Selecting a subset of channels to read with DAQmx

[AlexandrePB]

Hello,

I'm using a 32 channel NI9205 analog input module, connected with a NI cDAQ-9174 chassis and LABVIEW 2011.

I have created a task in the Measurement and Automation Explorer which has 16 analog input virtual channels (each one with it's input configuration, scale, calibration, etc..). Inside LABVIEW I open this task to do some data acquisition, but I just want do read some of the 16 channels (user-defined), not all of them, so before the "DAQmx Timing" I put a "DAQmx read node" and I set the ChannelsToRead with the names of the channels I want to read. It seems to work, but when I set the acquisition rate it comes the error.

The acquisition module supports a 250kHz rate (divided by each channel). So, if I set 5 channels to read, for example, it should work with a rate until 50kHz. But everytime I try to use a acquisition rate higher than 15,625kHz (which is 250kHz/16) I get an error which says that the maximum rate is 15,625kHz. Like if I were using the 16 channels of the task, but I'm not.. 

Below there is the screenshot of the channels to read and timing configuration.

Thanks
Alexandre

[odessy27]

You created a tak in MAX with 16 channels, and I am assuming that you are using that same task to start your daqmx, so you are preallocating channels and hence already divided the data rate 250/16 per channel. Try either creating new task with only 4 channels or just locally choose channels from your daqmx create task, and you should be able to reach max rate per chanel.

[Jamie_J · Accepted solution]

Hello Alexandre and welcome to the forums,

As odessy27 said you are not achieving the correct sampling rate as the task is being set-up to read 16 channels; despite you later specifying which channels to actually read.

To keep the configuration settings from your created task in Measurement and Automation Explorer (MAX) I recommend creating Global channels from these. A Global Channel will store your configuration for each physical channel and can be accessed from multiple tasks

Create a Global Channel by right clicking the physical channels in your task in MAX then selecting Create Global Channel. .

You can drag these across to your block diagram and wire into the task/channels in terminal of a DAQ-mx function as shown in the image below.

If you change this to a control by right-clicking>>Change To Control then the channels can be set by the user and the maximum acquisition rate will adjust as expected.

Regards

Jamie

[AlexandrePB]

Thanks Jamie and Odessy.

I've searched a little more and I found these global channels.. I changed my program to use them so now everything is OK.

Best Regards

Alexandre