Counter miss TELL pulses, NI DAQmx 9181 with NI 9402

Question

Hi,
I am trying to read TTL pulses from a function generator using a counter. I am applying a 1KHz signal to the counter. Every time counter reads the pulses correctly in first cycle of measurement, but it miss some counts in all following cycles.
I am using NI cDAQ 9181 Chassis and NI 9402 module with LabVIEW 2014 and NI Max 14.0.
My computer has Windows 8.1 operating system.
Please find the attached VI and images of Front Panel after run.
What could be the reason for that, please guide.
Thanks !
B. Sharma
Attachments:
Counter cDAQ Testing-LV 2014.vi ‏44 KB
Counter cDAQ Testing-LV 2014_Front Panel.png ‏68 KB
Counter cDAQ Testing-LV 2014_Block Diagram.png ‏70 KB

Answer 1

1.  The Elapsed Time VI starts counting when you first call it, so the first loop will take about the desired amount of time (although OS timing is not very reliable anyway).  The Express VI will reset itself as soon as you hit the time target.  Then, you wrap back around and reconfigure your task.  This takes time (during which the DAQ task is not counting, but the Elapsed Time VI has already been reset), and so all subsequent acquisitions will actually run some time less.  For example, if the configuration took 100 ms and you wanted to acquire 1 second of data, you would only get ~90% of the count.
2.  The time it takes to reconfigure the task is going to vary dramatically with the bus you are using due to latency (ethernet > USB > PCI/PCIe).  So it makes sense that the problem was not noticeable with PCI, and it was most severe with ethernet (9181) compared to USB (9171).
3.  Discarding the first sample on buffered counter measurements is a behavior change with STC3-based DAQ products (e.g. X Series, C Series, 621x) compared to older NI DAQ products (e.g. E Series, M Series, 9172).  In your case, when you use the E Series card the first measurement represents the count measured between the arming of the task and the first edge of the gating counter (which toggles almost immediately, you are very unlikely to receive an external 1 kHz clock edge during this time).  Since this measurement is often not desired and can be confusing, it is discarded in the newer NI hardware.
Here is the behavior from the M Series and X Series user manuals (I know not your exact hardware, but I know that the diagrams are in these manuals and your E Series and cDAQ systems respectively will behave the same).
From M Series User Manual:
From X Series User Manual:
Best Regards,
John Passiak