RE: help: python wrapper for comedilib

Dear Paul,

Thanks a lot for your reply and the code snippet. I'd really appreciate it if you could forward me the full 'cmd.py'. I have modified the test_'comedi.py' script and populated the cmd struct. It passes all the 'comedi_command_test(dev,cmd)' tests but it fails when i try execute the command ('c.comedi_command(dev,cmd)'). Your code will be very helpful.

Justin

>>> Paul Finnigan <paul_at_pfinnigan.plus.Com> 09/18/07 8:33 PM >>>
Justin

I am reading your posting on my mobile phone not ideal for looking at code. I hope this is readable.

You could be correct about your command construction. I had similar problems when I first looked at comedilib, which I use exclusivley with python as I cannot write C very well (it's about as good as my English).

When I first started out I did hit a few problems with Luc Lefebvre's scripts. It was difficult to come to terms with much of the documentation. While they emulate the functionality they are not copies of the C programs. I am attempting to rewrite all the demo programs in python, using the C program as the basis. 

I have cmd.py rewritten and a few others, but the documentation and comments need sorting out. I even have a common.py under development to make sure they are as similar as is possible! In cmd.c a routine called prepare_cmd_lib is used to populate the command structure. I have emulated this under python using:

def prepare_cmd_lib(dev,subdevice,cmd):
    """
    This prepares a command in a pretty generic way.  We ask the
    library to create a stock command that supports periodic
    sampling of data, then modify the parts we want
    """
    # This comedilib function will get us a generic timed
    # command for a particular board.  If it returns -1,
    # that's bad.
    ret = c.comedi_get_cmd_generic_timed(dev,subdevice,cmd,int(1e9/com.freq));
    if ret<0:
        print "comedi_get_cmd_generic_timed failed"
	return ret

    # Modify parts of the command 
    cmd.chanlist=chanl
    cmd.chanlist_len= com.n_chan
    cmd.scan_end_arg=com.n_chan
    if cmd.stop_src==c.TRIG_COUNT:
        cmd.stop_arg=com.n_scan
    return 0

# The com.freq comes from my common module 
# and is the Frequency (the default is 1000). 

# cmd in the arguments is just the cmd structure you created 
# using comedi_cmd_struct. If you want the whole of my cmd.py 
# code then just ask. Everyone else will have to wait until I 
# have had time to review my documentation.

-----Original Message-----
From: "JUSTIN ABRAHAM" <abrjus002_at_uct.ac.za>
To: comedi_at_comedi.org
Sent: 17/09/07 12:59
Subject: help: python wrapper for comedilib

i'm programming using python with the python wrapper for comedilib. i've modified the test script that comes with the python wrapper. but it doesn't seem to be reading anything in...?? i've run some of the demo c-code provided with the card and its working fine. this script for some reason is not reading anything in...

##################################################
#A test-application to demonstrate using the Comedilib API
# and streaming acquisition in particular, from Python.
#This script is completely untested!
# author bryan.cole_at_teraview.co.uk

#set the paths so python can find the comedi module
import sys, os, string
import comedi as c

#open a comedi device
dev=c.comedi_open('/dev/comedi0')
if not dev: raise "Error openning Comedi device"

#get a file-descriptor for use later
fd = c.comedi_fileno(dev)
BUFSZ = 10000

nscans=1000 #specify total number of scans

#three lists containing the chans, gains and referencing
#the lists must all have the same length
chans=[0]
gains=[1]
aref =[c.AREF_GROUND]

nchans = len(chans) #number of channels

#wrappers include a "chanlist" object (just an Unsigned Int array) for holding the chanlist information
mylist = c.chanlist(nchans) #create a chanlist of length nchans

#now pack the channel, gain and reference information into the chanlist object
#N.B. the CR_PACK and other comedi macros are now python functions
for index in range(nchans):
    mylist[index]=c.cr_pack(chans[index], gains[index], aref[index])


#construct a comedi command manually
cmd = c.comedi_cmd_struct()
cmd.subdev = 0
cmd.flags = 0
cmd.start_src = c.TRIG_NOW
cmd.start_arg = 0
cmd.scan_begin_src = c.TRIG_FOLLOW
cmd.scan_begin_arg = 0
cmd.convert_src = c.TRIG_TIMER
cmd.convert_arg = 1
cmd.scan_end_src = c.TRIG_COUNT
cmd.scan_end_arg = nchans
cmd.stop_src = c.TRIG_COUNT
cmd.stop_arg = nscans
cmd.chanlist = mylist
cmd.chanlist_len = nchans

#test our comedi command a few times.
ret = c.comedi_command_test(dev,cmd)
print "first cmd test returns ", ret
ret = c.comedi_command_test(dev,cmd)
print "second test returns ", ret
ret = c.comedi_command_test(dev,cmd)
if not ret: raise "Error testing comedi command"

#execute the command!
ret = c.comedi_command(dev,cmd)
if not ret: raise "Error testing comedi command"

#I think this will work but it's completely untested!
datalist=[]
data =  os.read(fd,BUFSZ)
print len(data)
##################################################

the final 'print len(data)' outputs a 0, which means it hasn't read in anything and i don't know why??? maybe one of my parameters of cmd is wrong?? any ideas??


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Received on 2007-09-19Z08:40:51