CODEWITHSUNDEEP
pythonnidaqmx
Dzz
Dzz

Reputation: 553

Read-Out of two channels of National Instrument USB 6211 with python

I'm trying to read out two channels simultaneously if an USB 6211 with python. To that end, I tried to adapt the example from http://www.scipy.org/Cookbook/Data_Acquisition_with_NIDAQmx by changing the line

CHK(nidaq.DAQmxCreateAIVoltageChan(
        taskHandle,
        "Dev1/ai0",
        "", 
        DAQmx_Val_Cfg_Default, 
        float64(-10.0),
        float64(10.0), 
        DAQmx_Val_Volts,
        None))

to

CHK(nidaq.DAQmxCreateAIVoltageChan(
    taskHandle,
    "Dev1/ai0:1",
    "", 
    DAQmx_Val_Cfg_Default, 
    float64(-10.0),
    float64(10.0), 
    DAQmx_Val_Volts,
    None))

But then, I keep receiving the error message that "nidaq call failed with error -200229: 'Buffer is too small to fit read data". Adding the line CHK(nidaq.DAQmxCfgInputBuffer(taskHandle, uInt32(10000000))) or increasing the length of the data array did not help...

Could someone point me to the right variable to change?

Upvotes: 0

Views: 5079

Answers (2)

Carl Houtman
Carl Houtman

Reputation: 165

Here is an object I use to do A to D with a USB-6009. Note: at the bottom is an example of the calling procedure.

#-------------------------------------------------------------------------------
# Name:       This is a object that takes data from the AtoD board
# Purpose:
#
# Author:      Carl Houtman
#
# Created:     12/10/2012
# Copyright:   (c) Carl Houtman 2012
# Licence:     none
#-------------------------------------------------------------------------------
from PyDAQmx import *
import numpy

class DAQInput:

    def __init__(self, num_data, num_chan, channel, high, low):
        """ This is init function that opens the channel"""
        # Declare variables passed by reference
        taskHandle = TaskHandle()
        read = int32()
        data = numpy.zeros((10000,),dtype=numpy.float64)
        sumi = [0,0,0,0,0,0,0,0,0,0]

        #Get the passed variables
        self.num_data = num_data
        self.channel = channel
        self.high = high
        self.low = low
        self.num_chan = num_chan

        # Create a task and configure a channel
        DAQmxCreateTask(b"",byref(self.taskHandle))
        DAQmxCreateAIVoltageChan(self.taskHandle,self.channel,b"",DAQmx_Val_Cfg_Default,
                                 self.low,self.high,DAQmx_Val_Volts,None)
        # Start the task
        DAQmxStartTask(self.taskHandle)

    def getData(self):
        """ This function gets the data from the board and calculates the average"""
        DAQmxReadAnalogF64(self.taskHandle,self.num_data,10.0,DAQmx_Val_GroupByChannel,
                           self.data,10000,byref(self.read),None)

        # Calculate the average of the values in data (could be several channels)
        i = self.read.value
        for j in range(self.num_chan):
            self.sumi[j] = numpy.sum(self.data[j*i:(j+1)*i])/self.read.value
        return self.sumi

    def killTask(self):
        """ This function kills the tasks"""
        # If the task is still alive kill it
        if self.taskHandle != 0:
            DAQmxStopTask(self.taskHandle)
            DAQmxClearTask(self.taskHandle)

if __name__ == '__main__':
    myDaq = DAQInput(100, 2, b"Dev1/ai0:1", 10.0, -10.0)
    result = myDaq.getData()
    print ("the average readings were {:.4f} and {:.4f} volts".format(result[0], result[1]))
    myDaq.killTask()

Upvotes: 1

Dzz
Dzz

Reputation: 553

I found an answer here: http://www.physics.oregonstate.edu/~hetheriw/whiki/py/topics/ni/files/ni-daq_ctypes_multichannel_adc_usb_6008.txt

In short, the arguments of nidaq.DAQmxReadAnalogF64() need the additional argument "-1" after taskHandle. The line should then look like this:

CHK(nidaq.DAQmxReadAnalogF64(taskHandle, -1,float64(1.0),
    DAQmx_Val_GroupByScanNumber,#DAQmx_Val_GroupByChannel,#DAQmx_Val_GroupByScanNumber
    data.ctypes.data,max_num_samples,
    ctypes.byref(read),None))

Upvotes: 2

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