Writing a Digital Pattern Using DAQmx in LabVIEW

This article demonstrates how to build and write a pattern of digital data on an NI-DAQmx device.
This approach is only suitable for short patterns consisting of edge changes within the micro-second (μs) range. For higher precision and accuracy, one of the following approaches is required instead:

• a PXI Digital Pattern Instrument alongside the Digital Pattern Editor software.
  • Refer to What Are PXI Digital Pattern Instruments?
• A device with digital I/O and an onboard FPGA.

Prerequisites

• An NI-DAQmx device with digital I/O and hardware timing capabilities.
  • A device that supports hardware timed sampling will list Sample Rate specifications in the Specifications document.
• A compatible version of LabVIEW and NI-DAQmx installed.
  • Refer to NI Hardware and Software Operating System Compatibility to determine a suitable software version.

For demonstration purposes, this article creates a digital pattern on a PXI-6534 with a sample rate of 25 μs. Modify the code accordingly to suit your device and desired sample rate.

1.  Launch LabVIEW and create a new VI by clicking File >> New VI.
2. From the Measurement I/O >> NI DAQmx palette, place a DAQmx Create Virtual Channel VI.

• Set the polymorphism to Digital Output.
• On the Lines input, create a Control. In the Control, specify a digital port that you wish to write to.
  • For example, if using a PXI-6534 named "PXI1Slot2" in NI Measurement & Automation Explorer (MAX), the Control should be populated with PXI1Slot2/portX where X is a port number.

3. From the same palette, place a DAQmx Timing VI.

• Connect Task Out from DAQmx Create Virtual Channel to Task In of DAQmx Timing.
• Connect Error Out from DAQmx Create Virtual Channel to Error In of DAQmx Timing.
• Create a Constant on the Samples Per Channel input. Set this value to the total number of samples that your pattern will consist of. Note: it's value must be a multiple of 4.
• Create a Constant on the Sample Mode input and set it to Finite Samples.
• Create a Constant on the Sample Rate input and set it to a rate supported by your device.
  • For example, the PXI-6534 Specifications document states that a maximum sample rate of 20 MHz is possible.
  • The inverse of the sample rate (1/sample rate) dictates the period of a single sample. For this example, a 40 kHz rate is chosen so that each sample is 25 μs.
• Create a Control on the Source input. Set it's value to a destination that has direct access to the device's timebase.
  • Refer to What Internal Routes are Available on My Device? to check which destinations are suitable.
  • Using the instructions in the linked article, we can see that for the PXI-6534, the /<PXI device>/Dig0/SampleClockTimebase destination has access to the /<PXI device>/20MHzTimebase source.

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

4. Place a DAQmx Configure Output Buffer VI.

• Connect Task Out from DAQmx Timing to Task/Channels In of DAQmx Configure Output Buffer.
• Connect Error Out from DAQmx Timing to Error In of DAQmx Configure Output Buffer.
• Create a Constant on the Buffer Size (in samples per channel) input. Set it's value to a multiple of 4 that is larger than the Samples Per Channel input of DAQmx Timing. 

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

5. In parallel to the DAQmx VIs, place a For Loop.

• Create a 2D array of unsigned byte integers and place it outside the For Loop. This array should have 1 row of 0s populated. This 2D array will hold all of the samples for your digital pattern where:
  • Each column represents a line in the port (8 lines per port).
  • Each row represents a sample for those lines.
• Connect the 2D array to the edge of the For Loop and replace it's tunnels with Shift Registers.
• Inside the For Loop, place an Insert Into Array function.
• Place an Increment function inside the For Loop and connect it's input to the Loop Iteration ("i" symbol).
• Connect the output of Increment to the Index 0 input of Insert Into Array.
  • This ensures that each sample is appended to the end of the array.
• Connect the 2D array to the N-dim Array input of Insert Into Array.
• Outside of the For Loop, create a 1D array of unsigned byte integers. This array holds the next samples to be written into the pattern.
• Connect the 1D array to the New Element/Subarray input of Insert Into Array.
• Expand the 1D array to display 8 rows. Each row is 1 sample of each of the lines (this array is transposed in comparison to the 2D array). Populate each row with a value of 0 or 1 (low or high respectively).
• Create a Constant on the N terminal of the For Loop. Set it's value to the number of samples to be written for this specific pulse.
  • For example, the snippet below shows a For Loop that iterates 100 times. Since a 40 kHz clock has been selected, each sample is 25 μs long. For 100 iterations, this pulse will be 2.5 ms long (100 x 25 μs = 2.5 ms).
• Connect the output of the Increment function to the right-edge of the For Loop. This wire will be used in subsequent For Loops.

