Titles in the series Practical Cleanrooms: Technologies and Facilities (David Conway) Practical Data Acquisition for Instrumentation and Control Systems (John Park, Steve Mackay) Practical Data Communications for Instrumentation and Control (John Park, Steve Mackay, Edwin Wright) Practical Digital Signal Processing for Engineers and Technicians (Edmund Lai) Practical Electrical Network Automation and Communication Systems (Cobus Strauss) Practical Embedded Controllers (John Park) Practical Fiber Optics (David Bailey, Edwin Wright) Practical Industrial Data Networks: Design, Installation and Troubleshooting (Steve Mackay, Edwin Wright, John Park, Deon Reynders) Practical Industrial Safety, Risk Assessment and Shutdown Systems (Dave Macdonald) Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems (Gordon Clarke, Deon Reynders) Practical Radio Engineering and Telemetry for Industry (David Bailey) Practical SCADA for Industry (David Bailey, Edwin Wright) Practical TCPIP and Ethernet Networking (Deon Reynders, Edwin Wright) Practical Variable Speed Drives and Power Electronics (Malcolm Barnes)Practical SCADA for Industry David Bailey BEng, Bailey and Associates, Perth, Australia MIPENZ, BSc(Hons), BSc(Elec Eng), IDC Technologies, Perth, AustraliaNewnes An imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington, MA 01803 First published 2003 Copyright 2003, IDC Technologies. All rights reserved No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holders written permission to reproduce any part of this publication should be addressed to the publisher British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 07506 58053 Typeset and Edited by Vivek Mehra, Mumbai, India (vivekmehratatanova.com) Printed and bound in Great Britain For information on all Newnes publications, visit our website at www.newnespress.comContents Preface xiii 1 Background to SCADA 1 1.1 Introduction and brief history of SCADA 1 1.2 Fundamental principles of modern SCADA systems 2 1.3 SCADA hardware 4 1.4 SCADA software 5 1.5 Landlines for SCADA 6 1.6 SCADA and local area networks 7 1.7 Modem use in SCADA systems 7 1.8 Computer sites and troubleshooting 8 1.9 System implementation 9 2 SCADA systems, hardware and firmware 11 2.1 Introduction 11 2.2 Comparison of the terms SCADA, DCS, PLC and smart instrument 12 2.2.1 SCADA system 12 2.2.2 Distributed control system (DCS) 15 2.2.3 Programmable logic controller (PLC) 15 2.2.4 Smart instrument 16 2.2.5 Considerations and benefits of SCADA system 17 2.3 Remote terminal units 17 2.3.1 Control processor (or CPU) 19 2.3.2 Analog input modules 19 2.3.3 Typical analog input modules 26 2.3.4 Analog outputs 27 2.3.5 Digital inputs 28 2.3.6 Counter or accumulator digital inputs 29 2.3.7 Digital output module 31 2.3.8 Mixed analog and digital modules 33 2.3.9 Communication interfaces 33 2.3.10 Power supply module for RTU 33 2.3.11 RTU environmental enclosures 33 2.3.12 Testing and maintenance 34 2.3.13 Typical requirements for an RTU system 35 2.4 Application programs 36 2.5 PLCs used as RTUs 36 2.5.1 PLC software 37 2.5.2 Basic rules of ladderlogic 38 2.5.3 The different ladderlogic instructions 40 2.6 The master station 46 2.6.1 Master station software 48vi Contents 2.6.2 System SCADA software 48 2.6.3 Local area networks 48 2.6.4 Ethernet 49 2.6.5 Token ring LANs 51 2.6.6 Token bus network 52 2.7 System reliability and availability 52 2.7.1 Redundant master station configuration 52 2.8 Communication architectures and philosophies 54 2.8.1 Communication architectures 54 2.8.2 Communication philosophies 56 2.8.3 Polled (or master slave) 56 2.8.4 CSMACD system (peertopeer) 59 2.9 Typical considerations in configuration of a master station 61 3 SCADA systems software and protocols 64 3.1 Introduction 64 3.2 The components of a SCADA system 64 3.2.1 SCADA key features 65 3.3 The SCADA software package 67 3.3.1 Redundancy 70 3.3.2 System response time 72 3.3.3 Expandability of the system 72 3.4 Specialized SCADA protocols 72 3.4.1 Introduction to protocols 73 3.4.2 Information transfer 74 3.4.3 High level data link control (HDLC) protocol 78 3.4.4 The CSMACD protocol format 80 3.4.5 Standards activities 81 3.5 Error detection 82 3.5.1 Causes of errors 83 3.5.2 Feedback error control 84 3.6 Distributed network protocol 87 3.6.1 Introduction 87 3.6.2 Interoperability 87 3.6.3 Open standard 87 3.6.4 IEC and IEEE 88 3.6.5 SCADA 88 3.6.6 Development 88 3.6.7 Physical layer 88 3.6.8 Physical topologies 88 3.6.9 Modes 89 3.6.10 Datalink layer 92 3.6.11 Transport layer (pseudotransport) 96 3.6.12 Application layer 97Contents vii 3.6.13 Conclusion 97 3.7 New technologies in SCADA systems 97 3.7.1 Rapid improvement in LAN technology for master stations 97 3.7.2 Man machine interface 97 3.7.3 Remote terminal units 98 3.7.4 Communications 98 3.8 The twelve golden rules 98 4 Landlines 100 4.1 Introduction 100 4.2 Background to cables 100 4.3 Definition of interference