Two NI PCIe7842R modules out of sync

[stevensung]

Hi,

We have built automatic testing system with FPGA code running on two NI PCIe-7842R modules. The synchronization between these two modules are handled (as we believed) by a RTSI cable. Recently we noticed actually the synchronization between these two modules are not as accurate as we thought. So I am wondering if there is additional configuration that we have to do to bring that back to what it is supposed to do. However, in the MAX we could only add the RTSI cable as a new device but that is pretty much all we could do. There is no other configuration, such as adding a device to the cable, available.

Could anyone give me some help on this issue?

Thanks.

[Gregory]

Hi Steven,

Did you do anything in your code to synchronize the cards? I'll warn you right off the bat, I'm not familiar with FPGA or your NI cards, but I have made two tasks use the same clock before. Using the DAQmx Timing (Sample Clock) VI you can share the clock from one card with the other. In my case, I want the AI to be triggered off the AO card, and did it like this:

[Miro_T]

gregoryj shows a good general idea how to synchronize two devices i.e devices need to share a common clock and start trigger.

However the code shown is for DAQ devices using DAQmx API, programming on FPGA is different. In order to synchronize two R series cards with a RTSI cable you will need to connect RTSI cable between two cards (this PLLs their on board clocks) and add a RTSI trigger as shown in this KB. 

[stevensung]

Thank you, both gregoryj and Miro_T.

My problem does not seem to be related to the software. Reason being that is, the FPGA codes running on these two FPGA modules performed without such off-sync issue on two PXI-7833R modules, the fairly old one. Problem of that only showed up after I recompiled them on the new module, PCIe-7842R. Difference between these two setups, as I could see, could be narrowed down to the following a few,

* The sync between two module cards are done with RTSI cable, while on PXI that is handled by the built-in PXI bus on backplane

* New FPGA module (PCIe-7842R) vs. old FPGA module (PXI-7833R)

I personally lean over to the first but don't know why that cause the off-sync between two modules. And I hope someone could help me to figure that out.

Thanks.