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Old 09-11-2012, 01:59 PM
mhead10 mhead10 is offline
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Join Date: Mar 2012
Posts: 40
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"
to
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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