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Continuous DAQ with NI USB 6008 and DAQmx
Does anyone have any experience with continuous data acquisition using a NI-DAQ using the ctypes module to access the NI-DAQ family of libraries for NI instruments? I'm trying to use the "cookbook article" from here from SciPy which translates a sample C program for analog signal acquisition into Python.
I changed it to sample two channels continuosly by setting Code:
Dev1/ai0" HTML Code:
Dev1/ai0:1" I also then changed the clock timing to continuous samples HTML Code:
DAQmx_Val_ContSamps I can obtain finite samples fine, however, if I change to "ContSamps", then my data values are read as zero. If I go back to "FiniteSamps", and then use a timer, I typically only get one or two repeated data arrays, and then nothing else happens. Typically nothing else will happen after the "after stark task" print statement. I will include all of my code in the hopes that others can benefit off of it because I think this would be a common task, and I have not found any similar complete code elsewhere. HTML Code:
import ctypes import numpy nidaq = ctypes.windll.nicaiu # load the DLL viz.go() int32 = ctypes.c_long uInt32 = ctypes.c_ulong uInt64 = ctypes.c_ulonglong float64 = ctypes.c_double TaskHandle = uInt32 read = int32() # constants DAQmx_Val_Cfg_Default = int32(-1) DAQmx_Val_Volts = 10348 DAQmx_Val_Rising = 10280 DAQmx_Val_FiniteSamps = 10178 DAQmx_Val_ContSamps = 10123 DAQmx_Val_GroupByChannel = 0 DAQmx_Val_GroupByScanNumber= 1 #interleaved # initialize variables taskHandle = TaskHandle(0) max_num_samples = 2 data = numpy.zeros((max_num_samples,),dtype=numpy.float64) print 'before error check' def CHK(err): if err < 0: buf_size = 100 buf = ctypes.create_string_buffer('\000' * buf_size) nidaq.DAQmxGetErrorString(err,ctypes.byref(buf),buf_size) raise RuntimeError('nidaq call failed with error %d: %s'%(err,repr(buf.value))) #def obtain_value(): #Main Program print 'entered main program' CHK(nidaq.DAQmxCreateTask("",ctypes.byref(taskHandle))) print 'after create task' CHK(nidaq.DAQmxCreateAIVoltageChan(taskHandle,"Dev2/ai0:1","",DAQmx_Val_Cfg_Default,float64(-13.0),float64(13.0),DAQmx_Val_Volts,None)) print 'after create voltage' CHK(nidaq.DAQmxCfgSampClkTiming(taskHandle,"",float64(2),DAQmx_Val_Rising,DAQmx_Val_ContSamps,uInt64(max_num_samples))); #(samples/sec/channel) #ContSamps print 'after clk' CHK(nidaq.DAQmxStartTask(taskHandle)) print 'after start task' CHK(nidaq.DAQmxReadAnalogF64(taskHandle,-1,float64(-1),DAQmx_Val_GroupByScanNumber,data.ctypes.data,max_num_samples,ctypes.byref(read),None)) print 'after read' print data if taskHandle.value != 0: nidaq.DAQmxStopTask(taskHandle) nidaq.DAQmxClearTask(taskHandle) print 'after main program' #vizact.ontimer(1,obtain_value) print 'end of loop' |
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