CLAD Data Acquisition & Instrument Control 4 — Questions and Answers
Question 1: What is the purpose of the NI-DAQmx DAQmx Control Task VI set to 'Commit' state?
- It programs the hardware and reserves resources before calling Start, reducing start latency (Correct answer)
- It permanently saves task configuration to MAX
- It commits acquired data to a TDMS file
- It locks the sample rate to the committed value
Correct answer: It programs the hardware and reserves resources before calling Start, reducing start latency
The Commit state programs hardware registers and allocates resources so the transition to Running (Start) is faster, useful for time-critical applications.
Question 2: In SCPI (Standard Commands for Programmable Instruments), what does the '*RST' command do?
- Resets the instrument to its factory default state (Correct answer)
- Reads the instrument status byte
- Triggers a single measurement
- Clears the error queue only
Correct answer: Resets the instrument to its factory default state
*RST is an IEEE 488.2 common command that resets an instrument to its defined factory default settings without affecting communication parameters.
Question 3: Which NI-DAQmx property allows you to apply a custom linear scaling (slope and offset) to convert raw DAQ voltages into engineering units?
- DAQmx Scale (Linear)
- DAQmx Channel Property (Custom Scale) (Correct answer)
- DAQmx AI Voltage with Excitation
- DAQmx Calibration (Self-Cal)
Correct answer: DAQmx Channel Property (Custom Scale)
A custom scale created in MAX or via DAQmx Scale VIs and applied through the channel's Custom Scale Name property converts raw voltage to physical units (e.g., °C, PSI).
Question 4: What is the effect of selecting 'RSE' (Referenced Single-Ended) mode when measuring a grounded signal source?
- It works correctly because both signal and device share the same reference ground (Correct answer)
- It causes a ground loop that adds 60 Hz noise to the measurement
- It applies differential amplification automatically
- It enables the signal conditioning stage
Correct answer: It works correctly because both signal and device share the same reference ground
RSE is appropriate for grounded signal sources because the signal already has a ground reference that matches the DAQ device's AIGND, avoiding ground loops.
Question 5: A user calls DAQmx Read inside a While Loop for continuous acquisition. What is the correct placement of DAQmx Start Task and DAQmx Stop Task?
- Start Task before the loop; Stop Task after the loop exits (Correct answer)
- Start Task and Stop Task both inside the loop on each iteration
- Start Task inside the loop; Stop Task outside the loop
- Omit both; DAQmx Read starts and stops automatically
Correct answer: Start Task before the loop; Stop Task after the loop exits
Start Task should be called once before the loop to arm the hardware, and Stop Task should be called once after the loop to cleanly terminate acquisition.
Question 6: What does the 'timeout' input on DAQmx Read control?
- How long DAQmx Read waits for the requested number of samples before returning an error (Correct answer)
- The maximum duration of the entire acquisition task
- The hardware inter-sample delay
- The USB transfer packet timeout
Correct answer: How long DAQmx Read waits for the requested number of samples before returning an error
The timeout input specifies the maximum time (in seconds) DAQmx Read will block waiting for the requested samples; if samples don't arrive in time, it returns an error.
Question 7: Which LabVIEW function palette contains the VIs for communicating with serial (COM port) instruments?
- Instrument I/O → Serial (Correct answer)
- Data Communication → Protocols
- Instrument I/O → GPIB
- NI-DAQmx → Device Configuration
Correct answer: Instrument I/O → Serial
The Instrument I/O → Serial palette contains VISA-based Serial VIs (Configure, Write, Read, Bytes at Port) for COM port communication.
CLAD Data Acquisition & Instrument Control 4
What is the purpose of the NI-DAQmx DAQmx Control Task VI set to 'Commit' state?