How to read multiple analog channels simultaneously

[Peter_09876]

Hi,

I am trying to read two thermocouple channels on the NI-9211 board with a single task. It works great when there is only a single channel ("Dev1/ai0").

But when I try to add a second channel ("Dev1/ai0:1", or "Dev1/ai0:Dev1/ai1", or "Dev1/ai0,Dev1/ai1"), neither of them read anything at all.

Here is my code - I am using ANSI C API with continuous sampling and a callback:

double temp;

int32 CVICALLBACK DataReadyCallback(TaskHandle taskHandle, int32 everyNsamplesEventType,
 uInt32 nSamples, void *callbackData);

void CreateTask()
{
TaskHandle taskHandle;
DAQmxCreateTask("", &taskHandle);

DAQmxCreateAIThrmcplChan(taskHandle, "Dev1/ai0:1", "", 75, 1500, DAQmx_Val_Kelvins,
		DAQmx_Val_K_Type_TC, DAQmx_Val_BuiltIn, 0, NULL);

DAQmxCfgSampClkTiming(taskHandle, NULL, 6.0f, DAQmx_Val_Rising, DAQmx_Val_ContSamps, 1);

DAQmxRegisterEveryNSamplesEvent(taskHandle, DAQmx_Val_Acquired_Into_Buffer, 1, 0,
		DataReadyCallback, NULL);

DAQmxStartTask(taskHandle);
}

int32 CVICALLBACK DataReadyCallback(TaskHandle taskHandle, int32 everyNsamplesEventType,
 uInt32 nSamples, void *callbackData)
{
 double buffer[4];
 int32 numRead;

 DAQmxReadAnalogF64(taskHandle, -1, 0.5, DAQmx_Val_GroupByChannel, buffer, 4, &numRead, NULL);

 temp = buffer[0];
}

Any advice appreciated!

[andmboa7 · Accepted solution]

Hey Peter_09876!

Did you built this code from scratch?

It looks it is fine, but, you can try with an example we have available in C...you just need to go to All Programs->National Instruments->NI-DAQmx->Examples-> ANSI C-> Analog In

Try with those and let me know how it goes.

-Andrea G

National Instruments

[Peter_09876]

Thank you Andrea - indeed, the example code works fine.

The problem with my code was that the sampling rate was too high, which I was able to find by plugging my parameters into the example and seeing what errors were reported. The maximum sampling rate on NI-9211 is dependent on the number of TC channels:

max_rate = 14.28 / (N +1)

where N is the number of TC channels being measured. So by setting it at 6.0 scans/s, I exceeded the 4.76 max rate for two channels.

[andmboa7]

Excellent Peter_09876 I am glad you manage to figure it out then!

-Andrea G

National Instruments