In the past the information devices traditionally used in control rooms normally had only a single area of application. The instrument was normally electrically connected directly to some form of measuring device. In certain cases switches were used, which allowed the same instrument to be connected directly to several measuring devices. For example, this was usually the case for temperature readings where it was common to have alarms connected to all the measuring points. One then had to turn a special switch in order to be able to read the temperature at all the measuring points.
In the modern computerised process control system, information is collected from many measuring points in the system. Information is stored in computers and subjected to further processes, which then pass on the information to various receivers, such as the different information devices in the control room. The information devices connected to the computer system are normally the type that can simultaneously carry out several functions, that is, which can receive different forms of information, such as cathode ray tubes (CRTs). This form of flexible display could formally be described in Chapter 4, but for historical reasons we have retained it in the current chapter.