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New Example for Time Based Synchronization #674

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Add Time Based example for scheduled acquisition
MAnoushe-NI Feb 24, 2025
b4316bb
Add Time Based example for scheduled acquisition
MAnoushe-NI Feb 24, 2025
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34 changes: 34 additions & 0 deletions examples/nidaqmx - time tased synchronization.py
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import nidaqmx
import time
from nidaqmx.constants import AcquisitionType
from datetime import timedelta, datetime, timezone

# Function to perform the measurement
def perform_measurement():
with nidaqmx.Task() as task:
task.ai_channels.add_ai_voltage_chan("Dev1/ai0")

# Configure finite sampling
num_samples = 1000
task.timing.cfg_samp_clk_timing(1000, sample_mode=AcquisitionType.FINITE, samps_per_chan=num_samples)
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@zhindes zhindes Feb 24, 2025 •
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With my suggested change to the read, this can be simplified

Suggested change
num_samples = 1000
task.timing.cfg_samp_clk_timing(1000, sample_mode=AcquisitionType.FINITE, samps_per_chan=num_samples)
task.timing.cfg_samp_clk_timing(1000, sample_mode=AcquisitionType.FINITE, samps_per_chan=1000)


# Calculate the start time (1 minute from now)
start_time = datetime.now(timezone.utc) + timedelta(minutes=1)
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@zhindes zhindes Feb 24, 2025

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1 minute is a lot...

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# Calculate the start time (1 minute from now)
start_time = datetime.now(timezone.utc) + timedelta(minutes=1)
# Calculate the start time (10 seconds from now)
start_time = datetime.now(timezone.utc) + timedelta(seconds=10)

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@bkeryan bkeryan Feb 24, 2025

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FYI: I was going to ask whether it's really necessary to specify UTC here, but then I looked up whether datetime.now() returns an "aware" datetime and now I am sad.

https://discuss.python.org/t/datetime-api-no-atomic-way-to-get-aware-current-local-datetime-when-moving-between-time-zones/55041/8
https://stackoverflow.com/questions/24281525/what-is-the-point-of-a-naive-datetime

print(f"Scheduled start time: {start_time}")

# Configure the time start trigger
task.triggers.start_trigger.cfg_time_start_trig(start_time)

print("Starting the task...")
task.start()

print("Waiting for the scheduled start time...")
# Wait until the scheduled start time
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@zhindes zhindes Feb 24, 2025

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Two things:

  1. Generally printing commentary isn't really necessary, but for something that must wait, I like it. But let's not overdo it.
  2. You don't need comments that repeat the code. I like the comment describing how the time delta stuff works out. These feel a bit less useful.
Suggested change
print(f"Scheduled start time: {start_time}")
# Configure the time start trigger
task.triggers.start_trigger.cfg_time_start_trig(start_time)
print("Starting the task...")
task.start()
print("Waiting for the scheduled start time...")
# Wait until the scheduled start time
print(f"Scheduled start time: {start_time}")
task.triggers.start_trigger.cfg_time_start_trig(start_time)
task.start()
print("Waiting for the scheduled start time...")

while datetime.now(timezone.utc) < start_time:
time.sleep(1)
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@bkeryan bkeryan Feb 24, 2025

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There is a DAQmx API for this, Task.wait_for_valid_timestamp, so you don't need to poll.
https://github.com/ni/nidaqmx-python/blob/master/generated/nidaqmx/task/_task.py#L1028

I think you can wait for either TimestampEvent.FIRST_SAMPLE or TimestampEvent.START_TRIGGER.

I assume you are doing this to avoid getting a read timeout. FYI, another solution is to increase the read timeout to account for the scheduled start time, but that's probably less clear.


# Read data after the task starts
data = task.read(number_of_samples_per_channel=num_samples)
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@zhindes zhindes Feb 24, 2025 •
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This is a more standard read from a finite task:

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data = task.read(number_of_samples_per_channel=num_samples)
data = task.read(READ_ALL_AVAILABLE)

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@zhindes zhindes Feb 24, 2025

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Note you'll need to import READ_ALL_AVAILABLE or fully specify it


# Perform the measurement
perform_measurement()
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@zhindes zhindes Feb 24, 2025

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Our examples usually prefer simplicity. I would remove this function and simply put your with ... task: block at the top-level

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