SCADA Communication Techniques and Standards Dennis J.Gaushell'and Wayne R.Block2 oCADA)yem Change of signal amplitude RTUs are interconnected (amplitude modulation or consist generally of a master sta- AM) tion (master)and a number of geo- to the master station Change of signal frequency graphically dispersed remote through a variety of (frequency shift keying or terminal units (RTUs).RTUs are interconnected to the master via a communication channels FSK) Change of signal phase (phase variety of communication chan- shift keying or PSK). nels,including radio links,leased A demodulator detects the lines,and fiberoptics (see Figure 1).Due to the limited change in the signal and outputs the transmitted informa- availability and high cost of communication channels. tion at the receiving end of the communication channel. the design of master and RTUs is profoundly affected. Information transmitted in only one direction is referred Communication channels limit the speed at which to as a simplex channel.Information transmitted in both data acquisition and control can be performed,thus directions,but not simultaneously,is called a half-duplex affecting the master user interface and applications soft- channel.Information transmitted in both directions, ware design.In addition,noise occurring randomly on simultaneously,requires a full-duplex channel. the communication channel requires additional master and RTU hardware and software design to guarantee Multiplexing that information is transferred correctly from master to It may be desirable to transmit many pieces of informa RTU,and from RTU to master. tion at the various transmitter locations to a receiver. Configurations of communication systems are dictat- One possible approach is to provide a separate dc-loop ed by: communication channel for each piece of information; ■Number of RTUs however,this would be a very costly approach due to Number of points at RTUs and required update the cost of communication channels,as well as multiple rates transmitters and receivers. ■Location of RTUs Communication facilities available Communication equipment and techniques avail- SCADA able. MASTER STATION This article provides an overview of SCADA communi- cation techniques (modulation,multiplexing,message formats,information transfer)and an update on relevant RTU RTU standards activities. RTU Modulation RTU In order to convey information from one point to anoth- RTU er via a communication channel,it is necessary to trans- RTU mit a signal (or change)at one point that is recognized at the other point.This may be accomplished by several methods: Radlo Links Leased Line ●●●Fiber Optic Westin Engineering Figure 1.Typical SCADA system communications Los Angeles Department of Water and Powe architecture 155N0895-015693,33.00019931EEE Juy199345