ABSTRACT
Digital communications have become, and will continue to be in the future, an important component of control system technology. This chapter explores the fundamentals of a digital communications system with particular concentration on the computer networking techniques used in large control systems. Open systems Interconnection is to provide a mechanism which will allow a variety of different systems from different manufacturers to be interconnected with one another for the purpose of communications. The topology of a local-area network (LAN) refers to the way in which the systems or devices are interconnected by the LAN cable. There are three topologies: star topology communications system, bus topology communications system, and ring topology communications system. The token bus manufacturing automation protocol relies on the existence of a special frame, called the token, which is transmitted from one system to another in a round-robin fashion. Fieldbus is a communications system standard for applications at the lowest levels in the manufacturing systems hierarchy.
