Data Acquisition with Comedi

Most scientists and engineers love data. The more data you can feed them, the more they smile. In a laboratory setting, data means everything. In order to spot trends, analyze strange phenomena and draw final conclusions, a lab person needs to make sure they have acquired a complete set of data.

The concept of data acquisition therefore encompasses a broad scope of ideas. Most scientists and engineers, however, agree that data acquisition is the result of the measurement of some natural process. This could be as simple as the measurement of a temperature, for example, or as complex as the measurement of impurities in molten steel.

In the computing world, data acquisition most commonly is done by measuring a voltage. To do so, it is necessary to have some sensor or measurement device that is capable of producing a voltage that the computer can measure. It's also important to know the correlation between the measured parameter and the sensor's voltage output. Ideally, the correlation is linear, as in a temperature sensor where 1 measured degree Celsius corresponds to .1 volts.

Modern motherboards have onboard sensors, such as National Semiconductor's LM78, which assess the overall health of the system. These sensors measure such conditions as cooling fan speeds, processor core voltages and temperatures and hard drive rotation speeds. This information is acquired by the chip and can be reported to the processor through a serial bus. The open-source project lm_sensors (secure.netroedge.com/~lm78) provides the software for monitoring many aspects of motherboards.

Typical personal computers have no common interface for analog data acquisition, however. In order to make some external voltage measurement, a new interface is necessary. Data acquisition (DAQ) cards designed for either the PCI or ISA bus fill this gap. Many manufacturers make cards well suited for taking external measurements.

Most Linux users have experienced firsthand the complications surrounding having a single type of system (a printer, for example) and multiple models, makes, vendors and drivers. Any attempt at standardization becomes a large project. If the project receives enough support, it becomes the standard. Some vendors, like National Instruments, have released Linux drivers for their DAQ products, while others have not.

Comedi, or Control and Measurement Device Interface, is the standard suite of data acquisition drivers and libraries for Linux. Started in 1996 by David Schleef, Comedi attempts to support multiple vendors and models of cards through a common interface. In fact, the overall API design is a balance between modularity and complexity. Like other Linux driver projects, some of the work is the result of a lot of reading of hardware manuals, some is the result of reverse engineering and some is the result of manufacturers' assistance in providing Comedi support for their products.

Comedi is separated into two parts. Comedi itself is the package of drivers that are loaded into kernel space, and comedilib gives user-space access to those drivers. It is through comedilib that the transparency of Comedi shines. Programs using Comedi can be written in C or C++. Perl and Python bindings also exist for Comedi.

Comedi breaks things down into channels, subdevices and devices. A channel is the lowest level of measurement or control. Multiple channels of the same type are grouped into a common set, called a subdevice. Then multiple subdevices are grouped together into a complete device. When using Comedi, first a Comedi driver is loaded into memory. Then, /usr/sbin/comedi_config is run to bind the driver to a Comedi device, such as /dev/comedi0. Finally, functions are available in comedilib to access the various devices on the DAQ card.