The aim of this package is to make Comedi devices available as the interfaces defined in the Orocos Device Interface. It gives a short introduction to configuring Comedi devices under Linux and then demonstrates how the Orocos Comedi classes can be used to access these devices.

For each device supported by comedi, you need to

The Orocos Debian repository contains all the above modules and tools in the libcomedi-dev and comedi-modules packages, but the same procedure can be used if you compiled Comedi from source.

2.3. Looking up the sub-device info

The last step you need to take is listing the subdevices and their number by using comedi_info -f /dev/comediN. For Our NI-6527 card, this would be:

  comedi_info -f /dev/comedi0
	

Now note that the digital Input subdevice is number 0. You will need this number to finally setup your Orocos Comedi device classes. Note each subdevice you will use.

	Note
	Subdevices for incremental encoders are called counters.

The last step is creating the necessary objects. First create a OCL::ComediDevice object for each configured card in your system:

  int minor = 0;
  ComediDevice* cdev0 = new ComediDevice( minor );
      

Where minor is the same number as your /dev/comedi0 device descriptor used earlier.

Next for each sub device on your card, create a ComediSubDeviceX object. For example, for digital inputs you write:

  int subdev = 0;
  DigitalInInterface* cdev0_DIn = new ComediSubDeviceDIn( cdev0, 0 );
      

The first argument is the ComediDevice created earlier, the second argument is the subdevice. An error will be printed if the subdevice is not a Digital input.

As you can see, the OCL::ComediSubDeviceDIn is an implementation of the RTT::DigitalInInterface class of the Real-Time Toolkit. These interfaces can be name served such that other parts of the application can pick the device up just by using its name. You need to provide a name argument upon construction of the ComediSubDevice object:

  int subdev = 0;
  DigitalInInterface* cdev0_DIn = new ComediSubDeviceDIn( cdev0, "Digital1", 0 );
      
  // Somewhere else in the application, for example in a component's
  // configureHook():
  bool MyComponent::configureHook() {

     // ... din is a DigitalInInterface* 
     din = DigitalInInterface::getObject("Digital1");
     if ( din == 0 ) {
        log(Error) << "Device 'Digital1' not found !" << endlog();
        return false;
     }
     return true;
  } 

Even better, you could make 'Digital1' an Orocos property such that you can reconfigure for using another subdevice.

This name serving allows that multiple components use the same subdevice such that you can optimally use all the channels of your devices.

Orocos provides two ways of writing or reading a channel: directly using the device interface class or using a helper object. Both are equivalent from a performance point of view, as the helper objects are inlined.