Taming Clock Drift With NI-Sync

Some data acquisition applications using LabVIEW require an ultimately stable clock reference. In these cases there are a number of options, such as GPS, for generating a clock reference that is more accurate than your DAQ board can attain on its own. Most, however, don't require this kind of absolute accuracy and we can rely on the internal clock of the DAQ board in use.

In our modular DAQ system, we had a number of different data sources. Some of these have no clock source at all and we had to rely on a globally broadcast time signal based on UDP to reference these devices. All of the high speed DAQ devices were setup in PXI chassis and used a common clock. This worked well for all of the devices in each chassis but our system had multiple PXI chassis connected via separate host PCs. For short tests, we can ignore the drift between clocks of these separate clock bases but a drift of less than 0.03% will result in a difference of 1 second over the course of an hour test!

In this case the 1 second drift over 3600 seconds is of no consequence to the test results but the data from all of the data sources needed to be in sync with each other. The solution was to use NI-Sync with a PXI-6652 and PXI-6651 modules to allow all PXI chassis to operate off of one clock source. As an added bonus, the PXI-6652 can accept a high accuracy time signal if the client ever wants to reference the master clock to a more accurate time base.

For more information on NI-Sync, see "Leveraging NI-Sync to Synchronize DAQmx Devices".