Finite pulse-trai​n generation with variable time lapses with NiDAQmx

Question

Being a newby in Labview and NiDAQmx, I find it tedious to program the application I need to drive my experiment : I want to generate a finite pulse train, with variable delays between two consecutive pulses. These delays are known before the experiment starts, summurized in a table (element 0 decribes the time lapse between pulse 0 and 1, element 1 between pulse 1 and 2 ...). I would like to use NiDAQmx to program this. I had in mind the following scheme : two counters, first one counting time between pulses, the second one generating the pulse once "counter one" has finished his job. What I don't figure out is how to reprogram "counter one"'s register, without introducing unwanted delays in the time
sequence of my finite pulse train. I am using a 6052E DAQ, and I want to generate delays in a same pulse train ranging from 1 microsecond up to 20 seconds.
Thank you for ANY help :-)

Answer 1

Hello !
Well, I will try to be more precise while answering your questions. But first let me thank you very much for your help : your comments, questions and remarks are very helpful to help me shape my project, as you shed light on particular problems I strictly had no idea before !
1) I can certainly get on well with digital output as my flash device is triggered by TTL signals.
2) The lighting device is home-made, and I think it will be better for my needs to produce light throughout the duration of the input at the “on” level. In any case it will be driven by TTL-like signals. The intensity is manually set, so no analog control is required.
3) I will use Labview to collect the experimental data. The signal will be sampled during the light pulse : the rising edge of the TTL-like output signal will trigger both the lighting device and the data acquisition, the latter being stopped on the falling edge. The light emitted by the lighting device is called a “probe” for the following reasons : the photons are absorbed by the photosynthetic sample and “immediately” re-emitted at a longer wavelength. A photodiode acts as a signal transducer, and I monitor the intensity of this fluorescence. Knowing precisely when light pulses occurred after the initial start excitation (which is also a brief pulse of light but of a much greater intensity than the “probe” light) and the intensity of light emitted during theses pulses, I can reconstruct the kinetic of fluorescence decay. This kinetic reflects the behaviour of my biological sample. Thus no continuous acquisition is required as between two light pulses there is nothing to monitor. And all the half-time reaction are timed relative to the same “start” excitation as they reflect a jump sequence of an electron extracted by the “start” excitation : first jumps are quick ones (half-time in the 10 �s range) and last jumps are slow ones (half-time in the 1 s range). With respect to these kinetics, I would like a 1 �s precision. If not achievable, I could skip the quickest jump (but it would be a pity) and focus on the second one which has a 10 ms half-time reaction and a 100 �s precision would then be enough. But as we say in French : “Qui peut le plus, peut le moins !” (roughly meaning that when you can achieve the best, you easily can do the least). This precision relates to the position of one flash with respect to the others. I don’t need to be very precise on the duration of each individual light pulse : a typical value is 100 �s duration, but I can get on well with 90 �s or 110 �s, my data being an average of the light intensity emitted during the light pulse). All I want to know precisely is when it is fired.
4) Concerning the hardware, I already have a NI 6052E DAQ board, and unfortunately my laboratory cannot allow me to spend more money on this. I also have a fast acquisition board (IMTEC T3012) I intended to use to acquire my photodiode signal in place of the 6052E A/D converter. It has an old Labview driver, and I managed to upgrade it to suit my needs. It has an onboard segmented memory and can keep track precisely (it has a 60 MHz sample clock) of a trigger event. I am wondering if I can overcome the timing precision issue with this board, knowing that I strictly don’t care that flash n�1 is triggered at (t0 + 10 �s) or (t0 + 11 �s), provided that I know it, allowing me to place precisely my data point on the time scale. I can program the number of memory segments (i.e. the number of light pulses) and each one is filled with data coming from the photodiode upon receiving the rising edge triggering the light pulse. The precise date at which each triggered is received is stored in the board memory, allowing time reconstruction.
I do hope this is much clearer than my previous attempts ! Believe me, it is quite hard to leave biology for a while and enter the cryptic world of data acquisition !
Gritche