What is the major difference between DCS and PLC?

PLC controls the machine for which it is programmed, where as DCS controls as well as supervise number of Controlles such as PLCs in network.

The main difference between Programmable Logic Controller (PLC) and Distributed Control System (DCS) are:

The number of I/O in the system. DCS is dealing with very large no. of I/O with less cost. also DCS comes with it's SCADA always. if DCS stopped, all system is stopped. if the system has less No. of I/O, then PLC is better from cost view. Also SCADA is optional. if system is controlled by no of PLC. if PLC is down. others continue working Normally. Historical, main difference was that DCS deal with Analogue signals but PLC deal with Discrete digital system. Now DCS has Digital Modules and PLC has Analogue modules.

Besides, PLC has a processor and input and output cards (I/O card could be digital and/ or analog) Processor has the software - basis the input it receives from field devices it gives out the output commands to control devices of the field. DCS is a group of individual smart controllers (again having embedded software and connections with field devices) doing their specified operation (duty) but interconnected.

I have read other people answer. Thet are good answers

dcs is dealing with alarge no,of i/o and haveit; scada and have anetwork and if it stop system stop

plc is better  in less input output system for cost and have ascada option

if system have many plcs if one down system continou