The document discusses Remote Terminal Units (RTUs) which are electronic devices used in SCADA systems located at remote locations to facilitate communication. It describes how RTUs collect data from sensors and send it to a central station for monitoring. The document outlines the evolution of RTUs from hardwired devices to microprocessor-based units, and describes the typical components of an RTU including its CPU, memory, power supply, and input/output modules. It also discusses communication interfaces and protocols used by RTUs to send data to the central SCADA master station.

1. Remote Terminal Unit (RTU) in
SCADA

2. • Definition: Remote Terminal Unit (RTU) in
SCADA systems is a microprocessor-based
electronic device present at geographically
distributed remote locations in order to
facilitate communication of various devices
within the distributed SCADA system. It
consists of input-output hardware and
communication interface for remote sensing
and controlling of ongoing processes.

3. • The sole purpose of an RTU present at the
remote site of the SCADA system is to send all
the collected data and information to the
central station with the help of sensors,
monitors, production processes, etc. so that it
can be stored and monitored.
• The remote terminal unit is sometimes known
as the remote telemetry unit or remote
telecontrol unit.

4. Evolution of RTU
• In the initial phases, even before the arrival of the
SCADA system, RTUs were used for remote telemetry
purposes with integration to the central computer.
• So, at that time, RTUs were hardwired devices that
were electronically designed for fixed functions such as
responding to the serially received numeric codes.
• So, the early remote terminal units were functioned to
read and transmit the current input and status values
and to generate contact outputs according to the
control message.

5. • But at that time, the developed systems only supported
accurate monitoring and not fulfilled the purpose of
effective controlling. However, further innovations in that
field made possible the conveying of change of status of a
process to remote monitoring station as well as printing it
with reported date and time.
• Further, with the advent of microprocessor-based
technology, the manufacturers incorporated the
microprocessor and microcomputer logic within the RTUs.
And this further enhanced the flexibility of industrial control
systems by providing better operational characteristics and
performance.
• The reason behind this was that microprocessor chips
offered fast computing features and reduced the overall
cost, this improvised the performance of the system.
•

6. RTU Architecture
• The figure here shows the architectural
representation of the Remote Terminal Unit:

7. • The major components of RTU are CPU, Memory
(volatile and non-volatile), Power supply module with
battery, Real-time clock, Watchdog Timer,
Communication Interface (serial port or onboard
modem with I/O interface).
• In the figure, a hardware module is shown where the
I/O modules are placed within the RTU panel and
communication is established with the master station
via the communication port. The I/O modules are
connected to the central processor in order to
communicate with the master station.

8. CPU
• The inbuilt central processing unit nowadays uses
16-bit or 32-bits microprocessor with the
memory capacity of 256 kilobytes.
• It consists of 3 communication ports with
ethernet links and the real-time clock is used for
the overall controlling of the system.
• The use of watchdog timer provides real-time
checking of regular execution of processes. This
watchdog timer gets reset on regular basis and in
case the resetting is delayed then the timer flags
error.

9. • As it is clear from the figure that dual CPUs are
incorporated here with dual power supply.
And among the two CPUs, one serves the
purpose of primary and other as standby. So,
in case the primary one fails, the system is
automatically triggered to the standby CPU.

10. Power Supply Module
• For the uninterrupted operation, continuous supply of power
is required by the RTU, but the unit is separated with a large
distance from the point where supply is actually present.
• So, to facilitate uninterrupted operation in case of power
failure, an alternate power source with battery backup is
provided to the unit. For low power RTUs, solar panels are
mostly used while sometimes thermoelectric generators are
used according to the availability of gas pipelines.
• Generally, lead-acid or nickel-cadmium batteries are used.
• The typically expected value for operation is around 110/40
V AC ± 10% with a frequency of operation 50 Hz or 11/14/48
V DC ± 10%.

11. Analog Input Module
• The applied analog input is generally a variable voltage
or current, varying in direct proportion to the process
under measurement. The different analog inputs are 0-1
mA, 0-10 V, ± 1.5 V, 5 V, etc.
• The analog inputs must specifically include a multiplexer,
signal conditioner, sample and hold circuit, ADC and bus
interface.
• The features of analog input modules are:
 There are 8, 16 or 32 analog inputs,
 It usually offers 8 to 12 bits of resolution,
 Generally, 4 to 10 mA range,
 The input resistance for the input module is 140 Kilo
ohms to 1 Mega ohm,
 The offered conversion rate is about 10 microseconds to
30 milliseconds.

12. Analog Output Module
• These are used to deal with variable quantities such as
graphic recording instruments or strip charts. It basically
transforms the digital value received from the CPU into
analog form using DAC. Further, this analog
representation controls the actuators.
• The basic features of analog output modules are:
 It has 8, 16, 32 analog outputs,
 It offers a resolution of 8 or 12 bits,
 The provided conversion rate is between 10 microseconds
to 30 milliseconds,
 The output ranges between 4-10 milliampere or 0 to 10 V.

