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List:       comedi
Subject:    RE: help: python wrapper for comedilib
From:       "JUSTIN ABRAHAM" <abrjus002 () uct ! ac ! za>
Date:       2007-09-19 9:40:51
Message-ID: 46F10AC4020000850003FB85 () SRVNOVNGW002 ! uct ! ac ! za
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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@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@uct.ac.za>
To: comedi@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@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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