plc is a programmable logic controller which can handle the binary input and output of the logic statement which are stored in its memory.its a digitally operating system which are logic stored in its memory by programming system.
DCS:-Distributed control system its control all types of process variable at a time through this we can control the whole plant,we can see the status of plant,and handling yhe unit.
A2)In PLC the Scan time is less.but the scan time of DCS is high.Because it Control the whole plant. So in safety loop we used PLC
A 3)First let's take a quick brief history about controllers:
1 - For analog control loops we have a stand alone PID controller.
2 - Then we get a networked PID controllers through a hub and HMI for configuring and monitoring.The PID itself contain a network port.SIEMENS introduced PID 353 & PROCIDIA software for it.
3 - Then we get DDC - DIRECT DIGITAL CONTROL, single CPU, I/O'S for analog instruments only & networked PC is an option.SIEMENS introduced APOGEE, FISHER introduced ROC.
4 - The next phase is DCS - Distributed Control System, and this is first introduced by HONEYWELL & YOKOGAWA.Ability for handling large scale of I/O'S, discrete and analog, complex control loops, PID algorithms calculations and executions.
5 - PLC is basically designed and produced for discrete on/off control, limited I/O'S.
but modern PLC with great progress in h/w technology introduced itself as a less expensive option for PID, with it's complicated calculations and algorithms.
6 - by the time with great achievements in H/W tech. and networking capabilities the single PLC can handle discrete and analog instruments with wide range.
7 - Now the man can make his own DCS with multible networked PLC'S, but it will need a great engineering and effortsto get near from DCS features and functionality.