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kishor_1003
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Join Date: May 2008
Location: jharsuguda ,orissa .India
Posts: 3

Difference between PLC AND DCS ????

05/04/2008 12:08 PM

WHAT IS MAIN DIFFERENCE BETWEEN PLC AND DCS

WHY WE CON'T USE PLC INSTEAD OF DCS??????

AND VISE VERSA ..

PLEASE REPLY ME

REGARDS

KISHOR KUMAR VALLURU

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 kishor kumar Instrumentation engineer power sector
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mangeshkakade
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Join Date: Apr 2008
Posts: 17
#1

Re: Difference between PLC AND DCS ????

05/04/2008 3:57 PM

Hi Kishor,

DCS is mainly used to handle anolog IO's and PLC's for digitaly . But as far as concern about the functionlity of DCs and PLC both are same.

Major worthy advantage of DCS is having centralised database server. Using same data base server you can edit in logic as well as on graphics screen. This feature we can not achieved in PLC.

IO handling capacity is large and so speed also in DCS.
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kishor_1003
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Join Date: May 2008
Location: jharsuguda ,orissa .India
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#4
In reply to #1

Re: Difference between PLC AND DCS ????

05/09/2008 2:06 PM

Hi

Dear friend ,

Thanks for your reply .

what r u doing now .

regarding

kishor kuma valluru

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instrumentationengineer
Active Contributor

Join Date: May 2008
Posts: 14
#7
In reply to #1

Re: Difference between PLC AND DCS ????

06/06/2008 6:16 AM

To tell you now a days there is no much difference btwn DCS & PLC. PLC also handles AI,AO.
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Macsimus
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Join Date: Jan 2008
Location: Surat
Posts: 43
Good Answers: 2
#2

Re: Difference between PLC AND DCS ????

05/05/2008 4:23 AM

1)DCS: Earlier DCS performed the control functions of analog panel instruments. It uses multi-variable proportinal Integral Derivative (PID) control to perfom any control. PLC: PLC dealth with contacts I/P and O/P by performing boolean logic caliculations. 2) DCS: Offers extra controllers, networks & I/Os. Which are "Built-in", also offers extra diagnostic features. PLC: All functions are defined by the user by using a "Written Logic" 3) DCS: It is provided with HMI (Interface console). PLC: We have to purchase HMI/SCADA systems seperately 4) DCS: We can access data (Process variable) of any particular instrument by using the Tag number. PLC: We have to remember the address or location of the hardware of that instrument. 5) DCS: Offers fractional second. However some DCS's provide interrrupt/Event triggered logic for high speed applications. PLC: PLCs are fast. They run Input-Compute-Output Cycle in milliseconds. 6) DCS: Multitasking operation is possible. You can download and run other application aside from real time control. Still do fraction of second control. PLC: Multitasking is possible but costly. 7) DCS: Data processing errors are not there in the central database of DCS PLC: There will be Data procesing error while (register exchange mechanism) getting data from and to various PLCs. 8) DCS: Bottle necking and timing problems are less. PLC: If so mnay PLCs try to communicate with too many PLCs result in bottle necking and timing problems. These are the major differences and however some more minor differences are there. We have to decide based on our application and type od I/p, O/P's. If you require more info search on the net or just leave a message here in this forum. Good Luck.

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kishor_1003
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Join Date: May 2008
Location: jharsuguda ,orissa .India
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#5
In reply to #2

Re: Difference between PLC AND DCS ????

05/09/2008 2:12 PM

Thank you my dear friend

regards '

kishor kumar valluru

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vicini
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Join Date: Dec 2007
Posts: 588
Good Answers: 13
#3

Re: Difference between PLC AND DCS ????

05/06/2008 10:03 PM

today, they are interchangeable. I call PLC's true DCS because logic and loop controls can be distributed into multiple PLC units, then a SCADA/HMI system links them together.

DCS are for snobs in the industry.
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Anonymous Poster
#6

Re: Difference between PLC AND DCS ????

05/15/2008 1:12 AM

PLC - They are standalone equipment , predominantly used in safeguarding (ESD) logic. It is wrong to assume that they deal only digital inputs/outputs. They also deal with anlalogue inputs/outputs. Imagine an equipment need to be tripped on high high pressure. I use a pressure switch and set for the required value. Unfortunately the switch sensing diaphram failed. This will not be known at all till disaster of the equipment.(Unrevealed failure). Modern trend is to see that all failures are revealed and avoid all disasters totally. Instead of a switch, use a transmitter to measure the pressure and send this to analogue input card of the PLC and compare this with the set reference and initiate an useful output.(Revealed failure). Failure of the transmitter is known by itsdiagnostics. This is the trend nowadays.

DCS - Plant Engineer is complete in charge of any one plant. Plant manager is in charge of all plants. This is called hierarchial system. The same is applied to plant insrumentation. An RTU/PLC (Plant Engineer) accepts field inputs and sends corrected field outputs using a predetermined strategy, downloaded in its dedicated operating memory.All its input/output status are all stored in its addrressable memory.(say A0 to A100)

In this way other RTUs/PLCs store their status values in (A101 to A200........A901 to A100)

DCS (Plant manager) reads only the status values from A0 to A1000, through serial communication and generates a graphical presentation to the control room operator.Time is very efficiently managed. This is the benefit of networking technologies. Imagine a pressure switch in one plant actuates a shutdown valve in yet another plant. In conventional system, it involves a lot of wiring. In this system, Plant1 pressure switch status changed. PLC1 reflects this to DCS by A0. DCS transfers to A1000 and PLC10 reads this and sends to shutdown valve,located in Plant10. Hierachial system remains the same, Plant1 and Plant 10 are located far away,linked through fibre optic link.
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