13 - Fieldbus and SCADA communications systems

Abstract

The chapter discusses the main features of three different types of Profibus—DP, PA, and fieldbus message specification (FMS)—and Foundations fieldbus (FF). The chapter also presents the main differences between FF and Profibus. The fieldbus message specification (FMS) is used for general data acquisition systems. DP is used when fast communications is needed, and PA is used in areas when intrinsically safe devices and intrinsically safe communications are required. It is very easy to connect Profibus FMS, DP, and PA versions together on the same system, as the main difference between them is only the physical layer. The physical layer in Profibus specifies the type of Profibus transmission medium. This allows the deployment of lower cost devices in most of the plant (FMS) and fast devices (DP) in parts of the plant that requires the speed. The concept behind Foundation fieldbus (FF) is to preserve the desirable features of the present 4–20 mA standard while taking advantage of the new digital technologies. The FF consists of three parts that correspond to OSI layers 1, 2, 7, and 8. Layer 8 of the OSI model corresponds to the “user” layer. The EIA-485 voltage standard is defined for the FMS and DP versions of Profibus. The data link layer is defined by Profibus as the fieldbus data link layer (FDL). The medium access control (MAC) part of the FDL defines when a station may transmit data. The MAC ensures that only one station transmits data at any given time. The communications stack as defined by the FF corresponds to open systems interconnection (OSI) layers two and seven, the data link and applications layers. The data link layer controls access to the bus through a centralized bus scheduler called the link active scheduler. The application layer in the FF specification is divided into two sublayers—the Foundation fieldbus access sublayer and the Foundation fieldbus messaging specification.