and noise on cables 101 4.4 Sources of interference and noise on cables 102 4.4.1 Electrostatic coupling 103 4.4.2 Magnetic coupling 104 4.4.3 Impedance coupling 105 4.5 Practical methods of reducing noise and interference on cables 107 4.5.1 Shielding and twisting wires 107 4.5.2 Cable spacing 108 4.5.3 Tray spacing 110 4.5.4 Earthing and grounding requirements 111 4.5.5 Specific areas to focus on 111 4.6 Types of cables 112 4.6.1 General cable characteristics 112 4.6.2 Two wire open lines 114 4.6.3 Twisted pair cables 114 4.6.4 Coaxial cables 116 4.6.5 Fiber optics 116 4.6.6 Theory of operation 116 4.6.7 Modes of propagation 118 4.6.8 Specification of cables 120 4.6.9 Joining cables 120 4.6.10 Limitations of cables 121 4.7 Privately owned cables 121 4.7.1 Telephone quality cables 121 4.7.2 Data quality twisted pair cables 122 4.7.3 Local area networks (LANs) 122 4.7.4 Multiplexers (bandwidth managers) 122 4.7.5 Assessment of existing copper cables 125 4.8 Public network provided services 125 4.9 Switched telephone lines 126 4.9.1 General 126 4.9.2 Technical details 126 4.9.3 DC pulses 128viii Contents 4.9.4 Dual tone multifrequency — DTMF 128 4.10 Analog tie lines 128 4.10.1 Introduction 128 4.10.2 Four wire EM tie lines 129 4.10.3 Two wire signaling tie line 130 4.10.4 Four wire direct tie lines 131 4.10.5 Two wire direct tie lines 131 4.11 Analog data services 131 4.11.1 Introduction 132 4.11.2 Pointtopoint configuration 132 4.11.3 Pointtomultipoint 132 4.11.4 Digital multipoint 133 4.11.5 Switched network DATEL service 134 4.11.6 Dedicated line DATEL service 134 4.11.7 Additional information 135 4.12 Digital data services 135 4.12.1 General 135 4.12.2 Service details 135 4.13 Packet switched services 136 4.13.1 Introduction 136 4.13.2 X.25 service 138 4.13.3 X.28 services 138 4.13.4 X.32 services 139 4.13.5 Frame relay 139 4.14 ISDN 139 4.15 ATM 141 5 Local area network systems 142 5.1 Introduction 142 5.2 Network topologies 143 5.2.1 Bus topology 143 5.2.2 Bus topology advantages 144 5.2.3 Bus topology disadvantages 144 5.2.4 Star topology 144 5.2.5 Ring topology 145 5.3 Media access methods 146 5.3.1 Contention systems 146 5.3.2 Token passing 147 5.4 IEEE 802.3 Ethernet 147 5.4.1 Ethernet types 148 5.4.2 10Base5 systems 148 5.4.3 10Base2 systems 150 5.4.4 10BaseT 151 5.4.5 10BaseF 153Contents ix 5.4.6 10Broad36 153 5.4.7 1Base5 153 5.4.8 Collisions 153 5.5 MAC frame format 154 5.6 Highspeed Ethernet systems 155 5.6.1 Cabling limitations 155 5.7 100BaseT (100BaseTX, T4, FX, T2) 156 5.7.1 Fast Ethernet overview 156 5.7.2 100BaseTX and FX 157 5.7.3 100BASET4 157 5.7.4 100BaseT2 158 5.7.5 100BaseT hubs 158 5.7.6 100BaseT adapters 159 5.8 Fast Ethernet design considerations 159 5.8.1 UTP Cabling distances 100BaseTXT4 159 5.8.2 Fiber optic cable distances 100BaseFX 159 5.8.3 100BaseT repeater rules 160 5.9 Gigabit Ethernet 1000BaseT 160 5.9.1 Gigabit Ethernet summary 160 5.9.2 Gigabit Ethernet MAC layer 161 5.9.3 1000BaseSX for horizontal fiber 162 5.9.4 1000BaseLX for vertical backbone cabling 163 5.9.5 1000BaseCX for copper cabling 163 5.9.6 1000BaseT for category 5 UTP 163 5.9.7 Gigabit Ethernet fullduplex repeaters 163 5.10 Network interconnection components 164 5.10.1 Repeaters 164 5.10.2 Bridges 165 5.10.3 Router 165 5.10.4 Gateways 166 5.10.5 Hubs 166 5.10.6 Switches 167 5.11 TCPIP protocols 169 5.11.1 The TCPIP protocol structure 170 5.11.2 Routing in an Internet 170 5.11.3 Transmission control protocol (TCP) 171 5.12 SCADA and the Internet 172 5.12.1 Use of the Internet for SCADA systems 173 5.12.2 Thin client solutions 173 5.12.3 Security concerns 174 5.12.4 Other issues 175 5.12.5 Conclusion 175x Contents 6 Modems 176 6.1 Introduction 176 6.2 Review of the modem 176 6.2.1 Synchronous or asynchronous 178 6.2.2 Modes of operation 179 6.2.3 Components of a modem 180 6.2.4 Modem receiver 180 6.2.5 Modem transmitter 181 6.3 The RS232RS422RS485 interface standards 182 6.3.1 The RS232C interface standard for serial data communication 182 6.3.2 Electrical signal characteristics 183 6.3.3 Interface mechanical characteristics 185 6.3.4 Functional description of the interchange circuits 185 6.3.5 The sequence of asynchronous operation of the RS232 interface 186 6.3.6 Synchronous communications 187 6.3.7 Disadvantages of the RS232 standard 188 6.3.8 The RS422 interface standard for serial data communications 188 6.3.9 The RS485 interface standard for serial data communications 190 6.4 Flow control 191 6.5 Modulation techniques 191 6.5.1 Amplitude modulation (or amplitude shift keying) 192 6.5.2 Frequency modulation (or frequency shift keying — FSK) 192 6.5.3 Phase modulation (or phase shift keying (PSK)) 192 6.5.4 Quadrature amplitude modulation (or QAM) 193 6.5.5 Trellis coding 194 6.5.6 DFM (direct frequency modulation) 195 6.6 Error detectioncorrection and data compression 196 6.6.1 MNP protocol classes 196 6.6.2 Link access protocol modem (LAPM) 197 6.6.3 Data