How do I simultaneo​usly measure two frequencie​s using the two counters on a CompactDAQ chassis?

Question

I am using two encoders as speed wheels to compare two different speeds simultaneously.  I have the encoders connected to the two counter inputs of my CompactDAQ chassis using NI 9423 cards in order to measure the frequencies produced be each and then I convert that data into speed.  How do I efficiently measure these frequencies simultaneously?  I say efficiently because the only way I can get it to work right now is to create a task for the first encoder, gather my frequency data, clear the task for that encoder and then move onto the next.  This is a slow process and therefore the measurements are not exactly simultaneous.  If I try to measure them simultaneously I get resource errors.

Answer 1

Hey Kevin,
Thank you for the information. The cDAQ-9172 has two counters on the chassis itself.  There are no counters on the actual module.  What this means is when you access Counter 0 using a module in slot5 (for example), you are actually accessing Counter 0 on the backplane of the chassis.  This is why you are getting the resource is reserved error.  If you have the other input frequency connected to the second counter of the chassis, you just need to access it using Counter 1 instead of Counter 0.
In your code, for the Counter 2 input, you have "cDAQ1Mod6/ctr0".  If you just change that to "cDAQ1Mod6/ctr1", this will get rid of your "resource is reserved error.  Also, to get both your tasks to be a little more closely synchronized, you should probably use a sequence structure to be sure that they start together. In the code below, I have used two seperate counters on two different modules and synchronized them.  If you are using the same module you would just need to change the counter inputs to represent the two different counters on the same module.
Hope that helps.
Regards,
Message Edited by Raajit L on 04-02-2007 11:00 AM
Raajit L
National Instruments
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Counter Parallel Tasks.jpg ‏66 KB