problems.

Back at the end of the summer I noted that I no longer had the PPS signal
on the DCD line.  Neither ppstest nor ntpd reported a signal.
I have two active GPS receivers, so I could trade them.  Same problem,
and the 1st worked just fine on the 2nd machine.

OK, so its the machine, not the GPS receiver nor the cables.

After some thought and no obvious conclusions, I did the simple thing
and bought a 4port tty card and plugged it into the machine.

I plugged the GPS card into it, and everything worked again, so
it SEEMED that it had to be the actual tty port that was bad.

But given a month, that stopped working too.
I tried a 2nd port on the 4port card, and it worked for a while, 
then stopped.  Mumph.

Since then, at one time or another, I have played with the problem,
and the port has worked for a short period of time, but then stopped.

I have NOT pulled out a scope to see the height of the PPS signal, but the
fact that it works effortlessly on the 2nd, or 3rd machine makes me think
that that experiment isnt work the effort.

So, what could be going wrong?
I can think of no way that the software (linuxPPS) could stop working,
but neither can I come up with a reason why both the onboard and a 4port
tty card should both stop working.  (this is an Oncore card, so there is
plenty of tty chatter in addition to the PPS, so one cant just look
at interrupts...)

On occasion it seemed that moving from one port to another of the 4port
card was enough to get things working again,- just doesnt make any sense.

So, any hardware (or kernel) gurus out there that can make any sense of
this???
-- 
                                        Reg.Clemens
                                        reg at dwf.com