National Electrical Manufacturers Association (NEMA) defines PLC as a digital electronic device that uses a programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting, and arithmetic operations to control machines and processes.  The PLC is an assembly of solid-state digital logic elements designed to make logical decisions and provide outputs.  PLC is a programmed interface between input sensors & output devices.

1. Distributed Control System

2. Control Systems Types
Control Systems Types
PLC : Introduced in the late 1960 to replace Relays
and Hard-wired Programming.
DCS : Introduced in the Mid-1970 to replace
pneumatic controls by using computers.
PC-Based Control : Introduced by the early 1980s to
avoid the proprietary PLC & DCS systems.
SCADA : Supervisory control & Data Acquisition
basically introduced for long distance material
transfer

3. DCS Evolution
DCS Evolution
 LOCAL CONTROL : Initially control was performed local to the
equipment control. The ADVANTAGE was low wiring
costs .DISADVANTAGES were - not much control, monitoring, alarming
& history.
 CENTRALISED CONTROL: With the advent of minicomputer, sensors
and actuators were into the Central Control (Computer).
 DISTRIBUTED CONTROL: With the advent of microcomputer,
Distributed control systems were installed in the plants near to the control
room via proprietary digital communications lines called as Data Hiway.
First DCS was developed by Honeywell,U.S in 1975.
The ADVANTAGES were greatly reduced wiring costs, much
more limited failure and less cost to add more points. The
DISADVANTAGES were that wiring costs were that wiring costs were
still significant and there was lack of interoperability among controllers of
various manufacturers due to the proprietary protocols. Hence the user
was locked into a single vendor.

5. DCS Software Architecture
DCS Software Architecture

6. DCS Evolution –Centralised Control
DCS Evolution –Centralised Control

7. DCS Evolution –Distributed Control
DCS Evolution –Distributed Control

8. DCS Evolution –Field bus Control
DCS Evolution –Field bus Control

9. DCS Evolution –Web Control Application
DCS Evolution –Web Control Application

10. Distributed Control System
Distributed Control System
 The importance of DCS systems to increase as global
competitive dynamics in food and beverage, specialty
metals, pulp and paper, pharmaceutical and fire
chemical processing.
 The DCS has networking capabilities which are useful
for business management.
 The DCS has capacity for processing large number of
I/O points.

11. CONVENTIONAL DCS
This is a pure “Process only” control system. Usually purchased from one
vendor. This DCS arranged into three categories:
 Small - Less than $ 100,000.
 Medium - Greater than $100,000 & Less than $500,000.
 Large - Greater than $500,000.
PLC Based DCS. This is a network of PLC’s used to perform the task of
conventional DCS and programmable functionality when required.
Hybrid DCS. Performs both process and sequential control.
Open DCS System.
This is Field-Bus Control. Advantages are lower wiring cost and less
failure, smaller expansion costs and multi vendor interoperability DCS and
PLC can be more closely and efficiently interconnected.
DCS Types
DCS Types

12. Popular DCS Systems Available
Popular DCS Systems Available
 Yokogawa – Centum Excel , CS 3000, CS 5000
 Fisher - Rosemant - Delta V
 Honeywell - TDC 3000, GUS
 ABB - Freelance 2000
 Moore - APACS
 Fox boro - I/A series