compression techniques 198 6.7 Data rate versus baud rate 201 6.8 Modem standards 202 6.9 Radio modems 203 6.10 Troubleshooting the system 207 6.10.1 Troubleshooting the serial link 207 6.10.2 The breakout box 208 6.10.3 Protocol analyzer 208 6.10.4 Troubleshooting the modem 209 6.11 Selection considerations 210 7 Central site computer facilities 212 7.1 Introduction 212 7.2 Recommended installation practice 212 7.2.1 Environmental considerations 212Contents xi 7.2.2 Earthing and shielding 213 7.2.3 Cabling 213 7.2.4 Power connections 214 7.3 Ergonomic requirements 215 7.3.1 Typical control room layout 215 7.3.2 Lighting 216 7.3.3 Sound environment 216 7.3.4 Ventilation 216 7.3.5 Colors of equipment 217 7.4 Design of the computer displays 217 7.4.1 Operator displays and graphics 218 7.4.2 Design of screens 219 7.5 Alarming and reporting philosophies 220 8 Troubleshooting and maintenance 223 8.1 Introduction 223 8.2 Troubleshooting the telemetry system 225 8.2.1 The RTU and component modules 225 8.2.2 The master sites 227 8.2.3 The central site 227 8.2.4 The operator station and software 227 8.3 Maintenance tasks 228 8.4 The maintenance unit system 230 9 Specification of systems 232 9.1 Introduction 232 9.2 Common pitfalls 232 9.3 Standards 233 9.4 Performance criteria 233 9.5 Testing 233 9.6 Documentation 234 9.7 Future trends in technology 234 9.7.1 Software based instrumentation 234 9.7.2 Future trends in SCADA systems 235 Appendix A Glossary 237 Appendix B Interface standards 258 Appendix C CITECT practical 262 Index 2731 Background to SCADA 1.1 Introduction and brief history of SCADA This manual is designed to provide a thorough understanding of the fundamental concepts and the practical issues of SCADA systems. Particular emphasis has been placed on the practical aspects of SCADA systems with a view to the future. Formulae and details that can be found in specialized manufacturer manuals have been purposely omitted in favor of concepts and definitions. This chapter provides an introduction to the fundamental principles and terminology used in the field of SCADA. It is a summary of the main subjects to be covered throughout the manual. SCADA (supervisory control and data acquisition) has been around as long as there have been control systems. The first ‘SCADA’ systems utilized data acquisition by means of panels of meters, lights and strip chart recorders. The operator manually operating various control knobs exercised supervisory control. These devices were and still are used to do supervisory control and data acquisition on plants, factories and power generating facilities. The following figure shows a sensor to panel system. Sensors Figure 1.1 Sensors to panel using 4–20 mA or voltage2 Practical SCADA for Industry The sensor to panel type of SCADA system has the following advantages: • It is simple, no CPUs, RAM, ROM or software programming needed • The sensors are connected directly to the meters, switches and lights on the panel • It could be (in most circumstances) easy and cheap to add a simple device like a switch or indicator The disadvantages of a direct panel to sensor system are: • The amount of wire becomes unmanageable after the installation of hundreds of sensors • The quantity and type of data are minimal and rudimentary • Installation of additional sensors becomes progressively harder as the system grows • Reconfiguration of the system becomes extremely difficult • Simulation using real data is not possible • Storage of data is minimal and difficult to manage • No off site monitoring of data or alarms • Someone has to watch the dials and meters 24 hours a day 1.2 Fundamental principles of modern SCADA systems In modern manufacturing and industrial processes, mining industries, public and private utilities, leisure and security industries telemetry is often needed to connect equipment and systems separated by large distances. This can range from a few meters to thousands of kilometers. Telemetry is used to send commands, programs and receives monitoring information from these remote locations. SCADA refers to the combination of telemetry and data acquisition. SCADA encompasses the collecting of the information, transferring it back to the central site, carrying out any necessary analysis and control and then displaying that information on a number of operator screens or displays. The required control actions are then conveyed back to the process. In the early days of data acquisition, relay logic was used to control production and plant systems. With the advent of the CPU and other electronic devices, manufacturers incorporated digital electronics into relay logic equipment. The PLC or programmable logic controller is still one of the most widely used control systems in industry. As need to monitor and control more devices in the plant grew, the PLCs were distributed and the systems became more intelligent and smaller in size. PLCs and DCS (distributed control systems) are used as shown below.Background to SCADA 3 Sensors A fieldbus PLC or DCS PC Figure 1.2 PC to PLC or DCS with a fieldbus and sensor The advantages of the PLC DCS SCADA system are: • The computer can record and store a very large amount of data • The data can be displayed in any way the user requires • Thousands of sensors over a wide area can be connected to the system • The operator can incorporate real data simulations into the system • Many types of data can be collected from the RTUs • The data can be viewed from anywhere, not just on site The disadvantages are: • The system is more complicated than the sensor to panel type • Different operating skills are required, such as system analysts and programmer • With thousands of sensors there is still a lot of wire to deal with • The operator can see only as far as the PLC As the requirement for smaller and smarter systems grew, sensors were designed with the intelligence of PLCs and DCSs. These devices are known as IEDs (intelligent electronic devices). The IEDs are connected on a fieldbus, such as Profibus, Devicenet or Foundation Fieldbus to the PC. They include enough intelligence to acquire data, communicate to other devices, and hold their part of the overall program. Each of these super smart sensors can have more than one sensor onboard. Typically, an IED could combine an analog input sensor, analog output, PID control, communication system and program memory in one device.4 Practical SCADA for Industry A fieldbus PC Ethernet IEDs Figure 1.3 PC to IED using a fieldbus The advantages of the PC to IED fieldbus system are: • Minimal wiring is needed • The operator can see down to the sensor level • The data received from the device can include information such as serial numbers, model numbers, when it was installed and by whom • All devices are plug and play, so installation and replacement is easy • Smaller devices means less physical space for the data acquisition system The disadvantages of a PC to IED system are: • More sophisticated system requires better trained employees • Sensor prices are higher (but this is offset somewhat by the lack of PLCs) • The IEDs rely more on the communication system 1.3 SCADA hardware A SCADA system consists of a number of remote terminal units (RTUs) collecting field data and sending that data back to a master station, via a communication system. The master station displays the acquired data and allows the operator to perform remote control tasks. The accurate and timely data allows for optimization of the plant operation and
Introduction and brief history of SCADA 1
This manual offers a comprehensive overview of the essential concepts and practical challenges associated with SCADA systems, focusing on their future applications It prioritizes understanding over technical details, intentionally omitting specific formulas and information typically found in manufacturer manuals to emphasize key concepts and definitions.
This chapter introduces the essential principles and terminology of SCADA, summarizing the key topics that will be explored in the manual.
SCADA, or supervisory control and data acquisition, has a long history that dates back to the inception of control systems Early SCADA systems relied on data collection through panels featuring meters, lights, and strip chart recorders, with operators manually adjusting control knobs to oversee operations These systems continue to play a vital role in managing and acquiring data from plants, factories, and power generation facilities, exemplified by the sensor-to-panel system depicted in the accompanying figure.
Sensors to panel using 4–20 mA or voltage
The sensor to panel type of SCADA system has the following advantages:
• It is simple, no CPUs, RAM, ROM or software programming needed
• The sensors are connected directly to the meters, switches and lights on the panel
• It could be (in most circumstances) easy and cheap to add a simple device like a switch or indicator
The disadvantages of a direct panel to sensor system are:
• The amount of wire becomes unmanageable after the installation of hundreds of sensors
• The quantity and type of data are minimal and rudimentary
• Installation of additional sensors becomes progressively harder as the system grows
• Re-configuration of the system becomes extremely difficult
• Simulation using real data is not possible
• Storage of data is minimal and difficult to manage
• No off site monitoring of data or alarms
• Someone has to watch the dials and meters 24 hours a day
Fundamental principles of modern SCADA systems 2
In contemporary manufacturing, mining, utilities, leisure, and security sectors, telemetry plays a crucial role in linking equipment and systems across vast distances, from mere meters to thousands of kilometers This technology facilitates the transmission of commands and programs while also receiving monitoring data from remote locations.
