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If a Multidevice Task cannot be used, Reference Clock Synchronization can be done manually.
When using multiple NI-DAQmx tasks that are acquiring at the same rate, complete the
following steps to synchronize the hardware.
1.
Specify PXIe_CLK100 as the reference clock source for all modules to force all the
modules to lock to the reference clock on the PXI Express chassis.
2.
Choose an arbitrary PXIe-4480/4481 master module to issue a sync pulse on one of the
PXIe Trigger lines. The sync pulse resets the ADCs and oversample clocks, phase aligning
all the clocks in the system to within nanoseconds.
3.
Configure the rest of the modules in your system to receive their sync pulse from the sync
pulse master module. This will ensure all ADCs are running in lockstep.
4.
Choose one module to be the start trigger master. This does not have to be the same module
you chose in step 2.
5.
Configure the rest of the modules in your system to receive their start trigger from the start
trigger master module. This ensures that all modules will begin returning data on the same
sample.
6.
Set the synchronization type of the Start Trigger slaves at
DAQmx Trigger»Advanced»
Synchronization»Synchronization Type
to Slave and that of the Master to Master.
7.
Query
DAQmx Timing»More»Synchronization Pulse»Synchronization Time
on all
modules being synchronized, choose the maximum value and set that as the
DAQmx
Timing»More»Synchronization Pulse»Minimum Delay To Start
on the module from
which the synchronization pulse originates.
8.
Commit all of the sync pulse slave module tasks using the DAQmxTaskControl
VI/Function. This sets them up to expect the sync pulse from the master.
9.
Commit the sync pulse master module task using the DAQmxTaskControl VI/Function.
This will issue the sync pulse.
10. Start all of the start trigger slave module tasks. This sets them up to expect the start trigger
from the master.
11. Start the start trigger master module task. You can now acquire data.
Tip
Consider using a Multidevice task when synchronizing multiple devices at the
same rate.
Tip
You can find example VIs in the NI Example Finder. Select
Help»Find
Examples
to launch the NI Example Finder.
Consider the following caveat to using reference clock synchronization:
•
The PXIe-4480/4481 automatically compensates for its filter group delay. However, some
other device families do not compensate for their filter delay. In this case, manually
compensate for group delay in the waveforms when you synchronize between device
families if this level of synchronization is required for your application.