synchronizing PCI-6220 and PCI-4474 sampling clock via RTSI

[msvilans2]

Hello,

I am putting together a data acquisition application, and I am thinking about using a PCI-4474 and PCI-6220 linked with an RTSI cable.

The 4474 will be responsible for acquiring 4 channels of 24-bit resolution data, and the 6220 will be responsible for up to 8 channels of 16-bit resolution data. As the measurements need to be synchronized with external devices, I would like to synchronize the sampling to an external timebase.

The external timebase I have available is a 9.216MHz crystal oscillator. The sampling rate is fixed at 36kHz. How is it possible to make sure that the 4474 and 6220 sample in lock step, taking into account the oversampling requirements of the sigma delta A/D's on the 4474?

I had the idea that with an RTSI cable, I may be able to use the counters on the 6220 to divide down the 9.216MHz clock, and route a sampling time base to the 4474 and 6220. However, I have read in numerous places that the 4474 apparently can't (easily) accept an external sampling clock. Can the PCI-4474 in fact accept a sampling time base from the RTSI in this way?

Some expert opinion on this would be very appreciated.

Thanks very much,

Markus Svilans.

[PBear]

Hi Markus,

I see what you are trying to do and the short answer is no, you can not use your external clock to synchronize these two boards.

There's a few reasons for this and they all revolve around your DSA setup. First, the DSA board must use it's own oversample clock. The DSA's sample clock is derived from the oversample clock and by exporting one of these clocks, you can synchronize your 6220 with the DSA device. If you're acquiring at the same speed, you'd export the sample clock. If you're acquiring at different speeds, you'd output the oversample clock and use this as your DAQ cards timebase.

If you were using a 446x or 449x, you could bring in your external clock into your DAQ board and then export this to a RTSI line. Your DSA board and DAQ board could then PLL to this external clock. The 447x line of DSA products do not support PLL.

Your option right now is to synchronize to the 4474's timing engine through RTSI.

[msvilans2]

PBear,

Thanks for your helpful reply.

I am a little bit surprised that the 447x boards do not allow external clocking. Having this ability would seem to greatly increase the flexibility and usability of the 447x boards in a much wider variety of applications.

I had a look at the PCI 4462. It looks like it should work more than well enough for my application. I am not familiar at all with using phase locked loops to synchronize clocks. Are there any special requirements on the reference clock signal?

Using C or C++ and NI-DAQmx, would it be difficult to set up phase locked loop clock synchronization between the 4462 and 6220 boards? If I used the 9.216MHz clock as the reference clock, would I still be able to sample at 36kHz on both boards, to keep in tight lock step with the external hardware?

Thanks very much,

Markus.

[PBear]

Hi Markus,

I apologize for an error I made earlier. Unlike many of our modular instruments, the DSA cards can only PLL through the backplane of a PXI chassis. PXI offers more flexibility for timing and synchronization because of it's dedicated timing backplane.

But let's take a step back. If the end result is to simply synchornize your two boards, then your current setup is fine. Synchronizing the two boards is fairly straightforward and simple by using a RTSI cable. But if you are trying to drive the two or synch the two using an external clock, you will need to use a PXI chassis.

Summary, you can synchronize the two's data acquisition. You can not synchronize the two to an external clock.

http://digital.ni.com/public.nsf/allkb/A133ED27DF9BCC5986256F2E004BA342?OpenDocument