SCADA, which stands for Supervisory Control and Data Acquisition, integrates telemetry and data collection to monitor and control processes It involves gathering data, transmitting it to a central location for analysis, and displaying the information on operator screens Necessary control actions are then communicated back to the system, ensuring efficient process management.
In the early days of data acquisition, relay logic was the primary method for controlling production and plant systems With the introduction of CPUs and electronic devices, manufacturers began integrating digital electronics into relay logic equipment Today, programmable logic controllers (PLCs) remain among the most prevalent control systems in the industry As the demand for monitoring and controlling more devices increased, PLCs evolved to become more intelligent and compact, leading to the widespread use of both PLCs and distributed control systems (DCS) in modern applications.
PC to PLC or DCS with a fieldbus and sensor
The advantages of the PLC / DCS SCADA system are:
• The computer can record and store a very large amount of data
• The data can be displayed in any way the user requires
• Thousands of sensors over a wide area can be connected to the system
• The operator can incorporate real data simulations into the system
• Many types of data can be collected from the RTUs
• The data can be viewed from anywhere, not just on site
• The system is more complicated than the sensor to panel type
• Different operating skills are required, such as system analysts and programmer
• With thousands of sensors there is still a lot of wire to deal with
• The operator can see only as far as the PLC
The demand for more compact and intelligent systems has led to the development of Intelligent Electronic Devices (IEDs), which integrate the capabilities of PLCs and DCSs These advanced sensors connect to a PC via fieldbus systems like Profibus, Devicenet, or Foundation Fieldbus, enabling them to collect data, communicate with other devices, and execute their designated functions within a larger program IEDs often feature multiple onboard sensors, combining elements such as analog input and output, PID control, communication systems, and program memory into a single device.
PC to IED using a fieldbus
The advantages of the PC to IED fieldbus system are:
• The operator can see down to the sensor level
• The data received from the device can include information such as serial numbers, model numbers, when it was installed and by whom
• All devices are plug and play, so installation and replacement is easy
• Smaller devices means less physical space for the data acquisition system
The disadvantages of a PC to IED system are:
• More sophisticated system requires better trained employees
• Sensor prices are higher (but this is offset somewhat by the lack of PLCs)
• The IEDs rely more on the communication system
SCADA hardware 4
A SCADA system is composed of multiple remote terminal units (RTUs) that gather field data and transmit it to a master station through a communication network The master station not only displays the collected data but also enables operators to execute remote control functions effectively.
Accurate and timely data enables the optimization of plant operations and processes, leading to more efficient, reliable, and safer operations This advancement results in significantly lower operational costs compared to traditional non-automated systems.
On a more complex SCADA system there are essentially five levels or hierarchies:
• Field level instrumentation and control devices
• The commercial data processing department computer system
The RTU provides an interface to the field analog and digital sensors situated at each remote site
The communications system serves as the essential link between the master station and remote sites, utilizing various technologies such as wire, fiber optic, radio, telephone lines, microwave, and potentially satellite connections To ensure efficient and optimal data transfer, specific protocols and error detection methods are implemented.
The master station and sub-masters collect data from multiple RTUs, offering an operator interface for monitoring and controlling remote locations In extensive telemetry systems, sub-master sites relay information from these remote sites back to the control master station.
SCADA software 5
SCADA software is categorized into two main types: proprietary and open Proprietary software, developed by companies to interface with their hardware, is typically sold as 'turnkey' solutions, which can create a significant dependency on the supplier In contrast, open software systems have become increasingly popular due to their interoperability, allowing for the integration of equipment from different manufacturers within the same system.
Citect and WonderWare are among the leading open software packages for SCADA systems available in the market Many of these packages now offer integrated asset management features within the SCADA system The essential components of a SCADA system are illustrated in the accompanying diagram.
Report Server Task Input / Output Server Task
In Out In Analog Digital
Key features of SCADA software are:
Landlines for SCADA 6
Despite the reduced wiring when implementing a PC to IED system, a typical SCADA system still involves significant amounts of wiring This extensive wiring can lead to challenges, primarily due to electrical noise and interference.
When designing and installing a data communication system, it is crucial to consider interference and noise, particularly electrical interference Noise is an undesired random signal that disrupts the original signal, and it can infiltrate cables or wires through various means Designers must create systems that minimize noise from the outset, especially since SCADA systems, which operate on low voltage, are particularly vulnerable to such interference.
Utilizing twisted pair shielded Cat5 wire is essential for most systems, as it significantly reduces noise interference By combining high-quality wiring with proper installation techniques, you can ensure optimal performance and reliability for your network.
Fiber optic cable is gaining popularity because of its noise immunity At the moment most installations use glass fibers, but in some industrial areas plastic fibers are increasingly used
Future data communications will increasingly rely on radio, fiber optic, and infrared systems, while traditional wiring will primarily serve to supply power As electronic devices become more energy-efficient, the demand for power will also diminish.
SCADA and local area networks 7
Local Area Networks (LAN) facilitate the sharing of information and resources among nodes in a SCADA network For effective communication, these nodes must be interconnected through a transmission medium, which is defined by the network topology It is essential that all nodes can access this medium without interrupting ongoing transmissions from established senders.
