Control engineering is
completely concerned with data processing. Data from the plant is collected and passed to the
controller. The controller decides on what action is necessary and then sends data to the
actuators to move the process in the desired manner. In order for this data to be passed
around there needs to be some sort of communication network linking the instruments to the
controller and the controller to the actuators.
In the early days of automatic control, all controllers were based on pneumatics (air pressure) and the communications ran through air pipes roughly 5mm in diameter. These pipes carried data in an analogue form: as an air pressure between 3 and 15 psig. The 3 psig was used as a zero to allow leaks in the pipes to be detected. These early systems required a very clean compressed air supply (called instrument air) free of dust, oil and water. Connecting instruments meant running new air pipes and was very expensive. Most plants had several small, local, control rooms to reduce the length of the pipes.
With the move towards electronic controllers came the switch to analogue electrical communications. Voltage signals are not particularly appropriate for communication since the voltage drop in the transmission line is a function of the line length (i.e. you would get different measurements if you changed the length of the wires connecting to the instrument). As a result, current signals became the standard for analogue electrical communication with the most common standard being 4-20mA. Again the zero was offset at 4mA to allow line breaks or instrument failures to be detected.
In the past 10 years there has been a move towards digital communication through a plant-wide 'data-bus'. A data-bus is simply a group of wires through which digital signals can be passed. The key advantage of the bus approach is that the same group of wires are used for all the instruments and all the actuators - it is no longer necessary to run individual cables between the elements. This works because all communication through the bus has an 'address' attached to it. Elements on the bus only respond to communications which are addressed to them. The digital bus makes it really each to attach new instruments to the system. Digital communication also allows much more flexibility in the type of communication e.g. controllers can instruct instruments to recalibrate themselves. Another advantage to digital communications is that they are highly resistant to transmission noise (random signals introduced as the message travels down the wires), although this isn't usually a big problem with the sorts of shielded wires used in chemical plants.