DCS, PLCs, and RTUs share basic hardware features but have differing functionalities. DCS are used for complex, integrated control of entire processes and can implement advanced control strategies. PLCs are best for discrete and simple loop control. RTUs interface field devices to SCADA/DCS systems, with strong communication capabilities and data logging for remote locations. Key differences are in communications, data handling, and control capabilities. DCS are most suitable for large, integrated processes while PLCs and RTUs are used for discrete and remote control applications.

1. DCS, PLC, and RTU
Common features and diffrences
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2. Overview
 DCS,PLCs and RTUs shares same basic features with
somewhat differing functionality.
 In this discussion ,we shall focus on hardware point of view,
indicating common features and differences
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3. Definitions
 Distributed Control Systems
“A design approach in which factory or machine control is
divided into several sub-systems, each managed
independently by a unique programmable controller or other
control system, yet all interconnected to form a single entity.
Individual subsystems may be interconnected via
communications networks.”
o Patrick-Turner Industrial Automation Dictionary, 1996
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4. Definitions (cont.)
 Programmable Controller
“A solid state industrial control device, member of the
computer family, designed specifically to be programmed to
perform relay logic functions, timing, counting, arithmetic,
and data manipulation, all for the purpose of implementing
control over industrial machines and processes. The
programmable controller consists of a central processor,
memory system, input/output system, and power supply, all
of which are designed to operate in harsh industrial
environments.”
o Patrick-Turner Industrial Automation Dictionary, 1996
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5. Definitions (cont.)
 Remote terminal unit
“A remote terminal unit (RTU) is a microprocessor-controlled
electronic device that interfaces objects in the physical world to
a distributed control system or SCADA (supervisory control
and data acquisition) system by transmitting telemetry data to a
master system, and by using messages from the master
supervisory system to control connected objects.”
o Wikipidea
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6. Definitions (cont.)
 SCADA
“Supervisory Control – “A control scheme whereby a computer
or controller monitors and intermittently downloads programs,
sets sub-goals, or adjusts control parameters of a lower level
automatic controller.”
Data Acquisition – “The process of collecting data from the
system through some manual or automatic means for the
purpose of producing printed reports for operating,
supervisory, maintenance, or accounting disciplines.”
o Patrick-Turner Industrial Automation Dictionary, 1996
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7. Basic Hardware Platforms
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Control Unit
Output
Circuits
Work Memory
Input
Circuits
Program Memory
Power Supply
Communication
ports

8. Basic Hardware Platforms
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Micro
Compact Modular
Control
Unit Output
Circuits
Work
Memory
Input
Circuits
Program
Memory
Power Supply
Comm.
ports

9. Redundancy
1/12/2022
Southern Cairo Electricity Distribution Company, Cairo,
Egypt
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 Simplex: Conventional PLCs ,RTUs, simple DCS
 Doublex
 CPU Hot standby : Siemens S7:400H, Schneider : Quantum
140 CPU 6xx,Allen Bradley : Redundancy module
 IOs :Allen Bradley: 1715 redundant Ios
 Triplex :Triconics, Hima
Triconics

10. Redundancy
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 Hot standby structure example

11. Redundancy
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 Redundant IOs example

12. Environment resistance
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 Normal hardware : working temperature 0-55 degree
 Harsh environment :
 Part number ends with H or C for Schneider, Siplus series for
Siemens.
 Working temperature : -40°C to +70°C.
 Coating provides protection against most, aggressive gas like
H2S, conductive dust.
 IP65 : suitable for some DCS and PLC distributed IOs and
RTUs installed in harsh environment.

13. PLC vs RTU
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 The major differences between PLCs and RTUs are in :
 remote communications and data handling capabilities.
 Low Power Operation
 Most RTUs provides a variety of wired and wireless
communication support.
 Most RTUs provide significant data logging capabilities so
that data is retained during communications outages or for
reporting purposes.
 Most RTUs consumes low power compared to PLCs

14. PLC vs RTU (cont.)
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 RTUs offer sophisticated RTU to SDADA/DCS,RTU
to IED and RTU to RTU communications:
 Multiple SCADA protocols, proprietary and standard.
 Multiple IED protocols for instrumentation, field controllers,
meters, sensor networks, analyzers, etc.
 Advanced peer to peer communications with store and forward
as well as dynamic message routing capability.

15. PLC vs RTU (cont.)
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 Standard polling
The master station continuously requests the real-time data
values.
 Exception reporting (RBE)
The RTU is polled but only reports values that have changed
since the prior poll
 Push Communications
The RTU initiates messages on and event or time basis.
 Peer to peer communications
RTUs can communicate with the master station and also with
each other (peer to peer) if there is a communications path

16. PLC vs RTU (cont.)
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 Store and forward communication
 Dynamic message routing

17. DCS vs PLC
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 Direct implementation of simple PID,Cascade ,
Ratio,Feedforward,etc.
 Possible implementation of advanced loop control :
Multivariable loops,adaptive control,model
predictive control,artificial intelligence based
control (ex:fuzzy and neural based control)
 Handling of large number of loops

18. DCS vs PLC
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 Fixed Loop sampling time
 Easy changing of loop configuration by plant trained
person
 Common database between Programming and HMI