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When Does the New Data Get Outputted Using Continuous Buffered Output (S-Series 6115/6120)?

Primary Software: LabWindows/CVI Development Systems>>Full Development System
Primary Software Version: 7.1
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Multifunction DAQ (MIO)>>S Series>>PCI-6115, Multifunction DAQ (MIO)>>S Series>>PCI-6120

Problem: I am using a PCI-6115/6120 S-Series board to do continuous waveform generation. I want to change the output waveform during the output operation. I am able to change the buffer data to generate different waveforms, but sometimes the output has to generate a whole cycle of the old data before the new data is seen. Why does this happen?

Solution: When the new data is seen on the output depends on whether or not the current write position is in the middle of the data positions that will be changed. For example, assume the portion between points 200,000 and 800,000 of a 1,000,000 point waveform are to be changed. If the call to DAQmxWriteRaw is made when the waveform is at point 150,000, the entire changed portion is observed in the current iteration. It takes approximately 20 msec to complete the DAQmxWriteRaw call. If the call to DAQmxWriteRaw is made when the waveform is at point 300,000, none of the changed portion is observed until the next iteration, not even the portion after the half-buffer (points 500,000 to 800,000). It takes hundreds of msec to complete the DAQmxWriteRaw call. If the call to DAQmxWriteRaw is made when the waveform is at point 600,000, none of the changed portion is observed until the next iteration. It takes less time to complete the DAQmxWriteRaw call at point 600,000 than making the call when at point 300,000. DAQmx analyzes where it currently is in the waveform cycle and waits until it is not generating any of the range to be changed and only then changes its internal memory buffer over the desired portion of the waveform data.



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Report Date: 10/19/2004
Last Updated: 10/22/2004
Document ID: 3EIF6CTY

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