13. Status Inputs
• These represent status or alarm signals for the
ongoing process. Generally, remote terminal units
hold input status card that has two state real-
world information.
• It has isolated voltage or current source that is
used to have a check-on over the position of
remote contact (i.e., whether it is open or closed)
at the RTU site. Depending on the contact
position the status of various devices like
electrical breakers, alarm conditions, etc. can be
determined.

14. Digital Output Module
• The digital output module of the RTU usually
drives the output voltage at each respective
output channel such as high current capacity
relays so as to switch the power on and off.
• The approaches used for driving are as follows:
Triac Switching,
Read Relay Switching,
TTL voltage outputs.
•

15. Communication Interfaces
• A remote terminal unit can be interfaced to
multiple control stations and intelligent electronic
devices using different communication media like
RS485, RS232, ethernet, microwave and satellite,
etc. When a communication interface is maintained
between two ends then either side can initiate the
data transfer so as to have synchronized data
transfer with the least possible traffic issues.
• Also, if the multiple RTUs within the SCADA system
wants to share a common communication line then
using multi-drop scheme this can be done. As
unique addressing is provided to each unit and
response will be understood depending on their
polls and commands.

16. SCADA: Master Station
• Master Station is one of the three components of a
SCADA system.
• The term in a small system refers to a single computer
responsible for communicating with the field
equipments.
• In a large SCADA system, master station consists of
multiple servers, distributed software applications and
disaster recovery sites.
• Master station server and software communicate with,
apart from the field equipments like RTU/PLC, HMI
software running in the workstation of control room or
at other place.

17. Presenting The Information
• Information to the operating personnel is generally presented
graphically by the SCADA system.
• This graphic representation is known as a mimic diagram.
• This enables the operator to have a schematic representation
of the controlled plant.
• For example, it shows a graphic of a pump connected to a pipe
from which the operator can see both running of the pipe as
well as the volume of fluid being pumped through the pipe at
the given moment.
• The operator can switch off the pump depending on the need.
• The flow rate of fluid in the pipe decreases over real time as
shown by the HMI software.
• Process elements are usually represented by schematic
symbols and line graphics of mimic diagrams.
• The process equipment is represented in animated symbols by
digital photograph.

18. HMI Workstation
• There is a drawing program in a typical SCADA system with
HMI package.
• The operators or system maintenance people use this
drawing program to make changes in the way of
representation of these points in the interface.
• An on-screen traffic light representing actual traffic light
state in the field is an example of a simple representation.
• A multi-projector display to represent in a skyscraper
position of all the elevators is an example of a complex
representation.
• Platforms like LINUX were not much used earlier on account
of its dynamic development environment.
• However now all major operating systems like LINUX, UNIX
and OPENVMS are used for master station servers and also
HMI workstations.

19. SCADA: Communication
Infrastructure
• Communication infrastructure is one of the
components of a SCADA system.
• For meeting requirements of communication,
combined radio and direct serial or other modern
connections have been used by SCADA systems.
• Railways and Power Stations which are large sites very
often prefer use of Ethernet or IP over Synchronous
Optical Networking (SONET).
• The term Telemetry is used when a SCADA system
performs function of remote management and
monitoring.

20. SCADA Communication Protocols
• While the earlier low bandwidth protocols are still persisting, customers
now mostly want SCADA data to transmit through preset corporate
network and sharing of the network with other applications.
• Protocol designs in SCADA are compact and are so designed as to send
information to master station only in case the RTU is surveyed for
information by the master station.
• Modbus, RP570 and Conitel are the traditional vendor specific SCADA
communication protocols.
• Standard communication protocols include IEC61850, DNP3, Profibus
and IEC60870-5-101 or 104.
• All major SCADA vendors recognize these protocols.
• Communication protocols with extensions can operate in internet
protocol TCP/IP.
• Modbus TCP/IP has now become standard for lot of hardware
manufacturers and is a widely accepted communication protocol.
• Although for safety and security of SCADA system, it is advisable not to
connect it to internet and expose it to risk, Ethernet TCP/IP has found its
way into industrial automation breaking the barriers in majority of
SCADA/HMI markets.

21. SCADA Control Protocols
• Before evolution of standards for interoperability in
industry, a lot of automatic control devices like RTU
were already developed.
• This resulted in proliferation of control protocols by
various developers.
• Besides to keep their customers hooked, the vendors
made their own protocols.
• Some communication between devices was originally
not designed to be part of industrial network.
• Now there are solutions gaining wide acceptance for
interconnecting diverse hardware and software
permitting communication even between devices not
part of original network.