Why does my AI acquisition slow down to my frequency measurement rate?

Question

I'm using Labview/Daqmx to program my SCXI 1520 and TC modules while at the same time using the multifunction capabilities of the E-card controlling the SCXI.  This has been fairly easy until I wanted to use the frequency and counter measurements (outputs/DIO works great).  I'm using a waveform with buffered acquisition for the read on the AI channels and obviously there is no waveform for the frequency measurement so I'm just combining the signals using the dynamic data.  This works great when my input frequency is higher than my acquisition frequency.  When my input frequency is lower than my acquisition frequency the acquisition slows down to that frequency and the AI read falls behind (buffer fills while waiting for another frequency measurement).  I'm assuming the frequency read is not keeping up with AI read.  I understand that as the frequency is reduced it takes more time to calculate the frequency.  So how do I "read" the frequency measurement at the same rate as the AI data?  I'm fine with using the last frequency measurement until a new frequency measurement is available but I can't even seem to be able to do that.
An example of the problem is measurement of vehicle pedal force (AI channel) with pulse input for vehicle speed.  Assuming 100 Hz acquisition and the vehicle stopped or going slow (very slow pulse train) I should be able to acquire data at 100 Hz even if I just use the last data point for the frequency channel until another point is available.
Please help, thanks.

Answer 1

Basically, the very fact that you're *doing* a frequency read suggests that you don't know the expected freq ahead of time, and probably suggests that it can vary.  The waveform datatype assumes a constant interval between samples which wouldn't generally be true for frequency measurement, and that's why you can't use it.  Since the actual timestamps of the frequency measurements are NOT the same as those in the analog waveform, it's likely a mistake to pretend they are.
My method is to create my own array of "timestamps" for the frequency data.  Create a For loop with a shift register initialized to 0 from outside the loop.  Let your freq array auto-index the loop.  On each element, perform (1/x).  Add result to the left-hand shift register value and write it to both the right-hand shift register and to an auto-indexing output.  When the loop completes, the output array will hold timestamps that correlate to your freq measurements.  From there you can choose whether to display on an X-Y Graph, or whether to perform some interpolations to calculate freqs at the analog sample times or vice versa.
-Kevin P.