Mechanism. The mechanism part of the device interface is a faithful representation of the bare functionality of the device, independent of what part of the functionality an application will use.

Policy. Once a device driver offers a software interface to the mechanism of the device, an application writer can use this mechanism interface to use the device in one particular fashion. That is, some of the data stuctures offered by the mechanism are interpreted in specific physical units, or some of them are taken together because this composition is relevant for the application. 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. 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.

API: devices that offer similar functionalities, should have the same software interface, and their differences should be coped with by parameterizing the interfaces, not by changing the interface for each new device in the family. However, the DAQ manufacturers have never been able (or willing) to come up with such a standardization effort themselves.

Architectural structure: many electronic interfaces have more than one layer of functionality between the hardware and the operating system, and the device driver code should reflect this fact. 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. Hence, “lower-level” device drivers for these PCI chips and parallel ports allow for an increased modularity and re-useability of the software. Finding the generic similarities and structure among different cards helps in developing device drivers faster and with better documentation.

Kernel space vs. User space. The Linux operating system has two levels that require different programming approaches. Only privileged processes can run in the kernel, where they have access to all hardware and to all kernel data structures. Normal application programs can run their processes only in user space, where these processes are shielded from each other, and from direct access to hardware and to critical data of the operating system; these user space programs execute much of the operating system's functionality through system calls.

Device drivers typically must access specific addresses on the bus, and hence must (at least partially) run in kernel space. Normal users program against the API of the Comedilib user-space library. Comedilib then handles the necessary communication with the Comedi modules running in kernel-space.

Device files or device file system. Users who write an application for a particular device, must link their application to that device's device driver. Part of this device driver, however, runs in kernel space, and the user application in user space. So, the operating system provides an interface between both. In Linux or Unix, these interfaces are in the form of “files” in the /dev directory. Each device supported in the kernel may be representated as such a user space device file, and its functionality can may be accessed by classical Unix file I/O: open, close, read, write, ioctl, and mmap.

/proc interface. Linux (and some other UNIX operating systems) offer a file-like interface to attached devices (and other OS-related information) via the /proc directories. These “files” do not really exist, but it gives a familiar interface to users, with which they can inspect the current status of each device.

Direct Memory Access (DMA) vs. Programmed Input/Output (PIO). Almost all devices can be interfaced in PIO mode: the processor is responsible for directly accessing the bus addresses allocated to the device whenever it needs to read or write data. Some devices also allow DMA: the device and the memory “talk” to each other directly, without needing the processor. DMA is a feature of the bus, not of the operating system (which, of course, has to support its processes to use the feature).

Real-time vs. non real-time. If the device is to be used in a RTLinux/GPL or RTAI application, there are a few extra requirements, because not all system calls are available in the kernel of the real-time operating systems RTLinux/GPL or RTAI. The APIs of RTAI and RTLinux/Free differ in different ways, so the Comedi developers have spent a lot of efforts to make generic wrappers to the required RTOS primitives: timers, memory allocation, registration of interrupt handlers, etc.

Digital signals are conceptually quite simple, and don't need much configuration: the number of channels, their addresses on the bus, and their input or output direction.

Analog signals are a bit more complicated. Typically, an analog acquisition channel can be programmed to generate or read a voltage between a lower and an upper threshold (e.g., -10V and +10V). The card's electronics may also allow automatically sampling of a set of channels in a prescribed order.

Pulse-based signals (counters, timers, encoders, etc.) are conceptually only a bit more complex than digital inputs and outputs, in that they only add some timing specifications to the signal. Comedi has still only a limited number of drivers for this kind of signals, although most of the necessary API and support functionality is available.

Channel: the lowest-level hardware component, that represents the properties of one single data channel; for example, an analog input, or a digital output.

Subdevice: a set of functionally identical channels. For example, a set of 16 identical analog inputs.

Device: a set of subdevices that are physically implemented on the same interface card; in other words, the interface card itself. For example, the National Instruments 6024E device has a subdevice with 16 analog input channels, another subdevice with two analog output channels, and a third subdevice with eight digital inputs/outputs.

The sequence has a start and an end. At both sides, the software and the hardware need some finite initialization or settling time.

The sequence consists of a number of identically repeated scans. This is where the actual data acquisitions are taking place: data is read from the card, or written to it. Each scan also has a begin, an end, and a finite setup time. Possibly, there is also a settling time (“scan delay”) at the end of a scan.

So, the hardware puts a lower boundary (the scan interval) on the minimum time needed to complete a full scan.

Each scan contains one or more conversions on particular channels, i.e., the AD/DA converter is activated on each of the programmed channels, and produces a sample, again in a finite conversion time, starting from the moment in time called the sample time in Figure 1 (sometimes also called the “timestamp”), and caused by a triggering event, called convert.

In addition, some hardware has limits on the minimum conversion interval it can achieve, i.e., the minimum time it needs between subsequent conversions. For example, some A/D hardware must multiplex the conversions from different input channels onto one single A/D converter. Thus the conversions are done serially in time (as shown in Figure 1). Other cards have the hardware to do two or more acquisitions in parallel, and can perform all the conversions in a scan simultaneously. The begin of each conversion is “triggered” by some internally or externally generated pulse, e.g., a timer.

Single acquisition: Comedi has function calls to synchronously perform one single data acquisition on a specified channel: comedi_data_read, comedi_data_read_delayed, comedi_data_write, comedi_dio_read, comedi_dio_write. In addition, the lower-level comedi_do_insn function can be used to perform an acquisition.

“Synchronous” means that the calling process blocks until the data acquisition has finished.

Mutiple synchronous acquisition: The comedi_data_read_n function performs (possibly multiple) data acquisitions on a specified channel, in a synchronous way. So, the function call blocks until the whole acquisition has finished. The precise timing between the acquisitions is not hardware controlled.

In addition, comedi_do_insnlist() executes a list of instructions in one single (blocking, synchronous) call, such that the overhead involved in configuring each individual acquisition is reduced.

Command: a command is sequence of scans, for which conditions have been specified that determine when the acquisition will start and stop, and when each conversion in each scan should occur. A comedi_command function call sets up the aynchronous data acquisition: as soon as the command information has been filled in, the comedi_command function call returns. The hardware of the card takes care of the sequencing and timing of the data acquisition as it proceeds.