USB-6211 setup for DA

[mhubel]

I am trying to use a USB-6211 in a very simple Visual Studio C++ system. The ultimate intention is to continuousely output data to the two channels at 180 samples per second. At this point, I have the following commands setup:

DAQmxErrChk (DAQmxCreateTask("",&taskHandle));
DAQmxErrChk (DAQmxCreateAOVoltageChan(taskHandle,
    devcmd,    //physical channel
    "",    //name assigned
    -10.0,    //min output value
    10.0,    //max output value
    DAQmx_Val_Volts,    //units of output
    ""    //custom scale name
    ));
DAQmxErrChk (DAQmxCfgSampClkTiming(taskHandle,
    "",    //source (default to OnboardClock)
    180.0,    //rate in samples per second
    DAQmx_Val_Rising,    //active edge
    DAQmx_Val_FiniteSamps,    //a finite number of samples (to be changed)
    samples_to_write    //samples to aquire
    ));
DAQmxErrChk (DAQmxStartTask(taskHandle));

On the last command I get the message:

Non-bufferered hardware-timed operations are not supported for this device and Channel type. I can not make any sense out of this. Any suggestions?

[KyleTTU]

Hi mhubel,

Are you getting any specific error code associated with this message? 

[mhubel]

The full error message is:

 DAQmx Error: Non-buffered hardware-timed operations are not supported
 for this device and Channel Type.

Set the Buffer Size to greater than 0, do not configure Sample Clock timing, or
set Sample Timing Type to On Demand.
Task Name: _unnamedTask<0>

Status Code: -201025

Note that the buffer is be set by samples_to_write which is set to 1800

If you wish to look at I have included the offending routine as an attachment. This message is generated by line 35, the DAQmxStartTask

[KyleTTU]

Hi mhubel, 

It seems you may be attempting to do a non buffered hardware write, which is not supported by USB devices.  One way to fix this is to create a buffer before you start writing.  Please refer to the below KB for more information.

http://digital.ni.com/public.nsf/allkb/EC1968728E660B288625780700570D06?OpenDocument

[mhubel]

I effectively removed the DAQmxStartTask as is implied and it appears to work. This seems counter intutitive. I will never have the time to really understand the whole NI structure as I only use two very simple devices in a simple application. I mostly wish there was a much more direct way to talk to these devices without all the "help" of the whole NI system.

Still the buffering does not work on subsequent calls.

I have spend too much time on this simple issue. It is time to give up and build a D-A that is simple to access.

[mhubel · Accepted solution]

One of your support people helped on the phone. It seems that a somewhat obscure command

DAQmxSetWriteRegenMode(taskHandle, DAQmx_Val_DoNotAllowRegen)

is required to get buffering to work properly. This is a very poorly documented command and there is no real indication that it is required.

[KyleTTU]

That's great to hear you got some good immediate support and I apologize about the poor documentation, I am going to go ahead and add this information to the Knowledge Base.

Happy Programming