Hi Joe,
There may be a few ways to code your application in LabVIEW. You have already been looking at the derivative VI, which will take an array input and give you an array output. The purpose of this VI would mainly be to take a derivative of a signal that has already been acquired. This does not sound like what you need because you are trying to determine the derivative as the data is acquired. Thus I believe you have touched on the best method for your application: you will need to examine the last sample taken and calculate the slope between these two points.
To do this, you will need to pass the most recent value read from your acquisition to a shift register. You can then calculate the slope by taking the current value minus the previous value (from the shift register), then divide by the time between samples.
I assume that you are doing a continuous acquisition in a loop and the result is an array. If this is the case, then you can access these two samples without a shift register. You can use the index array function to extract a single element. The index of the most recent sample would be [array size] -1. The previous element will be [array size] -2. With each of these elements, you can then calculate the slope as discussed above.
I hope this explanation makes sense for your application. Please post back if you are still having difficulty. Thanks!
Mike D.
National Instruments
Applications Engineer
