DAQmx

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DAQmx

Alistair Bailey – Field Engineer

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What is DAQmx?At it’s core, DAQmx is a powerful hardware driver for controlling data acquisition hardware.

Hundreds of DAQ Devices

DAQ Hardware

USB, PXI Express, PCI ExpressBus

NI-DAQmxHardware Driver

LabVIEW, C, .NETApplication Software

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DAQmx Core ConceptsGlobal Virtual Channel

Contains the physical channel and other channel-specific informationTask

A collection of one or more channelsFunctions

Performs a specific function in DAQmxProperties

Control or set specific properties of your DAQmx session

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1. DAQ Assistant• The DAQ Assistant is a

configuration-based interface for controlling your DAQ instruments

• You can configure things like:

• Channels• Voltage ranges• Sampling rate• …and many other aspects

of your measurement

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2. Create Virtual ChannelCreates a virtual channel and adds it to a DAQmx task. If a task doesn’t exist, it creates one.

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2. Create Virtual ChannelCreates a virtual channel and adds it to a DAQmx task. If a task doesn’t exist, it creates one.

Channel Expansion across modules in a PXI chassis

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3. TriggerConfigures a trigger to perform a specific action. The most common actions are a start trigger and reference trigger.

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4. TimingConfigures the timing for hardware-timed data acquisition.

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4. TimingConfigures the timing for hardware-timed data acquisition.

The DAQmx Task BufferTemporary storage in computer memory for acquired or to-be-

generated samples

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5. Start TaskStarts your task. The task is now in the running state.

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DAQmx Task State Model

Unverified Verified Reserved Committ

ed Running

AbortAbort

Abort

Create Verify Reserve Commit Start

Unreserve StopSetClear

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6. ReadReads samples from the acquisition task.

The Read function adapts to different data types and channel counts

Start your task and read data

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7. WriteWrites samples to the generation task.

The Write function adapts to different data types and channel counts

Write data, then start your task

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8. Wait Until DoneWaits for the data acquisition operation to complete before finishing.

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9. Clear TaskClears the task, and releases all of the resources. If the task needs to be reused, use Stop Task instead.

After the DAQmx read, clear the task.

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10. PropertiesAccess all of the properties associated with DAQmx. Properties can by written to or read.

DAQmx TimingQuery the actual sample rate

DAQmx ChannelSpecify which terminal pules

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Why Use the API?Advanced control over your measurement

system

Multiple Language Support

Task and State Management

Multi-device Synchronization

Advanced Application Architectures

Ability to provide even better performance

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Example 1 – Multi Channel Analog OutputTwo ChannelsDifferent Frequencies

Fundamentals:•The sample rate is the “update rate” of the DACThe only variable we can play with is the number of samples in our waveform.•Use “interpolation” and “decimation” to modify the frequency of one independently.

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Output Waveform FrequencyOutput waveform frequency depends on three factors:

• Update rate• Points in the buffer• Number of cycles

in the buffer

Signal Frequency = update rate

points in the buffer# of cycles in buffer X

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Output Waveform Frequency• Buffer size = 1000 pts• # of cycles in buffer = 1• Update rate = 1000 Hz

• Buffer size = 1000 pts• # of cycles in buffer = 1• Update rate = 2000 Hz

• Buffer size = 1000 pts• # of cycles in buffer = 2• Update rate = 1000 Hz

Signal frequency = 1 Hz



Signal Frequency = update ratepoints in the buffer

# of cycles in buffer X

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Demo

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Example 2 – Simultaneous AI & AOOptions:1. Independent of each other

1. All hardware with hardware timed AO2. cDAQ with STC-3

2. Start together1. Use the same hardware trigger for AI & AO on the

trigger VI3. Same clock (preserves phase relationship)

1. Use the same sample clock on the timing VI

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Demo


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Questions?

DAQmx

Documents

daqmx session

daqmx task

nidaqmx properties

daq hardware

common paradigms of

powerful hardware driver

specific properties

ni data acquisition