scxi synchronization

[LSMZ]

The sync example programs and the code wizard both work for synchronizing multiple PXI 6281 DAQ's. However, when the DAQ's are each connected to an independent SCXI chassis, the examples and the code wizard both fail to establish synchronization (multi-device error). If I run the DAQ/SCXI pairs independently, everything works fine. How do I establish sync?

[J-Barticus]

Hi LSMZ,

Below I have included a link to Knowledge Base that discusses synchronizing SCXI chassis using Traditional DAQ.

http://digital.ni.com/public.nsf/allkb/5C862136A158645D86256CF0007F457F?OpenDocument

In case you are using DAQmx, I have done a revision of this Knowledge Base that should yield the same results:

You must share two clocks to synchronize multiple chassis successfully. You share the Sample Clock to control the start of each scan, and you share the Convert Clock to control when to sample each channel in the scan. These clock signals must be shared between the data acquisition (DAQ) devices controlling each chassis through the PXI_Trig lines. You do this the same way you do when synchronizing two E Series devices.

First, select which chassis will be the master chassis. The master chassis sources the Sample and Convert Clocks and shares them to the slave chassis. Use the DAQmx Export Signal VI/function to route the AI/StartTrigger signal to PXI_Trig0 to share the Sample Clock. To share the Convert Clock, use the DAQmx Export Signal to route the AI/ConvertClock signal to PXI_Trig1.

To configure the slave chassis to receive the Sample Clock from the PXI_Trig lines, use the DAQmx Timing VI/function. Wire PXI_Trig0 to the Source input of the DAQmx Timing (Sample Clock). To configure the slave chassis to receive the Convert Clock from the PXI_Trig line, use the DAQmx Connect Terminal VI/function and wire PXI_Trig1 to the Source input and AI/ConvertClock to the Destination.

Completing these steps does not ensure that all the channels on the chassis are simultaneously sampled (unless you have a simultaneous sampling module), but each chassis will sample the nth channel in their scan list at the same time. You also might consider configuring each acquisition to have a start trigger to ensure they start at the same time.

-Jake B.

[LSMZ]

Hi Jake-

Thanks for the help. I am using DAQmx with Measurement Studio.

I can't seem to find the Connect Terminal function you refer to?

[J-Barticus]

LSMZ,

I believe it is the DAQmxConnectTerms function, I have linked the DAQmx C Reference Help for the function.

http://zone.ni.com/reference/en-XX/help/370471W-01/daqmxcfunc/daqmxconnectterms/

-Jake B.

[LSMZ]

Jake-

Thanks again. I actually now have the Multi-device .net example running. I commented out the line that was supposed to provide a synchronized start of the 2 scxi devices.

I realize that this means that the scxi's are not sampling at exactly the same real time point, but that is not important for my application. Now I need to get multi-function going.

To be exact, the first scxi chassis handles about 40 pressure gages and the second scxi handles about 55 thermocouples. I see I could consider them all as just a voltage input and

convert the thermocouples to degrees F and the pressure gages to psi with my own code. But I would like to use the thermocouple conversion offered by measurement studio.

All transducers in the app I working on change very slowly and I only need to sample at 10samples/sec.

LSMZ

[LSMZ]

Jake-

I now have a multi-device( 2 DAQ modules connected to 2 SCXI chassis), one function (voltage input) working and I have a multi-function (voltage input, thermocouple input), one device (1 SCXI xhassis), working.

I need a multi-function, multi-device combination working as depicted below. Hope this make sense.

                                 DAQ PXI 6281 --------------------------------->SCXI with several 1125's connected to pressure gages (voltage input)

PXI 1042 Chassis -----[

                                  DAQ PXI 6281 --------------------------------->SCXI with several 1125's connected to thermocouples

LZ