CLAD Data Acquisition & Instrument Control 3 — Questions and Answers
Question 1: In LabVIEW, which GPIB function sends a command string to an instrument and which receives the response?
- VISA Write then VISA Read (Correct answer)
- GPIB Write then GPIB Trigger
- VISA Find then VISA Open
- Instrument I/O Assistant then DAQmx Read
Correct answer: VISA Write then VISA Read
The standard VISA query pattern is VISA Write (to send the SCPI command) followed by VISA Read (to receive the instrument's response string).
Question 2: What is the role of the 'samples per channel' input on DAQmx Read?
- It specifies how many samples to read per call from the buffer (Correct answer)
- It sets the total acquisition duration in seconds
- It defines the ADC resolution in bits
- It controls the inter-channel delay
Correct answer: It specifies how many samples to read per call from the buffer
Samples per channel tells DAQmx Read how many data points to return for each channel in this single call from the hardware buffer.
Question 3: Which LabVIEW Express VI provides a guided interface for configuring DAQ measurements without manually wiring DAQmx VIs?
- DAQ Assistant (Correct answer)
- Instrument I/O Assistant
- Signal Express Step
- Measurement & Automation Explorer
Correct answer: DAQ Assistant
The DAQ Assistant Express VI presents a configuration dialog that generates the underlying NI-DAQmx code and returns data directly.
Question 4: A counter task is configured to measure the frequency of a digital pulse train. Which DAQmx counter measurement type should be selected?
- Frequency (Correct answer)
- Edge Count
- Pulse Width
- Period
Correct answer: Frequency
The Frequency measurement type counts edges over a known gate time to compute frequency (Hz) of the input signal.
Question 5: What is the NI-DAQmx task state machine order when performing a hardware-timed acquisition?
- Create → Configure → Start → Read → Stop → Clear (Correct answer)
- Open → Write → Trigger → Read → Close
- Initialize → Arm → Trigger → Acquire → Reset
- Configure → Commit → Reserve → Start → Read → Clear
Correct answer: Create → Configure → Start → Read → Stop → Clear
The standard NI-DAQmx flow is: create the task, configure channels and timing, start the task, read data, stop the task, and clear (free) resources.
Question 6: When using the NI-DAQmx Trigger VIs, what is a 'digital edge start trigger' used for?
- It causes acquisition to begin when a specified digital signal transitions on a chosen edge (Correct answer)
- It stops acquisition when a digital line goes high
- It routes the sample clock to a PFI terminal
- It synchronizes the DAC output to an analog threshold
Correct answer: It causes acquisition to begin when a specified digital signal transitions on a chosen edge
A digital edge start trigger holds the task armed until the chosen PFI or RTSI line sees the specified rising or falling edge, then begins acquisition.
Question 7: Which LabVIEW data type is returned by DAQmx Read when configured for multiple analog input channels with multiple samples?
- 2D array of double (rows=samples, columns=channels) (Correct answer)
- 1D array of waveforms
- Cluster of DBL values
- Single waveform with interleaved channel data
Correct answer: 2D array of double (rows=samples, columns=channels)
The 'Analog 2D DBL NChan NSamp' polymorphic instance returns a 2D double array where each column represents one channel and rows are sequential samples.
CLAD Data Acquisition & Instrument Control 3
In LabVIEW, which GPIB function sends a command string to an instrument and which receives the response?