Hi! please tell me what I'm doing wrong here, I'm confused about how to do continuous sample acquisition with multiple channels (using NI USB-6215). following code is python code but I'm sure that won't confuse you 🙂 (and it's only one part of my code so don't try it, it won't work 😉 buffer_size = 2000 #samples to readsampling_rate_hz = 20000channels = "Dev1/ai0,Dev1/ai1" #this can be one channel or many...CHK(nidaq.DAQmxCreateTask("",ctypes.byref(taskHandle))) CHK(nidaq.DAQmxCreateAIVoltageChan(taskHandle,channels,"",DAQmx_Val_Cfg_Default,float64(-10.0),float64(10.0),DAQmx_Val_Volts,None)) CHK(nidaq.DAQmxCfgSampClkTiming(taskHandle,"",float64(sampling_rate_hz),DAQmx_Val_Rising,DAQmx_Val_ContSamps,uInt64(buffer_size)))CHK(nidaq.DAQmxRegisterEveryNSamplesEvent(taskHandle,DAQmx_Val_Acquired_Into_Buffer,uInt32(1000),uInt32(0),EveryNCallback_func,None))CHK(nidaq.DAQmxRegisterDoneEvent(taskHandle,uInt32(0),DoneCallback_func,None))#and the callback function def py_EveryNCallback_func(self, taskHandle, event_type, nSamples): data = numpy.zeros((self.channel_amount,buffer_size,),dtype=numpy.float64) read = int32() CHK(nidaq.DAQmxReadAnalogF64(taskHandle,buffer_size,float64(10.0),DAQmx_Val_GroupByScanNumber,data.ctypes.data,buffer_size*number_of_channels,ctypes.byref(read),None)) With one channel everything goes fine, and voltage chart looks like this: buffer_size = 2000 , sampling_rate_hz = 20000 But if I use two channels, voltage chart looks like thisbuffer_size = 2000, sampling_rate_hz = 20000it looks like the sampling rate is higher or there is less values?!?, but with two channels the array with results is 2000*2 long and with one result it is 2000*1 long so it's not smaller most important(?) settings in my code:DAQmxCfgSampClkTiming "float64 rate": 20000 (sampling_rate_hz)DAQmxCfgSampClkTiming "uInt64 sampsPerChanToAcquire" : 2000 (buffer_size)DAQmxRegisterEveryNSamplesEvent "uInt32 nSamples" : 1000 (?!?!)DAQmxReadAnalogF64 "int32 numSampsPerChan": 2000 (buffer_size)DAQmxReadAnalogF64 "float64 readArray[]": [ [buffer_size]*number_of_channels ]DAQmxReadAnalogF64 "uInt32 arraySizeInSamps" : buffer_size*number_of_channels as you can see nSamples is a big question mark, but the problem still exist if I put buffer_size variable (2000) thereMessage Edited by Dazzler on 09-04-2008 05:00 AMMessage Edited by Dazzler on 09-04-2008 05:02 AM

Hi Dazzler, There isn't a multichannel example that ships with the driver but after a quick look at the code you are using in the your third post, everything seems to be configured correctly. The one thing I thing I had a question about was your plots. It looks like you are plotting the same number of points each time. If you are just plotting the data array straight from the read function, you would need to plot (buffer_size*number_of_channels) number of channels since the data returned is as an interleaved array. You may also choose to deinterleave the samples. More information about this can be found in the NI-DAQmx C Reference Help which is installed with the NI-DAQmx driver. Regards,KentApplications Engineer

Measurement Studio for VC++

continuous sample acquisition with multiple channels

Solved!

Author

Interleaving was indeed my problem, so changing "DAQmx_Val_GroupByScanNumber" to "DAQmx_Val_GroupByChannel" in DAQmxReadAnalogF64 solved my problem!

thank you very much 🙂

Message 11 of 11

(3,856 Views)

Measurement Studio for VC++

continuous sample acquisition with multiple channels

Re: continuous sample acquisition with multiple channels