2 - Fundamentals of SCADA communications

Abstract

The chapter describes the essential components of the supervisory control and data acquisition (SCADA) communications. A SCADA system consists of a number of remote terminal units (RTUs) that collect field data and send that data back to a master station via a communications system. To enable all the nodes on the SCADA network to share information, the nodes must be connected by some transmission medium. Connection of the SCADA network to the local area network allows anyone within the company, with the right software and permission, to access the system. A more complex SCADA system essentially have five levels or hierarchies—namely, (1) field level instrumentation and control devices, (2) marshalling terminals and RTUs, (3) communication system, (4) the master station(s), and (5) the commercial data processing department computer system. The key features of SCADA software include user interfaces, graphics displays, alarms, trends, RTU (and PLC) interface, scalability, and access to data. Citect and WonderWare are the two open software packages available on the market for SCADA systems. Typical RTU hardware modules include a control processor and associated memory, analog inputs, analog outputs, counter inputs, digital inputs, digital outputs, communication interface(s), power supply, as well as an RTU rack and enclosure. The chapter also lists different communication philosophies—namely, the polled approach and the contention approach—used in the SCADA systems. The two basic standards—namely, RS-232 and RS-485—are described in the chapter. These standards form the key elements in transferring digital information between the RTUs (or operator terminals) and the modems, which convert the digital information to the appropriate analog form suitable for transmission over greater distances. The two protocols commonly used for SCADA applications are HDLC and MODBUS. The chapter lists the key components of the Modbus protocol and explains the seven different layers of the open systems interconnection (OSI) model.