9237 9205 out of sync

[KAndrej]

In the cDAQ 9178 we are using the 9237 module for strain  9205 for voltage measurement. The channels are selected with the Daq assistant so all channel are in the same task. After some measurements we noticed around 25ms delay on the strain channels regarding the analog channels. In cDAQ  chassis tech info it is stated that all the channels in the same task are automatically synchronized but this is not true. Must / how we use some additional triggering (we can only select the 9205 as digital , edge trigger source) to get rid of the time delay?

[Brad_K · Accepted solution]

Hi KAndrej,

This knowledgebase explains the delay you're seeing, and describes some techniques for compensating for it: Why Is My Data Delayed When Using DSA Devices?

Brad

[MarkGrot]

Is your sample rate <= 1612Hz?  One thing to note is that the NI 9237 has discrete sample rates (50kHz/1..31).  If you set the sample rate to less than the slowest rate, DAQmx will coerce your sample rate up to the next valid value.

The NI 9237 data is delayed.  This is specified in the manual as the input delay.  For this module the input delay is 38.4/fs + 4.8us.

25ms = 38.4/fs + 4.8us

fs = 1536, next valid rate is 1612 Hz.

Your modules are synchronized though.  If you were to run the task multiple times, you would notice that the NI 9237 data delay is constant at that rate.  The fix is to shift your data by the input delay.

Edit:  Looks like Brad beat me.

[KAndrej]

Many thanks Brad for the solution.

[KAndrej]

We a aware of the sampling rate for the 9237 as 50kHz/31, we are using the factor of 30 -> 1.667kHz.