An issue with DAQmx Error messages

Question

Greetings,
I'm using a 6602 counter board with DAQmx 7.4, ANSI C API.  A strange issue concerning the errors due to faulty attribute values keeps occuring, that being no error is reported when the (faulty) attribute value is set but only when it is read back afterwards.  Shouldn't the faulty value be reported by the Set function?
To be specific, I'm creating a period measurement counter input channel with implicit timing, sample mode = finite, then setting the number of samples per channel to 0 (bear with me, I know the 0 value makes no sense here, the point is how the errors are being reported so that they can be handled in a reliable and consistent way).  While DAQmxSetSampQuantSampPerChan(taskHandle, 0) does not produce an error, calling DAQmxGetSampQuantSampPerChan(taskHandle, &SampPerChan) for verification on the very next line returns Error -200077 : "Requested value is not a supported value for this property".  So, my question is why the error is reported by the Getter instead of the Setter?  Is this normal behavior (if so why?) or is something amiss here?
Jeff

Answer 1

This is the expected behavior. Validating attributes is tricky when attributes are dependent upon other attributes. There are two main approaches that can be taken by NI-DAQmx.
One, when every attribute is set, NI-DAQmx could verify the value of that attribute in the context of the task (i.e., in the context of all other attributes). This is problematic for at least a couple of reasons. One, validating the task after every attribute is set is time consuming and not efficient. Two, validating the task after every attribute is set requires that customers set attributes in a specific order such that dependent attributes are set after their dependencies. This would dramatically decrease the usability of NI-DAQmx. In fact, if attributes are mutually dependent, this approach is impossible.
The second approach is that NI-DAQmx doesn't verify the task until it is forced to do so. Starting a task forces it to be validated. Querying an attribute also forces the task to be verified since the value of an attribute may be dependent upon the value of another attribute.
As you've noticed, we've taken the second approach with NI-DAQmx. This approach leads to a more efficient execution as well as allow customers to set attributes in an arbitrary order. If you want to force the task to be verified in order to check for errors, you can do so explicitly at the desired time. However, the need to check attributes for errors is most often needed when the application is under development and the NI-DAQmx error reporting features makes it easy to determine which attribute has been set to an invalid value even when that error is not reported immediately.
Now, in reality, the way NI-DAQmx handles attributes is a bit more complicated than what I just described. Since some attributes are not dependent on other attributes or, since some attribute values can never be valid regardless of the values of other attributes, these attributes are verified when they are set and errors are returned immediately. We refer to this as coarse attribute verification. For example, if you set the sample rate to 100 MHz on an E-Series device you will immediately get an error.
Hope this helps clarify the behavior.
geoff
Geoffrey Schmit
Fermi National Accelerator Laborary