For example, a analog output card can be used to generate voltages that are the inputs for the electronic drivers of the motors of a robot; these voltages can be interpreted as setpoints for the desired velocity of these motors, and six of them are taken together to steer one particular robot with six-degrees of freedom. Table of Contents 1. For these cards, the command functionality may be provided in software. Kcomedilib is a Linux kernel module distributed with the comedi package that provides the same interface as comedilib in kernel space, and suitable for use by real-time kernel modules. Writing a Comedi driver 6. Some of the other outputs of the same physical device can be used by another application program, for example to generate a sine wave that drives a vibration shaker. For example, many different interface cards use the same PCI driver chips, or use the parallel port as an intermediate means to connect to the hardware device.

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So, Comedi focuses only on the mechanism part of DAQ interfacing. The hardware of the card takes care of the sequencing and timing of the data acquisition as it proceeds. This type of cards are often called data acquisition or DAQ cards.

For example, one writes a driver for the parallel port, because one wants to use it to generate pulses that drive a stepper motor. Comedilib is a separately distributed package containing a user-space library that provides a developer-friendly interface to the Comedi devices.

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Finding the generic similarities and structure among different cards helps in developing device drivers faster and with better documentation. Comedi is a free software project that develops drivers, tools, and libraries for various forms of data acquisition: This is where the actual data acquisitions are taking place: Pulse -based signals counters, timers, encoders, etc.

The buffer sizes are user-adjustable.

The sequence fomedi of a number of identically repeated scans. At both sides, the software and the hardware need some finite initialization or settling time.

The sequence has a start and an end. A general DAQ device driver package 1. Comedilib is a user-space library that provides a developer-friendly interface to Comedi devices. The card’s electronics may also allow automatically sampling of a set of channels in a prescribed order.

Hardware Supported by Comedi

The project does not provide the policy parts, such as Graphical User Interfaces to program and display acquisitions, signal processing libraries, or control algorithms. Instructions for multiple acquisitions 4. Some devices also allow DMA: So, the hardware puts a lower boundary the scan interval on the minimum time needed to complete a full scan. Comedi provides the API to configure what triggering source one wants to comevi in liinux case.


Analog signals are a bit more complicated. One golden rule for the device driver writer is to separate mechanism and policy:. In general, not only the start of a conversion is triggered, but also the start of a scan and of a sequence. The Linux operating system has two levels ilnux require different programming approaches.

A device driver is a piece of software that interfaces a particular piece of hardware: Included in the Comedilib distribution is documentation, configuration and calibration utilities, and demonstration programs.

And because of the quite strict real-time requirements for a command acquisition, a real-time operating system should be used to translate the command specification into a correctly timed sequence of instructions.

The command functionality cannot be offered coedi DAQ cards that lack the hardware to autonomously sequence a series of scans. Writing Comedi programs 3. For example, a set of 16 identical analog inputs. Device files or device file system. Kcomedilib is a Linux kernel module distributed with Comedi that provides the same interface as Comedilib in kernel space, suitable for real-time tasks. However, reported bugs are usually quickly fixed.

It is the combination of three complementary software items: