null TaskHandle in Python?

[Arvont]

GaryS,

Have you tried using the python None type instead of using the null = ctypes.POINTER(ctypes.c_int)() syntax?

check out this example code

http://scipy.org/Cookbook/Data_Acquisition_with_NIDAQmx 

Message Edited by Arvont on 07-05-2009 07:39 PM

[Arvont]

GaryS,

Have you tried using the python None type instead of using the null = ctypes.POINTER(ctypes.c_int)() syntax?  I've had success using the DAQmx dll in windows and problems typically reduce down to figuring out the proper way of passing variables from the python script to the C function.

Learning the ctypes module is the key. 

nidaq.DAQmxWriteDigitalLines(enable,
                                                   ctypes.c_int32(1),        # numSamplsPerChan
                                                   ctypes.c_int32(1),        # autostart=1
                                                   ctypes.c_float64(1),     # timeout
                                                   DAQmx_Val_GroupByChannel,
                                                   ctypes.c_uint8(1),        # VALUE TO BE WRITTEN
                                                   ctypes.byref(sampsPerChanWritten),

                                                   None  )      # reserved, supposed to be NULL 

check out this example code as well

http://scipy.org/Cookbook/Data_Acquisition_with_NIDAQmx 

[Volker (N)]

Hello,

I just tried (not being aware of this thread) to do the same thing: wrapping DAQmxBase into python under SuSE Linux 10.3, python 2.5.

My approach was using swig, but I unfortunately ran into the same problem -> the taskHandle pointer was NULL after createTask.

This just for the records. I might now consider to try the presented solution (compile into python), or I just stay in c++ - don't know yet.

Cheers

Volker 

[JamieB]

Gary S,

Your problem is not the last parameter, but with the third last.  DAQmxWriteDigitalLines wants an array of samples.  If you only have one sample, you can try replacing:

ctypes.c_uint8(1)

with:

ctypes.byref(ctypes.c_uint8(1))

I think the way to handle it generically would be:

numSamples = 1 # or however many samples you want

valArray = (ctypes.c_uint8*numSamples)()

valArray[0] = 1

nidaq.DAQmxWriteDigitalLines(enable,
                                                   ctypes.c_int32(1),        # numSamplsPerChan
                                                   ctypes.c_int32(1),        # autostart=1
                                                   ctypes.c_double(1),     # timeout
                                                   DAQmx_Val_GroupByChannel,
                                                   ctypes.byref(valArray),        # VALUES TO BE WRITTEN
                                                   ctypes.byref(sampsPerChanWritten),
                                                   null  )