On demand sampling with sample rate

[timpact]

Dear community,

I'm relatively new to NI and it Python API, so bare with me. I'm working on the control of a tank system. In the pas,t I used a NI card in the computer and on demand measuring to get the data I needed.

Now, we switched to NI 9202 and NI9264. When setting up the program, I wanted to use:

.read_one_sample() as I did before, but got the following error message:

DaqError: Task contains physical channels on one or more devices that require you to specify the Sample Clock rate. Use the Sample Clock Timing function/VI to specify a Sample Clock rate.
You cannot specify a Sample Clock rate if Mode is set to On Demand.

 Based on that I switched the mode to sample rate and set it to 2 samples per channel. I call .read_one_sample() twice to get the newest data. It's working although I'm aware that it's not a nice solution. Furthermore, I realized that my controller loop is much slower now. I guess it wouldn't be a slow network problem. How can I set up a measurement method that is similar to on_demand while using a sample clock which seems to be needed by my hardware.

Regards,

Tim

[timpact]

Dear community,

I think I can ask my question more clearly now. I guess the question would be "how can I read the most recent sample?"

I found this in the API documentation:

OVERWRITE_UNREAD_SAMPLES= 10252

When an acquisition encounters unread data in the buffer, the acquisition continues and overwrites the unread samples with new ones. You can read the new samples by setting relative_to to ReadRelativeTo.MOST_RECENT_SAMPLE and setting offset to the appropriate number of samples.

I tried to implement it like this:

ni.constants.OverwriteMode = 10252 # overwrite unread samples
ni.constants.ReadRelativeTo = 10428 # read most recent sample

temp3=ni.Task()
temp3.ai_channels.add_ai_voltage_chan(Mod1 + "/ai1")
temp3.timing.cfg_samp_clk_timing(rate = sample_rate)
temp3.in_stream.offset = -1 # read relative to offset

temp3_reader = AnalogSingleChannelReader(temp3.out_stream)

temp3_reading = temp3_reader.read_one_sample()

Now, I get an error message that -1 as an offset doesn't work. I guess there is some fundamental that I don't understand.

Thank you,

Tim

[Kevin_Price]

I don't know details of that text API (Python?) , but what you're trying to do is generally supported.  I see where you set the property temp3.in_stream.offset=-1, but don't see where you set the corresponding property for ReadRelativeTo.  I see where you assign a value to a constant but you aren't configuring the data acq task with it.   Same for the OverwriteMode.

It also looks a little odd to me (though again, I don't know the API), that you create a Reader by passing in an "out_stream" rather than an "in_stream".  

-Kevin P

[timpact]

Hello Kevin,

thank you for trying to help me and I'm sorry that I couldn't ask my question in a better, clearer way.

I just fixed the mistake about the "in_stram" that you pointed out, but it didn't solve my problem. What exactly do you mean by "you aren't configuring the data acq task" ? I thought I set those constants and then they apply to all my tasks. Now, that I think about it, it makes sense that those constants (and therefore the attributes) can be set individually for different tasks.

I thought that the following line, sets all tasks to "read most recent":
ni.constants.ReadRelativeTo = 10428 # read most recent sample

As that seems to be wrong, what's the correct syntax? I know that you don't know the API, so maybe someone else can help me here.

Regards,

Tim

[Kevin_Price]

Help, anyone who knows this API?

Meanwhile, here are some best guesses.  I would *expect* you to need to set the OverwriteMode and ReadRelativeTo in a way very similar to the way you set the offset value.  The settings would attach to one specific data acq task.  Setting the value to some globally-applicable constant is not at all similar to the way one does this from LabVIEW.  In fact, I'm a little concerned to see code that assigns values to things that identify themselves as ni constants. 

    In any event, my guesses at code mods are seen below in red

---

# ni.constants.OverwriteMode = 10252 # overwrite unread samples
# ni.constants.ReadRelativeTo = 10428 # read most recent sample

temp3=ni.Task()
temp3.ai_channels.add_ai_voltage_chan(Mod1 + "/ai1")
temp3.timing.cfg_samp_clk_timing(rate = sample_rate)

temp3.in_stream.offset = -1 # read relative to offset

# these next two lines are merely guesses at the syntax

temp3.in_stream.OverwriteMode = 10252 # overwrite unread samples in this task 

temp3.in_stream.ReadRelativeTo = 10428 # overwrite unread samples in this task

temp3_reader = AnalogSingleChannelReader(temp3.in_stream)

temp3_reading = temp3_reader.read_one_sample()

---

-Kevin P