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1. Open-loop vs. closed-loop control: SCADA is predominantly open-loop; thatis, control decisions are made by a human (hence, the supervisory Controlelement. !y contrast, DCS is predominantly closed-loop, meanin" thatcontrol decisions # actions are mainly al"orithmic and e$ecute automaticallybased on the process state at any "iven point in time.

%. &on"-distance Communications: 'rue SCADA (i.e., not the actory )oorstu* that some people still call SCADA but that is no+ +idely no+n as S(manuacturin" e$ecution systems usually involves miles/ilometersbet+een the host and the 0eld elements ('2s, 3&Cs, etc.. ven a lar"eautomotive plant, or e$ample, havin" several acres under roo still doesn4thave the same operational environment or monitorin"/control issues as say,a cross-country pipeline or a po+er '#D net+or the latter bein" +hat 5 calltrue SCADA environments.

'here is also a third element that is a little more subtle yet increasin"lyambi"uous as both SCADA and DCS systems have been deployed +idely inboth areas. 'hat is, SCADA most oten deals predominantly +ith discreteprocesses +hereas DCS most oten involves batch or continuous processes.'hat is, traditional SCADA applications typically have operational dynamicsthat occur in seconds, minutes or hours as compared to DCS controlresponse dynamics, +hich are oten measured in sub-second increments;much too ast to +ait or a human decision and response. 6o+ever, lie theother t+o criteria, the ey +ord today is predominantly, because there areboth types o systems oten even sharin" a common host and/or 0eldelements installed in both environments.

'he prime purpose o a DCS is to automate a process.

'he prime purpose o a SCADA system is to provide a 7remote7 operatorvisibility o a process and permit that person to control the process.

'he term 7SCADA7 is used to cover a +ide ran"e o systems. 8hen reerrin"to a 7actory-automation7 SCADA, there is oten little di*erence bet+een DCSand SCADA. 8hen reerrin" to a system that interconnects remote sites(such as an electrical transmission substation net+or, a "as pipeline or a+ater distribution system then there are si"ni0cant di*erences.

'o di*erentiate them it may help to eep a sin"le simple a$iom in mind:

A SCADA system must continue to operate 7reasonably7 even i thecommunication system ails temporarily.

5n the 7local SCADA7 or actory automation end o the spectrum, 5 a"ree thatthere seems to be technical conver"ence bet+een these systems.

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9eep in mind that there are also some hybrid systems +here a SCADAcomponent interconnects multiple 7local DCS7 sites and +here SCADA isused to collect remote site data that is inte"rated into a DCS system.

Sometimes these systems can be hi"hly inte"rated, sometimes they can

appear to be separate interconnected systems.

DCS uses a common database or the control layer and the 65 layer so the65 is con0"ured usin" the actual control point entities. SCADA usesseparate database at each remote 3&C/'2 and the 65 paca"e, +ith someorm o cross-reerencin" the 3&C/'2 point +ith the entity con0"ured ordisplay on the 65.

A "ood e$ample o an inte"rated system is in the case o an oil re0nery 5+ored at usin" a DCS or the re0nery control system but uses a SCADAsystem or monitorin" and control o the output product pipeline supplyin"

multiple products to a maor city some 1

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sot+are and build its database rom the '2 or 3&C sot+are (or an 5/Odriver such as an O3C server. rom the operator=s point o vie+, they onlyhave a sin"le database, but the SCADA system is built rom the code up,+hilst the DCS is de0ned rom the top do+n.

&astly, 5=d lie to consider +hat happens i you pulled the po+er plu" on theserver - an event that 5 am sure +ould terriy many o you. 6o+ many opeople on the list have a system that could survive a total blac out o thecentral control room or similarly a complete loss o remote communicationsB

5 do not thin that DCS systems are desi"ned to survive this at all, but manySCADA systems, on the other hand, are. Certainly there are lar"e andcomple$ SCADA systems that reuire constant attention and a e+ minuteso dead time +ould rea people out. 5 e$pect that in most cases, +ithout anycentral control at all, the '2=s and 3&C=s +ould be capable o maintainin" astable system or a an e$tended period, +hile normality +as restored.

5t +ill be interestin" to note +hat systems can or cannot and i there areclear lines o division by application or si?e.