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.
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
it ios easy to work on dcs than plc as it is having many
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
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)
In closed loop control there are several “blocks” used to
show the interactions between the process and the control
system. These blocks are referred to as the “process” and
the “controller.” Describe the relationship between these
two blocks when creating a well tuned control loop.