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SCADA
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Comparison of the terms SCADA,DCS, PLC and smart instrumentA. Scada SystemA SCADA (or supervisory control and dataacquisition) system means a system consistingof a number of remote terminal units (or RTUs)collecting field data connected back to a masterstation via a communications system. Themaster station displays the acquired data andalso allows the operator to perform remotecontrol tasks. The accurate and timely data(normally real-time) allows for optimization ofthe operation of the plant and process. A furtherbenefit is more efficient, reliable and mostimportantly, safer operations. This all results ina lower cost of operation compared to earliernon-automated systems.There is a fair degree of confusion between thedefinition of SCADA systems and processcontrol system. SCADA has the connotation ofremote or distant operation. The inevitablequestion is how far ‘remote’ is – typically thismeans over a distance such that the distancebetween the controlling location and thecontrolled location is such that direct-wirecontrol is impractical (i.e. a communication linkis a critical component of the system).A successful SCADA installation depends onutilizing proven and reliable technology, withadequate and comprehensive training of allpersonnel in the operation of the system. Thereis a history of unsuccessful SCADA systems –contributing factors to these systems includesinadequate integration of the variouscomponents of the system, unnecessarycomplexity in the system, unreliable hardwareand unproven software. Today hardwarereliability is less of a problem, but theincreasing software complexity is producingnew challenges. It should be noted in passingthat many operators judge a SCADA system notonly by the smooth performance of the RTUs,communication links and the master station (allfalling under the umbrella of SCADA system) but
also the field devices (both transducers andcontrol devices). The field devices however falloutside the scope of SCADA in this manual andwill not be discussed further. A diagram of atypical SCADA system is given opposite.On a more complex SCADA system there areessentially five levels or hierarchies:• Field level instrumentation and control devices• Marshalling terminals and RTUs• Communications system• The master station(s)• The commercial data processing departmentcomputer systemThe RTU provides an interface to the fieldanalog and digital signals situated at eachremote site.The communications system provides thepathway for communications between themaster station and the remote sites. Thiscommunication system can be radio, telephoneline, microwave and possibly even satellite.Specific protocols and error detectionphilosophies are used for efficient and optimumtransfer of data. The master station (andsubmasters) gather data from the various RTUsand generally provide an operator interface fordisplay of information and control of the remotesites. In large telemetry systems, submastersites gather information from remote sites andact as a relay back to the control master station.SCADA technology has existed since the earlysixties and there are now two other competingapproaches possible – distributed controlsystem (DCS) and programmable logic controller(PLC). In addition there has been a growingtrend to use smart instruments as a keycomponent in all these systems. Of course, inthe real world, the designer will mix and matchthe four approaches to produce an effectivesystem matching his/her application.B. Distributed control system (DCS)In a DCS, the data acquisition and controlfunctions are performed by a number ofdistributed microprocessor-based units situated
near to the devices being controlled or theinstrument from which data is being gathered.DCS systems have evolved into systemsproviding very sophisticated analog (e.g. loop)control capability. A closely integrated set ofoperator interfaces (or man machine interfaces)is provided to allow for easy systemconfigurations and operator control. The datahighway is normally capable of fairly highspeeds (typically 1 Mbps up to 10 Mbps).C. Programmable logic controller (PLC)Since the late 1970s, PLCs have replacedhardwired relays with a combination of ladder–logic software and solid state electronic inputand output modules. They are often used in theimplementation of a SCADA RTU as they offer astandard hardware solution, which is veryeconomically priced.Another device that should be mentioned forcompleteness is the smart instrument whichboth PLCs and DCS systems can interface to.D. Smart instrumentAlthough this term is sometimes misused, ittypically means an intelligent (microprocessorbased) digital measuring sensor (such as a flowmeter) with digital data communicationsprovided to some diagnostic panel or computerbased system.This tutorial will henceforth consider DCS, PLCand smart instruments as variations
