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How output desired waveform by USB 6251?

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Dear all

1. I have problem like this: Now I use Laview 7.1, NI USB 6251, I want to output current with desired waveform (shown on the image attached). I want to know: Can USB 6251 perform this problem. If yes, can you advice me to solve it, I'm newer with Labview.

Details of current waveform: begining I want to output current of pulse train with amplitude decreasing, until amplitude of current touch 0, after that USB 6251 outputs current with waveform of saw-tooth (but sawtooths flat at the peak).

 Waveform of current like this: 



2. I want to know: Which maximum frequency  and amplitude of pules (or/and sawtooth) that USB 6251 can output?


Can you show me how upload file and Image to my message? I wat to upload file *.vi and image of waveform of current I write.


Thank in advance for any help!


Cuong. NM


Message Edited by nguyenmanhcuong on 10-15-2008 12:18 PM
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Message 1 of 5

Hi Cuong,


Thanks for posting to the National Instruments Discussion Forums! The USB-6251 is only capable of outputting voltage waveforms and not current. To output current, you would need the PCI/PXI-6238, PCI/PXI-6239, or NI-9265. In the case that you do wish to output a voltage waveform, you may do this using an analog output channel of this device. For the specific type of waveform you wish to create, you will need to somehow build this waveform either within your program or by importing it from a file, and this will be the hardest part of creating your application. My recommendation for easily creating this kind of complex waveform would be to use the NI Analog Waveform Editor, which is a program made especially for creating just this type of non-standard waveform.


Once you have your waveform created, you can begin by exploring one of the DAQmx example programs found in the LabVIEW Example Finder (Help » Find Examples) under Hardware Input and Output » DAQmx » Analog Generation » Voltage » "Gen Mult Volt Updates-Int". This program will update the output with the corresponding value from the waveform you create on every edge of the sample clock, which is exactly what you need to do. The maximum amplitude you may output is 10 V and the maximum update rate (sample clock) can be 2.86 MS/s.


How you create your waveform (such as the total number of samples in the waveform and the number of cycles in the waveform) and the rate at which you update will determine what the actual frequency of your signal will be. For example, if your waveform contains 100 samples and represents one cycle (period) of a sawtooth pattern, and you output it at an update rate of 1 kHz, the resulting frequency of the sawtooth wave would be 10 Hz. This is because there would be 10 periods of this sawtooth pattern created in one second since each period consists of 100 samples. Further information on how to determine your output frequency can be found at the online KnowledgeBase article titled "How Can I Calculate the Output Frequency of My Analog Output?". I hope this helps!

Daniel S.
National Instruments
Message 2 of 5

Thank Danie!

Your instruction is very very useful for me

Like you said, I need to buy other device as: PCI/PXI-6238, PCI/PXI-6239, or NI-9265 , I want to know besides these divices, which USB device can output current? My task runs on Laptop. In addition, I need 2 AnalogInput chanels, DigitalOuput chanels (USB 6251 meets these demands and NI 9265 not).

Best regards


Message Edited by nguyenmanhcuong on 10-17-2008 01:11 AM
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Message 3 of 5
Accepted by topic author nguyenmanhcuong

Hi Cuong,


The C Series modules, such as the NI-9265, can be used within an NI-9172 CompactDAQ chassis. This chassis is connected to the computer through USB and contains 8 slots for any C Series modules you wish. My recommendation for the type of setup you need would be to get an NI-9205 analog input module as well as an NI-9401 digital I/O module for use in the cDAQ-9172. For a complete list of C Series module and description of each, please refer to the KnowledgeBase titled "C Series Modules Supported in the NI cDAQ-9172 CompactDAQ".


Using the NI-9265, you will be able to use similar DAQmx example programs to accomplish your task, and these examples can be found in the Example Finder under Hardware Input and Output » DAQmx » Analog Generation » Current. Unfortunately, those three devices mentioned in my last post are currently the only options available for current output and the cDAQ chassis is the only USB solution. Hope this helps further,

Daniel S.
National Instruments
Message 4 of 5

Thank you so much!

Best regards,


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