DAQmx Counting the number of input pulses with input TTL as gate

[tyshen]

Hello,

I am trying to realize recording the number of input pulses during each high TTL level input using LabVIEW DAQmx (PXI-6251 + BNC-2110).

The input pulses to be counted come from the single photon detector, and the input TTL signal (as gate) is defined by user on-demand to collect the counting number within this finite duration. So, both of the input pulses and TTL signals are really random, or to say, not regular and not periodic.

I sketch the purpose as shown in the following diagram:

I succeeded in using pause trigger to pause counting at low TTL level, but cannot record the value at the end of each high TTL portion using 1 Channel N samples read VI. The reason seems that these two functions (pause trigger and sample clock) conflict because they use the same gate channel for M series DAQ.  

So I wonder if there is a good way to realize my aim in this kind of DAQ? 

By the way, I find from the user manual that "pulse-width measurement" looks automatically doing what I want. But I could only readout the pulse width from the task, and did not know how to readout the count. Is it possible to directly get the count values from the register or memory as mentioned below? 

Thank you very much in advance!

[Kevin_Price · Accepted solution]

If I understand correctly, it may be as easy as doing these 2 things, as shown using LabVIEW.

1. When initially configuring the task, set up Units=Ticks.  

2. When performing the reads, choose to read the (raw) count as a 32-bit integer

-Kevin P

P.S.  Actually, only the U32 read part is necessary.  I just like to follow the convention of declaring my *intention* to simply count integers during my initial configuration.

[tyshen]

Dear Kevin, 

Many thanks for your help. It works. And, just one more thing I would like to double check: 

In this case, here my input of "CI.CtrTimebaseSrc" should be the "external pulse signal to be counted", right? 

Then, if it is non-periodic, will the "CI pulse-width task" give the real correct number of pulses? 

[Kevin_Price]

Yes, you've got it -- and good catch on the need for the property node to define your external signal as the Timebase Source too!

Once set up this way, the hardware does all the work -- pulses get counted while the other external signal is digital high, count gets latched when the signal transitions low. 

One other item I didn't mention before is to set up a task buffer with a call to DAQmx Timing so that latched counts also get buffered up at each high-to-low transition:

-Kevin P

[tyshen]

Many thanks. It is working well now. 

And how the raising / falling edge of the pulse is defined / specified in the "CI pulse width" task?

I mean, what is the voltage change can be recognized as raising / falling edge?

I am using 3.3 V LVTTL and no problem. But I did not find this kind of requirements in the datasheet, so I really wonder what is the minimum acceptable value for raising / falling edge? (The maximum input should be 10 V, right?) Thanks.