A Local Area Network (LAN) serves as a communication pathway for computers, file servers, terminals, workstations, and various peripheral devices, enabling shared access among multiple users with full connectivity Typically owned and managed by private entities, LANs are confined to a specific geographical area, often within a group of buildings Ethernet stands out as the most commonly used LAN technology due to its affordability and ease of use Integrating a SCADA network with the LAN allows authorized personnel within the company to access the system, with user permissions controlling data visibility from the centralized database While security concerns are present, they can be effectively managed.
Ethernet used to transfer data on a SCADA system
Modem use in SCADA systems 7
PC to RTU using a modem
In SCADA systems, remote terminal units (RTUs) are often situated at varying distances, from tens of meters to thousands of kilometers away A cost-effective communication method for these long distances is through dial-up telephone connections This setup requires a PC, two dial-up modems, and an RTU with a built-in COM port By configuring the modems in auto-answer mode, the RTU can establish a connection by dialing in.
A PC can easily connect to an RTU using software provided by RTU manufacturers, and the necessary modems are conveniently available for purchase at local computer stores.
Line modems facilitate the connection of Remote Terminal Units (RTUs) to a network via a pair of wires, typically covering distances of up to 1 kilometer Utilizing frequency shift keying (FSK) for communication, these systems are essential for interfacing with RTUs when employing RS-232 or RS-485 protocols.
485 communication systems are not practical The bit rates used in this type of system are usually slow, 1200 to 9600 bps.
Computer sites and troubleshooting 8
Computers and RTUs can operate smoothly for extended periods with minimal intervention; however, regular maintenance is essential This maintenance may involve daily, weekly, monthly, or annual inspections Technicians or engineers should routinely check specific equipment to ensure optimal performance and prevent potential issues.
• The RTU and component modules
• Interface from RTU to PLC (RS-232/RS-485)
• Analog or digital data links
• The operator station and software
Two main rules that are always followed in repair and maintenance of electronic systems are:
• If it is not broken, don’t fix it
Technicians and engineers often create more issues than they resolve by engaging in unnecessary actions, such as cleaning equipment that is only slightly dusty or attempting to extract an additional 0.01 dB of power from a radio, which can lead to damaging the amplifier.
Components that could need maintenance in a SCADA system
System implementation 9
When designing a SCADA system, it's essential to integrate it with existing communication networks to minimize costs associated with new infrastructure This integration can utilize current LANs, private telephone systems, or radio systems for mobile communications Careful engineering is crucial to ensure that the SCADA system overlay does not disrupt or degrade existing network facilities.
Front panel display of SCADA software and its block diagram
When implementing a new system, it is essential to prioritize the quality of the installation while balancing economic factors with performance and integrity requirements Companies must operate within budget constraints, making it crucial to evaluate these aspects to ensure a successful system outcome Additionally, the reliability of equipment and the availability of communication links play a significant role in setting realistic performance expectations for the system.
This book will thoroughly explore all the key factors related to designing, specifying, installing, and maintaining an effective telemetry and data acquisition system tailored for industrial environments By systematically connecting these elements, readers will gain the knowledge needed to implement robust solutions in their specific settings.
SCADA systems, hardware and firmware
Introduction 11
This chapter explores the fundamentals of telemetry systems, defining key terms such as SCADA, distributed control systems (DCS), programmable logic controllers (PLC), and smart instruments, and contextualizing their use within this manual.
The chapter is broken up into the following sections:
• Definitions of the terms SCADA, DCS, PLC and smart instrument
• Remote terminal unit (RTU) structure
• Control site/master station structure
When configuring a master station in SCADA systems, it is essential to consider various factors, including master station software and communication protocols Telemetry, which involves transferring remote measurement data to a central control station via a communication link, plays a crucial role in this process While measurement data is typically collected in real-time, it may not always be transmitted immediately The concepts of SCADA, DCS, PLC, and smart instruments all utilize telemetry to enhance operational efficiency and data management.
Comparison of the terms SCADA, DCS, PLC and smart instrument 12 1 SCADA system 12
Distributed control system (DCS) 15
A Distributed Control System (DCS) utilizes multiple microprocessor-based units located near the controlled devices or data-gathering instruments to perform data acquisition and control functions These systems have advanced to offer sophisticated analog control capabilities, such as loop control They feature a well-integrated set of operator interfaces, facilitating easy system configuration and operator management Additionally, the data highway typically supports high-speed communication, ranging from 1 Mbps to 10 Mbps.
Programmable logic controller (PLC) 15 2.2.4 Smart instrument 16
Since the late 1970s, programmable logic controllers (PLCs) have effectively replaced traditional hardwired relays by utilizing ladder-logic software alongside solid-state electronic input and output modules Their cost-effective design makes them a popular choice for implementing SCADA remote terminal units (RTUs), providing a reliable standard hardware solution.
