Continuous DAQ with NI USB 6008 and DAQmx

[mhead10]

Hello,

I'm using Python to obtain continuous data from my USB 6008 DAQ. I can sucessfully obtain finite samples, but when I change "DAQmx_Val_FiniteSamps" to "DAQmx_Val_ContSamps" in the "DAQmxCfgSampClkTiming" function, my data is read as zeros instead of the actual voltage it should be reading.

Therefore, I have changed back to "DAQmx_Val_FiniteSamps" and tried obtaining continuous data from a while loop. Unfortunately, I can only obtain a maximum of 4 loops worth of voltage values before my program hangs on trying to read data further.

I have altered what functions to loop as well as the sample rate, number of samples to obtain, and the number of channels to obtain data from with no sucess yet.

Below is my example code. Thanks in advance or any help.

import ctypes
import numpy
import viztask
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()
# the 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)))

    
#Main Program
#def obtain_value(): #used with while loop
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(1000),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_GroupByChannel,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 '      end'

#obtain data from a while loop
"""i = 0
while i < 5:
    i += 1
    print i
    obtain_value()"""

[Patrick_NI]

Hello mhead10,

It looks like you are creating a task, creating a channel, setting up timing, reading the voltage, stopping the task, and closing the channel all inside the while loop.  Ideally, only the read voltage shouls be inside the while loop.  I would try moving the task, channel, and timing setup to before the while loop and moving the task and channel close to outside the while loop.

Also, what exactly does your program do when it hangs up?  Does it give an error message, does just python freeze, or does your OS freeze?

Regards, 

[mhead10]

PattyD,

I revised my code as you suggested. Unfortunately, when using "ContSamps" I get : "'Measurements: Task specified is invalid or does not exist.'" Therefore, I put all the code in the loop to avoid this error. I have yet to learn how to keep the loop as you suggested and not get the error.

As for what happens after the program runs, nothing besides seeing the print statements and the zero voltage values. No errors are given and the program isn't frozen because it still responds. I can continue to work in python an on my OS. 

[Patrick_NI]

Hello mHead10, 

Unfortunately, the DAQmx driver isn't supported by Python. Therefore, even if you've used the proper sequence of commands, it's possible that the code will still not execute as it hasn't been fully tested.

That being said, please see the following posts to see examples of progress others have made using Python:

http://forums.ni.com/t5/Multifunction-DAQ/Python-Solution-to-Multi-Channel-Analog-Reads-using-USB-60...

http://forums.ni.com/t5/LabVIEW/Wrapping-LabVIEW-and-NI-DAQmx-libraries-for-Python/m-p/921924/highli...

http://forums.ni.com/t5/Automotive-and-Embedded-Networks/Python-calling-nican-dll-issues/m-p/2023690...