[LinuxPPS] (OT) A question about DCD.

[Hal V. Engel]

On Tuesday 27 January 2009 19:24:32 clemens at dwf.com wrote:
> 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,

Can you give us some details on how the Oncore is interfaced to the serial 
port?   Are you driving the DCD line through a MAX type TTL to serial chip 
with +-12V or are you using the TTL output either directly or through an 
inverter?

Are you loosing everything on a port or just the PPS pulse?

Hal