Re: [gpsd-users] Enable PPS Echo

[Reto Zbinden ST]

Hello Gary

As far as I can see in the log, GPSD is using KPPS, so we're fine with that:

gpsd:INFO: KPPS checking /sys/devices/virtual/pps/pps0/path, /dev/ttyS0
gpsd:INFO: KPPS RFC2783 fd is 6
gpsd:INFO: KPPS caps 1133
gpsd:WARN: KPPS kernel PPS will be used

>From my understanding this PPS echo stuff doesn't really require any special 
>GPS hardware. The feature should be provided by the PPS API and simply clears 
>the RTS line of the serial port as soon as the PPS event is timestamped. For 
>the serial port PPS echo should be available:

# cat /sys/devices/virtual/pps/pps0/path
/dev/ttyS0
# cat /sys/devices/virtual/pps/pps0/echo 
1

I've found some further explanations here:
http://www.ntp.org/ntpfaq/NTP-s-config-tricks.htm#AEN4296

But with my rather basic (linux) knowledge, it isn't clear to me if and how 
echo can be enabled for the PPS device.

Regards
Reto


-----Ursprüngliche Nachricht-----
Von: Gary E. Miller [mailto:address@hidden 
Gesendet: Dienstag, 5. April 2016 22:18
An: Reto Zbinden ST
Cc: 'address@hidden'
Betreff: Re: [gpsd-users] Enable PPS Echo

Yo Reto!

On Tue, 5 Apr 2016 20:11:00 +0000
Reto Zbinden ST <address@hidden> wrote:

> Many thanks for your fast response. I've tried to catch a lot of 
> samples and did some statistics to get a more usable result, but an 
> internet source isn't really a good choice for such a comparison.
> That's why I'm looking for a way to do some more accurate measurements 
> (using some commonly available tools).

If you find some, tell us, computers on a chip makes it hard to pin down 
latency issues.

> My feeling is, that the delay could be introduced by the PPS / 
> interrupt processing on the ALIX board or the serial port itself.

Yup, and if you care, your are using KPPS, and not just PPS.

> I thought that the PPS echo function which generates an event on the 
> RTS line could be handy to verify my theory.

It could, if your GPS supports it.  I have a pile of GPS and I don't think I 
have any with that function.  Does yours?  If so, I'd like the model number.

We'd need to create a tool to send the echo and measure the return, both as PPS 
and as KPPS, but that would not be hard.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588