This document discusses the differences between programmable logic controllers (PLCs) and distributed control systems (DCSs) in order to help determine which type of system is best suited for different applications. It outlines seven key questions to consider regarding the manufacturing process, product value, system requirements, operator needs, engineering expectations, and whether the application is hybrid in nature. PLCs are generally better for discrete and simple batch control, while DCSs are more suitable for complex batch processes and facilities that require flexibility and recipe management where system availability is critical. A hybrid system may be needed if an application requires both fast logic control and regulatory analog loop control.

1. Decision :
PLC or DCS
Vivek Koshti

2. Selecting the technology and the right supplier can
help your company :
• To respond quickly to changing market conditions in
a way that creates a sustainable competitive
advantages.
• Minimize Total cost of ownership over the life of plant
• Create a system which is easily maintainable /
upgradeable for the long term.
• Achieve its future goals & vision.
• Selecting an automation system based on a review
of available products is the typical course of action
for someone in the market for a new automation
system.

3. The hybrid application typically requires a process
Control system that can deliver both PLC & DCS
Capabilities.
A successful evaluation should start by developing a
Clear picture of the requirements of your application
& the needs of your engineering, maintenance and
Operations personnel.
This presentation outlines the seven key questions
that will lead you to make the right choice.

4. Typical DCS System Architecture

5. • In DCS Architecture diagram, redundancy is often
Employed for I/O Controller, Networks, HMI servers.
• Since redundancy adds cost & sometimes complexity
DCS users must carefully evaluate their need for
redundancy in order to achieve their required system
availability and to prevent unplanned downtime.
• DCS is typically better suited for complex batch
manufacturing facilities that require a high level of
flexibility and recipe management.
• Plant operator can’t see the product which is within
the vessel and may be hazardous in nature. There
is usually a large amount of simple as well as complex
analog control i.e. PID or loop control, although the
response time is not exceptionally fast (100 ms or < )

6. Typical PLC System Architecture

7. • PLC Architecture diagram illustrate one of its most
common applications, the control of discrete devices
such as motor & drives.
• To effectively control motors and drives requires that
the controller be able to execute at high speeds
(10-20 msec scan rate) and that the electrical
technician responsible for maintaining it be able to
read and troubleshoot the configuration in a language
that he is familiar with (relay ladder logic).
• From a technology point of view, one can see that
PLC & DCS are not that different. We must look
beyond technology that is built into these system by the
supplier, so that we can better understand the “sweet-
spots” where each is best applied.

8. At first glance, system architectures look very similar.
Both system share following Components :
• Field Devices
• I / O Modules
• Controllers
• Human Machine Interfaces (HMIs)
• Engineering
• Supervisory Control
• System Integration
The differences become more apparent when you
consider the nature & requirement of an application.

9. The seven questions to ask yourself
before Choosing a system :
PLC or DCS

10. Q. 1 – What are you manufacturing & how?
• No. of Products manufactured : Single / Multiple
• Recipe parameter : Constant or Variable
• Procedure : Single or Different
• Equipment Utilization : Fixed or Flexible
• Frequency of changes to formula & Recipe :
Never or Often

11. PLC DCS
Manufacturing or assembly of Involves the combination & / or
specific items Transformation of raw material
Product is visible & it moves Impossible to visually see Product
through the process as it moves thought the process
High speed logic control Regulatory / Analog loop control
Simple Batch Control Complex Batch Control

12. Q. 2 – What is the value of the product being
manufactured and the cost of downtime?
• If the value of each independent product being
manufactured is low, & / or downtime results in
lost of production, but with additional cost or
damage to the process, the PLC is likely choice.
• If the value of the batch is high, either in raw
material cost or market value, & the downtime
not only results is lost production but potentially
dangerous and damaging conditions, the
selection should be DCS.

13. PLC DCS
Value of individual component being The value of batch can be very
Manufactured is relatively low High.
Downtime mainly results in lost Downtime not only results in lost
Of production Production but can result dangerous
Or hazardous conditions.
Downtime does not typically Downtime can result in process
Damage the process equipment Equipment damage.
Return to steady state production Return to steady state production
after an outage is short & relatively after an unplanned outage ca be
straightforward long, expensive & difficult.

14. Q. 3 – What do you view as the “heart” of the
system?
• PLC is the heart of the factory Automation
control system, which contains all of the logic
To move the product through the assembly line.
HMI is often an on machine panel or a PC
based station.
• Operational information resulting from data
analysis is also a requirement for factory
automation applications – driving demand for a
more sophisticated HMI

15. • The environment is process automation can be
volatile & dangerous.
• In this scenario, the HMI is a central control
room console that provides the only complete
“window” into the process, enabling operator
to monitor & control the process which are
occurring inside pipes & vessels located through
out the plant.
PLC DCS
Typically, heart of the system is the Typically, hear t of the system is
Controller. The HMI

16. Q. 4 – What does the operator need to be
successful ?
• In PLC environment, status information &
exception alarming help to keep the operator
aware of what is happening in the process.
• The DCS plant require an operator to make
decision and continuously interact with the
process to keep it running.
• In fact, operators process knowledge is often
critical to operational excellence & keeping the
process running optimally

17. • Operators has to keep an eye during product grade
changes & when adjusting the process changes to
overall production environment.
• The operator will change the set-points, open/ close
valves or to make manual addition to move a batch to
the next stage of production.
• Within the HMI, faceplates & analog trends provide a
critical view into what is really happening in the
production process, while alarm management system
focuses operators attention on areas where he must
intervene to keep the process running within its target
performance.
• In the event of HMI failure, the plant could be forced to
shutdown in order to keep the people & equipment
safe.

