When using a synchronized acquisition over NI-DAQmx devices, the setup often includes a master device which receives a trigger, and slave devices which look to the master device for their own trigger to begin acquiring.  If the slave devices look to the master device via an exported signal (using the DAQmx Export Signal.vi), a problem may occur if the slave devices are armed before the master device, because when the master device is armed it asserts the trigger destination line.  The assertion of this line will trigger the acquisition for the slave devices, causing the slave devices to no longer be synchronized to the master device.  The attached TriggeredMasterAcquisition.vi and Acq&Graph Voltage-Int Clk-DigRef-Slave.vi demonstrate this behavior when run together.  If the slave device VI starts running before the master device VI,  Acq&Graph Voltage-Int Clk-DigRef-Slave.vi will finish as soon as the master VI TriggeredMasterAcquisition.vi begins running.  This is because when DAQmx Start Task.vi is run in TriggeredMasterAcquisition.vi, the destination line has a low-to-high transition, triggering the slave which is already looking for a low-to-high transition to begin acquisition.  To avoid this problem, start the master VI first, then start the slave VI, and then trigger the acquisition on the master side. 

For other solutions, see KnowledgeBase 4XD8PT1V: False Triggers When Arming Slave Device before Arming Master Device in NI-DAQmx.