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False Triggers When Arming Slave Device before Arming Master Device in NI-DAQmx

Primary Software: LabVIEW Development Systems>>LabVIEW Base Package
Primary Software Version: 8.6.1
Primary Software Fixed Version: N/A
Secondary Software: Driver Software>>NI-DAQmx
Hardware: Multifunction DAQ (MIO)

Problem:
I am performing a multi-device triggered acquisition where the master device receives a trigger, and uses DAQmx Export Signal.vi to export the trigger to other physically connected (slave) devices. The slave devices use this exported signal as their own start trigger for acquisition. When I arm the slave devices before the master, the slave devices seem to receive a false trigger and finish acquisition before the master even receives its trigger. Why is this happening?

Solution:
The behavior described above is consistent with many NI-DAQmx devices. When exporting a trigger to another line, the device usually asserts the destination line to either low or high as soon as DAQmx Start Task.vi is run to arm the device. The assertion of this line can cause slave devices which are already armed for the trigger to start acquiring data before the master. The example code in Developer Zone Community: Synchronization Considerations for NI-DAQmx Programming demonstrates this issue.

To avoid this problem there are a couple of solutions. One is to arm the master device before the slave devices.

The only problem with this is that you must arm the slave devices immediately after the master device, so that the risk of the master device receiving a trigger before the slave devices is minimized. Another viable option is to just use the master device’s “ai/Start Trigger” or “ai/Reference Trigger” as the start trigger for the slave devices. The only caveat here is that a route must exist on a PXI backplane or RTSI cable for the signal connection between the master and slave devices. One example that demonstrates this is Multi-Device Synch-Analog Input-Cont Acquisition.vi, which is found in the LabVIEW Example Finder (Help » Find Examples…) under Hardware Input and Output » DAQmx » Synchronization » Multi-Device.

In this example, the slave device is armed before the master device (to completely avoid the risk of missing the master device’s trigger), and no false trigger is sent to the slave device when the master is armed.

ArmMasterFirst.jpg
ExampleVIPic.jpg

Report Date: 05/14/2009
Last Updated: 05/21/2009
Document ID: 4XD8PT1V

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