18. PLC DCS
The operator primary role is Operator interaction is required to
to handle exceptions Keep the process in its target
Performance range
Status information (ON/OFF, RUN/ Faceplates & analog trends are
STOP) is critical information for the Critical to see what is happening to
operator The process.
Exception based alarming is key Alarm management is key to safe
Information for the operator Operation of the process
Manufacturing might be able to run Failure of the HMI could force the
“light-outs” shutdown of the process

19. Q. 5 – What system performance is required ?
• The speed of logic execution is a key differentiator.
• Fast scan rates are necessary to be able to
effectively control the operations involving
motion control, high-speed interlocking, control
of motors and drives.
• The DCS does not have to be that quick.
• Increasingly, PLC is capable of delivering simple to
complex PID control, but DCS is the choice for
applications with advanced analog control, cascade,
Feed forward, ratio, model predictive control etc…

20. PLC DCS
Fast logic scan rate(10ms) is Control Loops require deterministic
required to perform motion control Scan execution at speed 100-500ms
Redundancy may not be cost System redundancy is often
justified required
System can be taken offline to Online configuration changes often
Make configuration changes required
Analog Control – Simple PID ONLY Analog Control – Simple to Advanced
PID upto Advanced Process Control
Diagnostic to tell you when something Asset Management alerts you to what
Is broken Might break before it does

21. Q. 6 – What degree of customization is required ?
• PLC delivers a “toolkit” of functions & elemental
building blocks that can be custom developed and
chained together to address the requirements of an
application.
• Powerful Programming languages are typically available
to facilitate the creation of custom code from scratch.
• DCS Pre-engineered solutions consists of standards,
templates & extensive libraries.
• The highest priority of DCS is to deliver reliability &
availability, which often results in a design which trades
unlimited functionality for repeatability and
dependability.

22. PLC DCS
High Level Programming Languages Custom Logic created from
Are available for creating custom existing function blocks
logic
Customized routines usually Many algorithms are complex (PID)
required & do not vary among applications
Standard libraries considered Function Blocks and faceplates
As nice features Are expected
Provisions must be available to Entire system is expected to
Integrate functions / products into an Function as a complete solution
Integrated architecture

23. Q. 7 – What are your engineering expectations ?
• Factory automation engineers want customizable
control platform, which offer the individual components
that can be quickly programmed together to accomplish
the task at hand.
• PLC engineer demands flexibility & open
architecture.
• DCS engineers focus on upfront design is a key to
minimizing costs, compressing the project schedule &
creating an application that can be maintained by plant
personnel over the long term.

24. • The process engineers controlling entire plants
with a DCS require more intuitive programming
platforms, which utilize pre-defined & pre-tested
functions to save time & drive repeatability.
• Think about in this way –
The PLC is controlling a machine.
The DCS is controlling the plant.

25. PLC DCS
Program / configure individual Upfront design of complete system
Components, integrate later Before implementation begins.
Desire customizable platforms to Looking for significant “out-of-box”
build upon Functionality
System designed to be flexible System designed to make it “easy”
To engineer to process application
Use of Ladder logic to configure Use of FBD for configuration of
applications applications

26. Do you have a Hybrid Application?
What is a “ Hybrid ”?
• The marriage of the discrete functions, which PLC
handles so simply and economically, with the
sophisticated analog continuous control capabilities
of the DCS.
• Defined based on the industries in which system
works & serves like oil & gas, refinery, pharmaceutical
food & beverages
• The architectural marriage of the PLC simplicity and
cost with the sophisticated operator displays, alarm
management & easy configuration capabilities of DCS.

27. How to select a Process Control System for
Hybrid Application ?
• Controller : can execute fast scan logic (10-20 msce), such that
required for motor control and slow scan logic (100-500 msec)
such as required for analog control.
• Engineering Configuration Language: Provides ladder logic,
Functional Block Diagram and a powerful programming language
for creation of custom logic from scratch.
• Flexible Modular Redundancy: Offers the option of tailoring the
level of system redundancy to deliver the required system
availability by balancing up-front cost versus the cost of
unplanned downtime.
• Modular batch from simple to complex – Provides modular
batch capability to cost effective address of simple to complex
batch application.

28. How to select a Process Control System for
Hybrid Application ?
• Alarm Management : Offers power alarm management tools to
help operators respond effectively to plant upset conditions
• System Diagnostics & Asset Management: Provides both a
rich set of built in system diagnostics, as well as asset
management of all critical assets (transmitters, valve Positioner,
motors, drives, MCC’s, heat exchanger etc.) in the plant
• Scalable Platform: Hardware, software & licensing supports
smooth & economical scale up from small all-in-one system
(10’s of I/O’s) up to a large client / server system (1000’s of I/O’s)

29. In Nut-shell
Characteristics PLC DCS
Market Introduction 1960 1975
Replacement of Electromagnetic Replays Pneumatic & SLC
Application Automotive Refinery
Type of Control Discrete Regulatory
Engineering Mindset Programming Configuration
Size Compact Large
System Open Closed
Operator Interface Simple Graphics Sophisticated Graphics
Upfront Cost $$ $$$$
Operation interaction Exception Basis Man in the Loop

30. References :
[1] Bob Nelson, “Making the Right Choice for
Process Industry”, Control Engineering Asia,
October-2008.
[2] “Process Automation”, SIEMENS Energy &
Automation.
[3] “Hybrid Control Identity Crisis: What's in a
Name?”, In-Tech Sept. 2007.
[4] Rich Merritt, “Does DCS have A Future?
If so, is it HMI/SCADA”, Control- Dec. 2008.