Why the use of DCS rather than PLCs moreover in industries(Oil and Gas company)?

            DCS                                VS                                        PLC

1.   Single vendor                                                                    Mutiple

2.    Small Area /LAN                                                            Large Area/WAN

3.    Many I/O                                                                         Less I/O

4.  Use for Monitoring                                                       Used for Controlling

PLC mainly used for controlling fast, repetitive actions like packaging and conveying system where mostly dealin with Digital IO. DCS concentrates on Analog function with loop control which is not suitable in conventional PLCs. Nowdays PLC and DCS are merging and with some modifications both can be used. Still scantime, memory limit, ladder logic etc affect PLC use in highly complicated process, where DCS is preferred. 

Usage of more than one PLC's is called DCS.

PLC  Programmable Logic Controller. Its main purpose is to replace the relay logic controls which is "On" or "Off". And DCS "Distributed Control Systems" its emphasis is Fast analog handling because of communications through Bus systems, networking and etc.