Synchronizing multiple cDAQ-9188 ehternet chassis

[lis@jiang]

Good morning Yo001,

Another option you have is to create a counter out task to use as the master timebase for your DSA modules.  Again, this should be done in LabVIEW rather than SignalExpress.

Thanks,

Lisa

[John_P1]

The problem with synchronizing multiple delta-sigma modules across chassis is the inability to share a clean timebase between them.

NI's delta-sigma C Series modules use either a 12.8 MHz (e.g. the 9236) or a 13.1072 MHz (e.g. the 9234) oversample clock--this clock frequency comes from an oscillator which is on the actual module itself.  When the modules are in the same chassis, it is possible to share oversample clocks between modules cleanly across the backplane, so synchronization is not much of a problem.  However, when you are using multiple chassis, the limiting factor is that there is no current C Series module with the appropriate bandwidth to cleanly export and import the DSA oversample clocks between chassis.

So, to synchronize modules in multiple chassis, you still can use a common start trigger, but the sample clocks will drift over time since they are not derived from the same source.  If you are running relatively short finite acquisitions then this probably wouldn't be noticeable (the 13.1072 MHz oscillator is accurate to 50 ppm, the 12.8 MHz oscillator is accurate to 100 ppm).  Over longer acquisition periods however, the drift might start to become a problem.  If this is the case, you can generate a slower frequency (e.g. using a counter) to share between the chassis which can be used as a shared oversample clock.

Note that while you can define your start trigger source in SignalExpress, you cannot define an external timebase for your delta-sigma modules without the use of the lower level driver functions (which are available in the LabVIEW, ANSI C, or .NET APIs).  So, in SignalExpress the best you'll be able to get is a shared start trigger between chassis, which results in a worst-case drift of 200 ppm (if the task in each chassis is using the 100 ppm 12.8 MHz oscillator).  In development environments which expose the full DAQmx API, you can utilize a slower oversample clock which may be shared across chassis if desired.

Best Regards,