ANSI C Create Task vs. Reconfiguring

[FBrings]

Greeting fellow data acquirerers,

I have a question regarding the NI DAQmx C API. I have a very basic application in Qt (for GUI) to start, stop, reconfigure and plot a measurement with a USB-6255 DAQ. The program is built up as follows:

• At the start of the program I create a task
• The task is configured with some initial parameters
• Upon button press this task can be started and stopped
• There are button for changing the sample rate, samples per channel and dynamic range
• Upon changing a value in any of the aforementioned controls the task is stopped, the parameters reconfigured and started again

The initial initialization consists of:

• DAQmxCreateAIVoltageChan to add the required channels to the task
• DAQmxCfgSampClkTiming to configure the sample clock
• DAQmxSetBufInputBufSize to 10 times the samples per channel value
• DAQmxRegisterEveryNSamplesEvent to configure the callback

The problem seems to be, when I choose certain parameters (for example 10000 Hz rate and 10000 samples per channel) DAQmx issues an error that my application can't keep up with the acquisition. I reconfigure the task in the following way:

• If the task was running, stop it
• Call DAQmxCfgSamlClkTiming with the new parameters
• Call DAQmxSetAIMax/Min with the new parameters
• Call DAQmxSetBufInputBufSize to set the buffer to 10 times the new samples per channel value
• Unregister my EveryNSamplesEvent
• Register an EveryNSamplesEvent with the new parameters
• Restart the task, if it was running before

Now comes the twist. If, instead of stopping, reconfiguring and restarting the task, I clear the task and just perform the initial configuration with the new parameters everything works well. So my question in short would be: What happens between deleting, recreating and configuring the task that calling the configuration functions repeatedly does not do? In my opinion I am performing the same configuration steps (obviously without the channel creation) in the initialization and in the reconfiguration, so it seems odd to me that I get different results.

I would be happy about any suggestions. Thanks in advance,

FB.