Programmable logic controller (PLC) system
Another device that should be mentioned for completeness is the smart instrument which both PLCs and DCS systems can interface to
The term often refers to an intelligent digital measuring sensor, like a flow meter, which is based on a microprocessor and features digital data communications to a diagnostic panel or computer system.
Typical example of a smart instrument
This book will henceforth consider DCS, PLC and smart instruments as variations or components of the basic SCADA concept.
Considerations and benefits of SCADA system 17
Typical considerations when putting a SCADA system together are:
• Ratio and number of analog to digital points
• Speed of control and data acquisition
• Speed of communications/update time/system scan rates
Obviously, a SCADA system’s initial cost has to be justified A few typical reasons for implementing a SCADA system are:
• Improved operation of the plant or process resulting in savings due to optimization of the system
• Increased productivity of the personnel
• Improved safety of the system due to better information and improved control
• Protection of the plant equipment
• Safeguarding the environment from a failure of the system
• Improved energy savings due to optimization of the plant
• Improved and quicker receipt of data so that clients can be invoiced more quickly and accurately
• Government regulations for safety and metering of gas (for royalties & tax etc)
Remote terminal units 17
Control processor (or CPU) 19 2.3.2 Analog input modules 19
This is generally microprocessor based (16 or 32 bit) e.g 68302 or 80386 Total memory capacity of 256 kByte (expandable to 4 Mbytes) broken into three types:
1 EPROM (or battery backed RAM) 256 kByte
3 Electrically erasable memory (flash or EEPROM) 128 kByte
A mathematical processor is a useful addition for any complex mathematical calculations This is sometimes referred to as a coprocessor
Communication ports – typically two or three ports either RS-232/RS-422/RS-485 for:
• Communications link to central site (e.g by modem)
Diagnostic LEDs provided on the control unit ease troubleshooting and diagnosis of problems (such as CPU failure/failure of I/O module etc)
A real-time clock with full calendar functionality, including leap year support, is essential for accurate event time stamping This clock must maintain its updates even during power outages, ensuring reliability and precision in timekeeping.
A watchdog timer is essential for monitoring the regular execution of the RTU program It resets at consistent intervals, and failure to do so within a specified timeout period triggers an error condition, potentially resetting the CPU.
There are five main components making up an analog input module They are:
• The sample and hold circuit
• The bus interface and board timing system
A block diagram of a typical analog input module is shown in Figure 2.7
Block diagram of a typical analog input module
Each of the individual components will be considered in the following sections
A multiplexer is a device that sequentially samples multiple analog inputs, typically 16, and directs them to a single output This output is usually connected to an A/D converter, which reduces the necessity for individual converters on each input channel, leading to significant cost savings Key parameters associated with multiplexers include their sampling rate, channel count, and switching speed.
• Crosstalk The amount of signal coupled to the output as a percentage of input signals applied to all OFF channels together
• Input leakage current The maximum current that flows into or out of an OFF channel input terminal due to switch leakage
Settling time refers to the duration required for a multiplexer output to stabilize within a specified percentage, typically 90% or ±1 LSB of the input value, as a single input transitions from -FS (full scale) to FS or from +FS to -FS This stabilization is crucial, as the output must achieve a precision within ±1 LSB of the input range before the A/D converter can accurately convert the analog input voltage.
A similar parameter to settling time, it specifies how long the multiplexer output takes to settle to the input voltage when the multiplexer is switched from one channel to another
The throughput rate refers to the maximum speed at which a multiplexer can transition between channels, constrained by the longer of the settling time or the switching time.
• Transfer accuracy Expresses the input-to-output error as a percentage of the input
To accurately digitize low-level voltages, amplification is essential to align the signal with the input range of the board’s A/D converter; otherwise, precision loss occurs While some boards feature built-in amplification, those equipped with a Programmable Gain Amplifier (PGA) allow users to select varying gain levels for different channels through software, enhancing conversion flexibility.
An ideal differential input amplifier exclusively reacts to the voltage difference between its two input terminals, independent of any common voltage present at both terminals However, in practical applications, common mode voltages can lead to erroneous output signals in real-world amplifiers.
An important characteristic is the common mode rejection ratio, CMRR, which is calculated as follows
CMRR = 20log (V cm / V diff ) [dB] where:
V cm is the voltage common to both inputs
V diff is the output (error) voltage when V cm is applied to both inputs
An ideal value for CMRR would be 80 dB or greater
Drift is another important amplifier specification; it depends on time and temperature
An amplifier calibrated to output zero for zero input at a specific temperature will experience changes in output over time and with temperature fluctuations Time drift and temperature drift are typically quantified in parts per million (PPM) per unit time and PPM per degree Celsius, respectively For a 12-bit board, 1 least significant bit (LSB) corresponds to 1 count in 4096, equating to 244 PPM Consequently, within an operational temperature range of 0°C to 50°C, a drift of 1 LSB is observed.
When selecting a component, it is essential to verify that the board's time and temperature drift specifications align with your required precision across the entire operating temperature range, especially considering that temperatures can rise significantly within the RTUs enclosure.
