programming logic control & digital logic control

PLC is Progamable Logic Controller & DCS is Distributed
control system. In brief PLC using for perticular mechine or
production unit. DCS usinging for monitoring & controlling
of entire plant, including PLC.

DCS is a number of PLC's Distributed over a Control Network
over the entire plant.

Plc is basically oriented for one job only,suppose if plc 
fails then whole system will shut down,but in distributed 
control system,you have a redundant plc for each plc so 
that if primary system fails then its standby plc takes 
over the charge.

PLC is used Commonly with On\Off (Digital) Control and may
be expanded with Analog I\O Modules for Analog Control and
used for a Unique Task .
DCS is used Commonly with Process Control Systems as it  is
Combined of Many control Cards, PLCs and Can Mange a Large
Scale Systems .

plc-programmable logical control,in plc program executes 
searliy but in dcs all programs runs parallay.
dcs includes controller,communication and scada but plc is 
a controller communication and scada softwares are to be 
provided.
it ios easy to work on dcs than plc as it is having many 
functions.

plc is a progrramable logic controllers & dcs is a 
distributed control systems. plc is a on/off controller.it 
controls the only machine on or off. dcs controls the many  
plc at the time.

1.PLC is for the system which need quick response.Thats why 
PLC is for control of a single system with multiple control 
loop,i/p and o/p
2.The response time of PLC is in millisec which is much 
less than the DCS response time

The difference between DCS and PLC is that, PLC is 
primarily a small scale control system which only executes 
only digital function. When we say DCS, It is a networks of 
PLC's,it is a large scale control system which is 
distributed all over the plant,and is located in a control 
room.

In DCS, field I/O points can be away from the control 
processor,(where as program/logic resides)its a integarted 
system includes other PLC's and Stand-alone controllers for 
application specific mechine or process and having large 
memory for trending & Historical alarms.PLC must be a fast 
scan cycle application oriented, simple system without 
human meachine interface(such as process mimics/graphics)