analog output digital start trigger c api

[PatrickR]

Hi, I am attempting to start analog output based off a digital trigger (either PFIO, or a PXI line)  I can do this easy in LabVIEW.  However with the C API (through Python wrappers), the problem isthat when I call DAQmxBaseWriteAnalogF64, the write will always timeout as the acquisition has not been triggered.   However, I cannot call this after the trigger occurs, as obviously that will be too late.

I cannot find any C API examples where analog output is triggered off a digital trigger.  I can find them for analog input, but that is fundamentally different as you can performan read anytime after the trigger occurs.

Python code as follows (the functions are equivalent ot C API, though you do not need ot pass the task handle as it maintained as part of the Task object)

# create analog output task
analog_output = Task()
analog_output.CreateAOVoltageChan("Dev1/ao0","",-10.0,10.0, DAQmx_Val_Volts, None)
analog_output.CfgSampClkTiming("",outputRate, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, numSamples)
analog_output.CfgDigEdgeStartTrig("/Dev1/PFI0", DAQmx_Val_Rising) 
analog_output.StartTask()
analog_output.WriteAnalogF64(numSampsPerChan=numSamples, autoStart=False,timeout=1.0, dataLayout=DAQmx_Val_GroupByChannel, writeArray=data, reserved=None, sampsPerChanWritten=byref(samplesWritten))
print("Analog output: Wrote %d samples" % samplesWritten.value)

# create digital trigger 
dig_out = Task()
dig_out.CreateDOChan("Dev1/port0", "", DAQmx_Val_ChanForAllLines)

# create digital trigger function
highSamples = 1000
numpts = 3 * highSamples
doData = np.zeros((numpts,), dtype=np.uint32)
doData[highSamples:2*highSamples] =  2**32 - 1

# send digital trigger    
doSamplesWritten = c_int32()
dig_out.WriteDigitalU32(numSampsPerChan=numpts, autoStart=True, timeout=1.0, dataLayout=DAQmx_Val_GroupByChannel, writeArray=doData, reserved=None, sampsPerChanWritten=byref(doSamplesWritten))
print("Digital output: Wrote %d samples" % doSamplesWritten.value)

[David22JRA · Accepted solution]

Hi PatrickR,

You can review some of the text based NI DAQmx (ANSI C) code examples on generating an analog output using a digital start trigger. If you included/checked text-based support dusing your NI DAQmx driver installation, you can navigate to Windows Start>All Programs>National Instruments>NI DAQ>Teaxt-Based Code Support>ANSI C Examples>Analog Out>Generate Voltage>Mult Volt Updates-Int Clk-Dig Start. If you have any questions/concerns regarding the NI hardware.

[PatrickR]

Thanks, yeah I eventually found the examples which were in C:\Users\Public\Documents\National Instruments\NI-DAQ\Examples\DAQmx ANSI C\Analog Out\Generate Voltage\Mult Volt Updates-Int Clk-Dig Start

My mistake was that the AnalogWrite() had to occur before StartTask()  was called.  This fixed the issue

[David22JRA]

Good Job 😃