Real-Time Concepts

Real-Time

In addition to delivering correct results, real-time systems are required to meet time constraints. In general, the requirement for real-time can be subdivided into the requirement for timeliness, simultaneity and determinism. Timeliness requires the timely availability of a result. For example, to meet the required cycle time of a PLC, the program must be processed in time. The requirement for simultaneity means that when several real-time critical tasks are processed in parallel, all time specifications are met. For example, a PLC must control several axes simultaneously when controlling machines. Determinism describes the fact that the state of a system is defined at any point in time. For example, communication in real-time Ethernet is deterministic, but not in standard Ethernet. In real-time, a violation of the requirement for timeliness leads to a total failure of the system.

Hard and Soft Real-Time

In the literature, a distinction is sometimes made between hard and soft real-time. Hard real-time corresponds to the above definition, whereas soft real-time relaxes the requirements for timeliness. Missing a desirable deadline does not lead to a total failure, but only to a reduction in the quality of the system.