stream_readers implementation in python

[KelvinTao]

 Hey there!

I am looking into implementing the stream readers into my python code. The Docs have this interesting note:

 Consider using the nidaqmx.stream_readers and nidaqmx.stream_writers classes to increase the performance of your application, which accept pre-allocated NumPy arrays.

I cannot find an example of the implementation for this. See the code I have below for a 5 minute record. What benefits would I get with the stream reader and how to setup??

Using a NI 9234, with a high sensitivity accelerometer.

Thanks

Example Code:

import nidaqmx
from nidaqmx.constants import AcquisitionType
import matplotlib.pyplot as plt

sample_time = 60*5  # units = seconds
s_freq = 2**12
num_samples = sample_time*s_freq
dt = 1/s_freq

with nidaqmx.Task() as task:
    task.ai_channels.add_ai_accel_chan("cDAQ1Mod1/ai0",
                                       sensitivity=9948.0,
                                       max_val=0.5,
                                       min_val=-0.5)
    task.timing.cfg_samp_clk_timing(4096,
                                   sample_mode = AcquisitionType.CONTINUOUS)
    data = task.read(number_of_samples_per_channel=num_samples, timeout = nidaqmx.constants.WAIT_INFINITELY)

[serlucov]

 I would also be very interested to learn about stream readers. Currently trying to implement them.

[gaving]

From what I see the closest we have to an example is https://github.com/ni/nidaqmx-python/blob/master/nidaqmx/tests/test_stream_analog_readers_writers.py

Just pull out the writer parts if you are only doing input.

[ChadMat]

I am trying to implement the use of a streamreader within a task.register_every_n_samples_acquired_into_buffer_event.  I got the  read function work.  

Here is what I have for the input.  

from nidaqmx import stream_readers as sr

readings=np.ndarray((4,16384))
print(readings)
sr.AnalogMultiChannelReader.read_many_sample(readings,16384)

This it the error I get. 

Traceback (most recent call last):
File "_ctypes/callbacks.c", line 234, in 'calling callback function'
File "CodeFileGraphStreamReader.py", line 27, in data_collect
sr.AnalogMultiChannelReader.read_many_sample(readings,16384)
File "C:\Users\chad_finkbeiner\AppData\Local\Programs\Python\Python36-32\lib\site-packages\nidaqmx\stream_readers.py", line 327, in read_many_sample
self._task._calculate_num_samps_per_chan(
AttributeError: 'numpy.ndarray' object has no attribute '_task'

The documentation clearly states to use a numpy array.  

What am I missing? 

[sallott831]

Hi,

did you ever figure the stream_reader ADI out? I also would like to use it but the documentation is unclear. Any help appreciated.

[brian.lee.cc]

Hey there,

The `read_many_sample` method is not a static method on `AnalogMultiChannelReader`. You need to instantiate an `AnalogMultiChannelReader` object and pass it the `in_stream` object from the `nidaqmx.Task` object.

You can find an example of doing this here:

https://github.com/ni/nidaqmx-python/blob/master/nidaqmx/tests/test_stream_analog_readers_writers.py...

[michaelarf]

I just wanted to provide an updated link for the github code, since the paths have been changed and the links above are broken.

https://github.com/ni/nidaqmx-python/tree/master/tests/legacy