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

6. Place another For Loop after the first For Loop.

• Similar to before, create a Constant on the N terminal of the For Loop to define how many samples to write.
  • The snippet below writes another 100 samples (i.e. a 2.5 ms pulse) of 1s.
• Place an Insert Into Array function.
• Connect the 2D array wire from before to the N-dim Array input of this Insert Into Array function. Note: remember to replace the array tunnels with Shift Registers.
• Connect an Increment function to the Loop Iteration ("i" symbol) terminal.
• Place an Add function.
• Connect the output of Increment to the X input of Add.
• Connect the output of Increment from the previous For Loop to the Y input of Add. This ensures that the next series of samples will still be appended to the end of the array.
• Create another 1D array of unsigned byte integers with 8 rows. Set each row to a value of 0 or 1, depending on your pattern requirements.
• Connect the 1D array to the New Element/Subarray input of Insert Into Array.
  • The snippet below writes 100 samples of 1s on all 8 lines. Since the first For Loop wrote 100 samples of 0s, the combined output of both For Loops will appear as a square wave with a duty cycle of 50%.

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

7. Repeat step 6 until you have completed the required pattern.
8. Once the pattern is complete, place a Build Digital Data function.

• Connect the 2D array output from your last For Loop to the Data input of Build Digital Data.
• This function converts your pattern (2D array) to a digital waveform data type.

9. Place a Build Digital Waveform function.

• Connect the output of Build Digital Data to the Y input of Build Digital Waveform.
• Set the value of dt to 1/sample rate. For example, if using a 40 kHz sample clock, dt should be set to 25E-6.

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

10. Place a DAQmx Write VI after DAQmx Configure Output Buffer.

• Connect Task Out from DAQmx Configure Output Buffer to Task/Channels In of DAQmx Write.
• Connect Error Out from DAQmx Configure Output Buffer to Error In of DAQmx Write.
• Set it's polymorphism to Digital >> Single Channel >> Multiple Samples >> Waveform.
• Connect the output of Build Digital Waveform to the Data input of DAQmx Write.
• Create an Indicator on the output of Build Digital Waveform.

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

11. Place a DAQmx Start Task VI.

• Connect Task Out from DAQmx Write to Task/Channels In of DAQmx Start Task.
• Connect Error Out from DAQmx Write to Error In of DAQmx Start Task.

12. Place a DAQmx Wait Until Done VI.

• Connect Task Out from DAQmx Start Task to Task/Channels In of DAQmx Wait Until Done.
• Connect Error Out from DAQmx Start Task to Error In of DAQmx Wait Until Done.

13. Place a DAQmx Stop Task VI.

• Connect Task Out from DAQmx Wait Until Done to Task/Channels In of DAQmx Stop Task.
• Connect Error Out from DAQmx Wait Until Done to Error In of DAQmx Stop Task.

14. Place a DAQmx Clear Task VI.

• Connect Task Out from DAQmx Stop Task to Task In of DAQmx Clear Task.
• Connect Error Out from DAQmx Stop Task to Error In of DAQmx Clear Task.
• Create an Indicator on the Error Out of DAQmx Clear Task.

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

15. Run the VI and observe the digital pattern on the Front Panel.

For the example shown above, a digital pattern consisting of 2.5 ms off and 2.5 ms on on 8 lines of a digital port is created. After running the VI, the pattern appears as follows.