How to count frequency of two signal at the same time ?

Question

Hello every one
please help me with this one i am really stuck.
The situation   : I have two square wave signal, and i want to measure the frequency of both of themat the same time.
the problem : Error -50103 occurred at DAQmx Read (Counter DBL 1Chan 1Samp).vi:3
Please see the attachement.
i also have 3 analog inputs to be measured.
but i dont have any problem with the analog inputs,,,multiple analog  inputs can easily be measured,,,,,but the prob lem is in the digitl (square wave ) signals. 
Attachments:
i want to use two counter input.GIF ‏54 KB
two counter tasks1.vi ‏250 KB

Answer 1

NI has an excellent overview of the accuracy and tradeoffs of the different freq measurement methods titled  "Making Accurate Frequency Measurements."  To boil down the essence of it, the (possibly) increased accuracy of the 2-counter method depends on a relatively long collection / accumulation time which would in turn typically limit the rate at which you could update your measured freq value.
Now let me offer some friendly but blunt advice.  LabVIEW makes it easy to collect data.  However, there are many other factors affecting the usefulness of that data.  I support the dictum: "It is hard to measure *well*."  Choice of sensors, their sensitivity to environment, various sources of noise, an understanding of the decision to be made from the data, programming methods, etc.  And even more etc.
What I mean is that just because a hardware freq measurement can be performed that is precise to 1 part per million, it would be very RARE in a test that ALL of those 6 significant digits prove useful.  In your app, the measurements are presently only approximately correlated in time.  And the gap of blind time between measurements will usually represent a much longer period of time than the measurement itself.
Analogy: suppose you had a 100 minute film at 25 frames/sec for a total of 150_000 frames.  Now start selecting individual frames that are separated by some random # of frames between about 200 and 500.  In the end, you may end up with a sequence of 400 frames in the correct order.  Each individual frame is VERY accurate, but the collection of all of them can zip by in 16 seconds and not give you a much understanding about the movie.
What would usually work out much better is to sacrifice some accuracy in order to collect a greater quantity of frames.  You trade off the instantaneous accuracy for the overall coherency.
Sorry, got on a roll...   Back to your question.  My own preference is almost always to avoid consuming hardware resources when not truly necessary.  I've been toying with counters for many years and have still never once deployed an app using a 2-counter frequency measurement.  I do the 1-counter measurement and save the other counter(s) for other purposes. 
If you've got an M-series board, the 80 MHz timebase allows you to measure a 500 kHz pulsetrain with a quantization error of only about 0.06% (1 part in 160).  There's a pretty fair chance that whatever physical process emits that pulsetrain isn't appreciably more stable than that
-Kevin P.