error getting string size

[Misfittech]

I was looking for a forum to post this question but could not find one.

I am trying to get size of string for the device names using the following function. I copied from a NI C++ example.
import ctypes
import PyDAQmx as pydaq
import numpy as np

n=pydaq.DAQmxGetSystemInfoAttribute(pydaq.DAQmx_Sys_DevNames,None)

I get the following error:

Traceback (most recent call last):
File "C:\HMRI\Calorimeter\python\main.py", line 9, in
print pydaq.DAQmxGetSystemInfoAttribute(pydaq.DAQmx_Sys_DevNames,None)
File "", line 2, in function
File "C:\Anaconda2\lib\site-packages\PyDAQmx\DAQmxFunctions.py", line 34, in mafunction
raise DAQError(error,errBuff.value.decode("utf-8"), f.name)
PyDAQmx.DAQmxFunctions.DAQError: NI-488: Interface has lost power.
in function DAQmxGetSystemInfoAttribute

[clade]

This is a bug that should be corrected : by default all the functions are supposed to return an error code. An exception was made to a generic case of function (see DAQmxFunctions.py the catch_error function) but not to DAQmxGetSystemInfoAttribute

You should be able to get your attribute by passing a long enough string (create_string_buffer(xxx)).

Second options : use the DAQmxGetSysDevNames functions. This function will have the correct behaviour :

n = DAQmxGetSysDevNames(None, None)
buffer = create_string_buffer(n)
DAQmxGetSysDevNames(buffer, n)

[Misfittech]

I was working on trying to record multiple channel data from NI 9220. It
took me awhile as I did not realize you could pass multiple channel string
to the CreateAIVoltageChan() function. Here is a quick example of what I
coded up. This would/could make a nice example for the documentation.

from ctypes import *
import PyDAQmx as pydaq
import numpy as np
import time
import threading
import datetime
from itertools import imap
import os

class NI_9220:
def init(self):
self.thread=None
self.filename=None
self.pydaqTask=None
self.event=None
self.fs=0;
self.numChannels=0
self.file=None

#channels is a string like 'cDAQ1Mod1/ai0:4' to record first 4 channels
def startRecording(self, filename,channels,fs,

numChannels,comments=""):
#init member variables
self.fs=fs;
self.numChannels=numChannels;
self.filename=filename

    #open file for writting
    self.file=open(filename,"wt")

    #setup the DAQ to fill buffer once a second
    self.pydaqTask=pydaq.Task();

self.pydaqTask.CreateAIVoltageChan(channels,"",pydaq.DAQmx_Val_Cfg_Default,-10.0,10.0,pydaq.DAQmx_Val_Volts,None)

self.pydaqTask.CfgSampClkTiming("",fs,pydaq.DAQmx_Val_Rising,pydaq.DAQmx_Val_ContSamps,1)

    self.file.write("#comments=%s\n\r" % comments);
    self.file.write("#channels=%s\n\r" % channels);
    self.file.write("#samples per second=%s\n\r" % fs);
    self.file.write("#numchannels=%s\n\r" % numChannels);
    self.file.write("#units=volts\n\r");
    #start the data recording
    self.pydaqTask.StartTask()
    t=datetime.datetime.now().time();
    self.file.write("#date=%s\n\r" % datetime.date.today())
    self.file.write("#time=%s\n\r" % t);


    #setup thread to hanndle the processing of the data
    self.event=threading.Event();
    self.thread=threading.Thread(target=self.writeToFile,

args=(self.event,))
self.thread.daemon=True
self.thread.start()

    print "starting"

def writeToFile(self, event):
    data = np.zeros((self.fs*self.numChannels,), dtype=np.float64)
    print "running thread"
    t=0;
    while not event.isSet():
        read = pydaq.int32()

self.pydaqTask.ReadAnalogF64(-1,10.0,pydaq.DAQmx_Val_GroupByChannel,data,self.fs_self.numChannels,byref(read),None)
n=read.value;
x=data[0:(n_self.numChannels)].reshape(self.numChannels,n)

        x= x.transpose()
        for y in x:
            s=",".join(imap(str,y))+"\n\r"
            #print s
            self.file.write(s)

        #flush the file to disk once a second
        if (time.time()-t)> 1:
            self.file.flush();
            t=time.time()
            print("%s - %d bytes" %

(self.filename,os.path.getsize(self.filename)))

        time.sleep(.1)
    print "thread done"
    self.file.close()

def stop(self):
    print "stopping"
    self.event.set();

analog=NI_9220()
analog.startRecording('test.txt','cDAQ1Mod1/ai0:1',10000,2)
time.sleep(10)
analog.stop();
exit()

On Mon, Sep 19, 2016 at 8:20 AM, Pierre Cladé notifications@github.com
wrote:

This is a bug that should be corrected : by default all the functions are
supposed to return an error code. An exception was made to a generic case
of function (see DAQmxFunctions.py the catch_error function) but not to
DAQmxGetSystemInfoAttribute

You should be able to get your attribute by passing a long enough string (
create_string_buffer(xxx)).

Second options : use the DAQmxGetSysDevNames functions. This function
will have the correct behaviour :

n = DAQmxGetSysDevNames(None, None)
buffer = create_string_buffer(n)
DAQmxGetSysDevNames(buffer, n)

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