I want to count pulses coming from an avalanche photon detector (APD)
and reference them to the intensity of a light source. I have written a
small vi (using Labview 8.0, attached) to read a counter (to which I have
connected the output of the APD) at intervals synchronized with the
acquisition of analog data (on which I measure the output of another
detector).
The counter is the card PCI-6602, while I collect the analog data with a PCI-6052 (installed on the same computer).
The
vi executes successfully about 95% of the times, but in the other 5%
the counter does not get sampled the number of times that I require and
the DAQmx Read vi that reads the buffer times out (when the vi times out
usually all the requested samples are available except the last two or
three, for example if I request 5000 samples I may get 4997 and if I
request 100 samples I may get 97). I attach the vi here in the hope that
someone can tell me what is wrong.
The vi is quite
straightforward but let me add a few lines of description so that it is
clear what I have in mind when writing it - it is quite possible that I
am not doing what I think I am doing and that could be the source of the
problems.
First of all I create two DAQ tasks, one
"Analog input" and the other "Counter input". I believe I do that
correctly, the problem is not likely there.
Then I
request the synchronization using the DAQmx Timing vi, Sample Clock. For
doing this I wire an instance of the vi at the output of each of the
tasks I created. I set the source of the Sample Clock for the analog
input task to "Onboard clock" (I understand that by doing so the analog
input channel sampling takes its clock from an internal clock of its own
board), and I set the source of the Sample Clock for the counter task
to /Dev1/ai/SampleClock (Dev1 is the name of the PCI-6052 board); if I
understand correctly, this is the same source as the "Onboard clock" for
the analog input, so this should already guarantee that the two
samplings are performed at the same time if I set the rate for the two
instances to be the same; and if I would set the rate of the clock for the counter at one half the rate of the clock for the analog input, then I would get one counter sample every two analog samples (please let me know if I am understanding
wrong).
Both instances of the Sample Clock vi are set
to "Finite Samples", and the number of samples is entered as a variable
(the same for both). I start the counter task, and I apply to each of
the tasks the DAQmx Read set to the proper case.
Sometimes
the DAQmx Read fails with error code -200284, and the size of the
vector it reads is smaller than what it should be by a couple of units.
The curious thing is that if I select Analog Wfm NChan NSamp instead of Analog Wfm NChan NSamp then the timeout problem becomes worse, happening about 10 to 15% of the time.
What could be the problem? I am doing something wrong? Thanks in advance for all of the answers.
Giovanni
