Introduction

Early automation networks were called serial "buses." The term field- bus evolved from that thinking. Because each supplier's business model was different and focused on its own technology, one manufac- turer's bus turned out to be different from another manufacturer's bus. This caused confusion among end-users, so standards committees attempted to fix the problem. However, standardizing on one or two technologies just didn't work, so today there are many industrial com- munications technologies.

Ethernet, the clear winner in the information technology (IT) market, became the basis for the newest evolution of industrial automation net- works at least at the high-performance end. Lower-level networks con- nect sensors and actuators with different technologies.

Exchanging information through any communications method requires both the transmission end and receivers to share a common agreement on the type of electrical signals used, the organization of the data, and the processes used to assure successful and error-free transmission of the information. Formalization of that agreement is called the commu- nications protocol.

All network architectures are described by the International Standards Organization (ISO) standard Open Systems Interconnection (OSI) basic reference model: standard ISO/IEC 7498-1:1994.

Network Topology

The network architecture, or wiring plan, is called the "topology" designed to interconnect network devices.