Tiny Core Extensions > TCE Talk
Time Synchronization
MTCAT:
Hi Rich,
I've been having problems posting since Friday, been dying to talk about all this, maybe I'm babbling on too long on each one....
I was glad to see that gpsd was at least trying to look for the PPS, but this is the error I get,
gpsd: PROG PPS Create Thread gpsd_ppsmonitor
gpsd: PROG PPS can not connect chrony socket: /tmp/chrony.ttyACM0.sock
To me, this looks like gpsd is trying to get the PPS off the USB port (ttyACM0), when the PPS signal is actually on the serial port (ttyS0) ?
David
MTCAT:
Hi Rich,
I'm using a U-Blox EVK-M8T, it has both USB and serial port outputs. Right now, the old power-hungry setup used the USB port for the NMEA strings, and the serial port for the PPS, for lack of knowing any better, this is how I've been trying to do it on the VortexDX3 as well.
There's "U-Control" software that comes with the U-Blox units as well, with that I think I could configure the PPS **and** NMEA strings to come out on the serial port, maybe that would work with everything I have installed "as-is", as long as gpsd I got from TCZ repository is happy with reading both the PPS and NMEA strings from ttyS0. According to gpsd document, this would be like "software" PPS, or good to something like 5 usec. I think that's what this fellow is doing;
https://www.lammertbies.nl/comm/info/gps-time
To get the "holy-grail" of kernel level PPS, around 1 usec, I think I would need to re-build the kernel with CONFIG_PPS enabled (could also enable GPIO support, can input PPS that way too), but I would also need to build and install "pps-tools" and add "ldattach" capability (to attach pps to serial port), so would be a lot of extra work...
But the serial port PPS and NMEA would be a lot better than trying to do it all over USB, apparently USB adds a fair amount of extra jitter along with a 100 usec plus offset.
https://blog.dan.drown.org/pps-over-usb/
Maybe I will try the PPS and NMEA strings on the serial port first, if that works with my TC3.8.4 "as-is", that would probably be the way to go... I think chrony was close to doing it actually....
By the way, I did get the "kernel" level PPS going in Lubuntu, kind of, I installed pps-tools and was able to see both NMEA strings on ttyACM0, and the PPS on ttyS0 (or actually /dev/pps1), but ntp was being difficult for me, ignoring my ntp.conf in /etc, I guess ntp looks for dhcp clients first and ignores ntp.conf in that case, that was about 4 hours of Googling to find that out, but, I almost got the kernel level PPS going in Lubuntu....
Thanks for letting me talk to you about all this,
David
Rich:
Hi MTCAT
Look at page 6 of this manual:
http://www.generalstandards.com/specs/pc104p24dsi12_spec_011111.pdf
Does your board have this connector?
Pins 29,30 and 33,34 of row B are listed as clock and sync outputs respectively.
Pins 29,30 and 33,34 of row A are listed as clock and sync inputs respectively.
Isn't their purpose for syncing multiple data acquisition cards by daisy chaining the outputs from one card to the
inputs of the next?
MTCAT:
Hi Rich,
That would be good to make use of that except that even the "roving" recorders are typically quite far apart, maybe 100 m at best, and more typically 200 m to 300 m, maybe even 1000 m apart sometimes. The fixed (for a given survey) base station can be even further. maybe something like 5 to 10 km away. Having "hard-wired" connection is pretty tough/awkward in the field, even for the closer roving stations, a lot of cabling would be required (heavy).
I think the 24DSI12 does have a GPS input though, which I don't really understand what it's used for.....would be nice if we could just sync all the A-D's to PPS directly and just use the NMEA strings to give ntp or chrony milli-second type of accuracy....
Thanks,
David
Rich:
Hi MTCAT
I didn't realize they were that far apart. I'm pretty sure 100 meters too far to run cabling for the LVDS outputs.
--- Quote from: MTCAT on March 28, 2021, 04:54:57 PM --- ... I think the 24DSI12 does have a GPS input though, which I don't really understand what it's used for.....would be nice if we could just sync all the A-D's to PPS directly and just use the NMEA strings to give ntp or chrony milli-second type of accuracy....
--- End quote ---
Have you read section 3.12 (page 35) of this:
http://www.generalstandards.com/user-manuals/pc104p_24dsi12_man_091113.pdf
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version