Multithreading with PCIe-6363 AI channels

[dwang@aurora.tech]

I'm trying to figure out how I can use the AI channels with multithreading in Python. Here is my experiment: I created two threads, and each thread would create a task to read one analog input channel. There would be an error of "nidaqmx.errors.DaqReadError: The specified resource is reserved". But it doesn't happen if I use the two threads to operate on other channel combinations, like

analog input + analog output,

analog output + analog output, 

analog input + digital input,

.....

So far I only found that error happens with analog input + analog input. 

I wonder why is that? Are the analog input channels share any resources (like clock)? Why that error doesn't happen with other combinations?

Thanks!

[santo_13]

Fundamental rule: on 6363, you can have only one DAQmx Task per board per type for hardware-timed operations on AI, AO, DI, and DO. So, when the first thread creates a hardware-timed task for AI, the next thread cannot create another DAQmx task for AI unless the first thread releases.

On the other hand, when each thread creates Task for a different type, there is no issue as it doesn't exceed the 1 task per type rule.

This applies to most NI DAQ hardware and by hardware design.

[ft_06]

Here is how we leveraged mult-threading:

- only 1 thread per card for AI can be registered

task.register_every_n_samples_acquired_into_buffer_event(xxx, callback_nidaq)

def callback_nidaq(..., task_handle, every_n_samples_event_type, number_of_samples, callback_data):
        task = .... # some magic to get the task easily, see thread mentionning functools.partial
        
        # read task: all AI channels are read containing a list of list
        data = task.read(number_of_samples_per_channel = number_of_samples) 

- copy the data in a specific place

- start various processing threads from this thread, here you can have 1 thread per channel if you want as you have copied them internally

[dwang@aurora.tech]

Is there any official document describing "only one DAQmx Task per board per type for hardware-timed operations on AI, AO, DI, and DO"?

It's weird as I was able to use two threads to process two digital lines, as well as two analog outputs at the same time.

[santo_13]

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dwang@aurora.tech wrote:

Is there any official document describing "only one DAQmx Task per board per type for hardware-timed operations on AI, AO, DI, and DO"?

It's weird as I was able to use two threads to process two digital lines, as well as two analog outputs at the same time.

---

You can use it as long as the hardware timed task is not running simultaneously or you were using software timed single point, i.e., doesn't use a sample clock.

Here you go,

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019KWYSA2

Check out the Hardware-timed section, i.e., if it uses a sample clock.

Another related article, https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P8kmSAC&l=en-US