9225 gps synchronization

[Sam_Rudneff]

Hi!

I'm working on the project in the field of enrgy/ power quality analisys.

The thing to do is synchronize voltage measurements in two cRIO systems with NI 9225 modules, that are placed far from each other (no cable connection is available).

Test system: NI cRIO 9024 controller, 9118 chassis, 9225 AI module in slot 2, S.E.A. GPS module in slot 1. LV 2010 SP1, Real Time 2010 SP1, FPGA 2010 SP1, PC with Win7.

What I'm trying to do:

1) at first I've tried to synchronize two AI modules by sending PPS signal from GPS module (PPS-Pulse per second, Boolean signal, have high value once a second precisely (1 microsecond). I'm using this pulse to start 9225 (link PPS to START property value of 9225). As 9225 is delta-sigma ADC-based module, i have a phase shift between two waveforms (about 20 microseconds at 5 kHz sampling rate ).

2) So, i've decided to use a code from article DSA Module Synchronization Reference Design for Multiple cRIO Chassis. There is a resampling algoritm to re-link samples with sample clock pulses.

3)A support engineer adviced me to try start module before the PPS rising, after that take measurement data from FIFO by PPS signal.

Do you have any thoughts by this problem? I've tried a method from 2 point, but I get an error while compiling a code from article. I'm working on it, but, however, want to know about alternate sync methods (i need a sync precise about 1-2 microseconds).

So, what is the ability to synchronize delta sigma ADC module without resampling data.  I hardly understand how to sync measurement data , taken from FIFO after PPS rising (point 3).

All answers will be appreciated!

[Blayne_K]

Hi Vyachelav,

From what I can gather, the SEA GPS module interfaces with LabVIEW via SEA's cRIO Sync Toolkit which is available at http://www.sea-gmbh.com/en/products/labview-addon-toolkits/crio-sync-toolkit/ . I believe the best way to gain information on why there might be a skew of 20 us between your two cRIO system acquisitions would be to contact SEA directly if you are using SEA's cRIO Sync Toolkit. On the other hand, from a hardware perspective one could likely achieve a PPS synchronization with the NI 9225 and an additional NI 9401 digital module that would utilize the digital PPS signal from the GPS module to act as a "start trigger" for an acquisition of the 9225. Given this approach, one would have to architect a solution that would continually use the PPS signal as a reference point of acquisition to prevent a drift in acquisition over time.

Best,

Blayne K 

[Sam_Rudneff]

Hi , Blayne!

Thanks for you answer!

I recently compiled a code from article mentioned above, and used PPS signal from S.E.A. module as a reference signal to trigger reference clock for resampling algorithm. Everything just works well! So it's Ok As for S.E.A. sync toolkit, as I've understood, it uses time synchronization instead of PPS sync, so it would work well. The only difficulty in this task was sync the DSA modules based on sigma-delta ADC, which can't be directly clocked.

Shrotly say, the problem is decided.

Best regards, Slava.