Linux Comedi#

The Control and Measurement Device Interface.

Intro#

The Comedi project develops open-source drivers, tools, and libraries for data acquisition.

Comedi is a collection of drivers for a variety of common data acquisition plug-in boards. The drivers are implemented as a core Linux kernel module providing common functionality and individual low-level driver modules.

Comedilib is a user-space library that provides a developer-friendly interface to Comedi devices. Included in the Comedilib distribution is documentation, configuration and calibration utilities, and demonstration programs.

Kcomedilib is a Linux kernel module (distributed with Comedi) that provides the same interface as Comedilib in kernel space, suitable for real-time tasks. It is effectively a “kernel library” for using Comedi from real-time tasks.

Handbook#

Setup#

http://www.comedi.org/doc/install.html

uname -a
Linux lablab 4.2.0-1-amd64 #1 SMP Debian 4.2.3-2 (2015-10-14) x86_64 GNU/Linux

aptitude install libcomedi0 python-comedilib

/etc/modprobe.d/comedi.conf:

options comedi comedi_num_legacy_minors=4

if l /dev/comedi* shows no devices, issue:

for i in `seq 0 15`; do mknod -m 666 /dev/comedi$i c 98 $i; done;

::: modprobe comedi_test

dmesg:

[5527065.958917] comedi: module is from the staging directory, the quality is unknown, you have been warned.
[5527065.959659] comedi: version 0.7.76 - http://www.comedi.org
[5527125.524709] comedi_test: module is from the staging directory, the quality is unknown, you have been warned.

lsmod | grep comedi
comedi_test            16384  0
comedi                 65536  1 comedi_test

cat /proc/comedi
comedi version 0.7.76
format string: "%2d: %-20s %-20s %4d", i, driver_name, board_name, n_subdevices
no devices
comedi_test:
 comedi_test

Configure device#

comedi_config /dev/comedi0 comedi_test

dmesg:

[5528564.009252] comedi comedi0: comedi_test: 1000000 microvolt, 100000 microsecond waveform attached

cat /proc/comedi
comedi version 0.7.76
format string: "%2d: %-20s %-20s %4d", i, driver_name, board_name, n_subdevices
 0: comedi_test          comedi_test             2
comedi_test:
 comedi_test

Operation#

Without access to appropriate hardware, let’s use channels from the comedi_test device. http://lxr.free-electrons.com/source/drivers/staging/comedi/drivers/comedi_test.c

comedi_test#

… generates fake waveforms

http://www.comedi.org/doc/lowleveldrivers.html#idm140692783667536

Description:

This driver is mainly for testing purposes, but can also be used to
generate sample waveforms on systems that don't have data acquisition
hardware.

Configuration options:
  [0] - Amplitude in microvolts for fake waveforms (default 1 volt)
  [1] - Period in microseconds for fake waveforms (default 0.1 sec)

Generates a sawtooth wave on channel 0, square wave on channel 1, additional
waveforms could be added to other channels (currently they return flatline
zero volts).

$ comedi_test -t info
I: Comedi version: 0.7.76
I: Comedilib version: unknown =)
I: driver name: comedi_test
I: device name: comedi_test
I:
I: subdevice 0
I: testing info...
rev 1
I: subdevice type: 1 (analog input)
  number of channels: 8
  max data value: 65535
  ranges:
    all chans: [-10,10] [-5,5]
I:
I: subdevice 1
I: testing info...
rev 1
I: subdevice type: 2 (analog output)
  number of channels: 8
  max data value: 65535
  ranges:
    all chans: [-10,10] [-5,5]

$ comedi_board_info
overall info:
  version code: 0x00074c
  driver name: comedi_test
  board name: comedi_test
  number of subdevices: 2
subdevice 0:
  type: 1 (analog input)
  flags: 0x00119000
  number of channels: 8
  max data value: 65535
  ranges:
    all chans: [-10 V,10 V] [-5 V,5 V]
  command:
    start: now
    scan_begin: timer
    convert: now|timer
    scan_end: count
    stop: none|count
  command structure filled with probe_cmd_generic_timed for 1 channels:
    start: now 0
    scan_begin: timer 1000
    convert: now 0
    scan_end: count 1
    stop: count 2
subdevice 1:
  type: 2 (analog output)
  flags: 0x00120000
  number of channels: 8
  max data value: 65535
  ranges:
    all chans: [-10 V,10 V] [-5 V,5 V]
  command:
    not supported

Crashes machine:

$ comedi_test -t read_select
I: Comedi version: 0.7.76
I: Comedilib version: unknown =)
I: driver name: comedi_test
I: device name: comedi_test
I:
I: subdevice 0
I: testing read_select...

packet_write_wait: Connection to xx.xx.xx.xx: Broken pipe

Try Python

aptitude install cython python-numpy libcomedi-dev

python -m venv .venv
source .venv/bin/activate
#pip install numpy
#pip install pycomedi

# https://github.com/wking/pycomedi
wget https://github.com/wking/pycomedi/archive/master.zip