Most A/D converters necessitate a fixed duration, known as aperture time, during which the input signal must remain stable to ensure accurate A/D conversion This stability is crucial because any changes in the input signal during this period could lead to erroneous readings To address this, a sample-and-hold device is employed at the input of the A/D converter, which rapidly samples the output signal from the multiplexer or gain amplifier and maintains it constant throughout the A/D's aperture time.
The standard design approach is to place a simple sample-and-hold chip between multiplexer and A/D converter
The A/D converter is the heart of the module Its function is to measure an input analog voltage and to output a digital code corresponding to the input voltage
There are two main types of A/D converters used:
Integrating or dual slope A/D converters are ideal for low frequency applications, typically up to a few hundred hertz, offering exceptional accuracy and precision, often reaching up to 22 bits Commonly used in thermocouple and RTD modules, these converters provide advantages such as low cost and reduced noise, as their design minimizes mains pickup The A/D conversion process involves charging a capacitor with the input signal for a predetermined duration, followed by measuring the time it takes for the capacitor to discharge, which is directly proportional to the input voltage.
Successive approximation A/D converters offer high sampling rates, achieving up to a few hundred kHz with 12-bit resolution, while remaining cost-effective The conversion process resembles a binary search, beginning with a comparison of the input voltage against a reference voltage generated by an internal D/A converter, which represents half of the full-scale range If the input is in the lower half, the first digit is set to zero; if in the upper half, it is set to one This halving process continues, narrowing down the input range until the desired bit accuracy is reached It is crucial that the input signal remains stable throughout the conversion to ensure accurate results.
The specifications of A/D converters are discussed below
This value refers to the maximum analog error; it is referenced to the national bureau of standards’ standard volt
This is the maximum deviation of an actual bit size from its theoretical value for any bit over the full range of the converter
• Gain error (scale factor error)
The difference in slope between the actual transfer function and the ideal function in percentage
Unipolar offset refers to the initial transition point occurring 1 LSB above the analog common The unipolar offset error indicates the difference between the actual transition point and the ideal first transition point This error can typically be calibrated to zero using software and a trimpot on the board Additionally, this parameter often comes with a specified temperature drift.
The transition from FS/2-ẵ LSB to FS/2 (from 7 FFh to 800 h on a 12-bit A/D) should take place at ẵ LSB below the analog common The bipolar offset, typically adjustable with a trimpot, along with the temperature coefficient, defines the initial deviation and the maximum error variation due to temperature changes.
In A/D converters, gain, offset, and zero errors are often manageable through calibration, making them less critical However, linearity errors, including differential non-linearity (DNL) and integral non-linearity (INL), hold greater significance as they cannot be corrected once present.
Is the difference between the actual code width from the ideal width of 1 LSB
Typical analog input modules 26 2.3.4 Analog outputs 27 2.3.5 Digital inputs 28
These have various numbers of inputs Typically there are:
• Range of 4–20 mA (other possibilities are 0–20 mA/±10 volts/0–10 volts)
• Conversion rates typically 10 microseconds to 30 milliseconds
• Inputs are generally single ended (but also differential modes provided)
To reduce costs and minimize data transfer over radio links, a typical configuration involves eight single-ended 8-bit channels that read voltages from 0 to 10 volts, with each analog point having a conversion rate of 30 milliseconds.
Sampling signals at the correct frequency is a critical yet often overlooked aspect of analog input boards According to the Nyquist criterion, a signal must be sampled at least twice its highest frequency component, necessitating that the analog-to-digital system operates at a sufficiently high sampling rate Failing to do so may require filtering to mitigate input frequency components, but this is frequently disregarded due to the added expense of installation, leading to inaccurate measurements It's important to note that software filtering cannot compensate for inadequate hardware filtering or insufficient sampling rates; while it may smooth the signal, it does not accurately reproduce the original analog signal in digital form.
Typically the analogue output module has the following features:
• Conversion rate from 10 à seconds to 30 milliseconds
• Outputs ranging from 4–20 mA/± 10 volts/0 to 10 volts
Care has to be taken here on ensuring the load resistance is not lower than specified (typically 50 kΩ) or the voltage drop will be excessive
Analog output module designs generally prefer to provide voltage outputs rather than current output (unless power is provided externally), as this places lower power requirements on the backplane
Status and alarm signals are essential for monitoring valve operations For instance, a valve may utilize two limit switches: one with a closed contact indicating an open status and the other indicating a closed status If both contacts are closed simultaneously, it suggests the valve is in transit, which could signify a malfunction if both indicate an open condition Additionally, a high-level switch serves to signal alarm conditions.
It is important with alarm logic that the RTU should be able to distinguish the first alarm from the subsequent spurious alarms that will occur
Digital input boards typically offer configurations of 8, 16, or 32 inputs per unit To accommodate a higher number of digital points, it may be necessary to install multiple boards when the input capacity of a single board is surpassed.
For alarm systems, standard normally open or normally closed converters can be utilized effectively Typically, normally closed digital inputs are preferred for indicating alarm conditions within a circuit.
When selecting an input power supply, it is crucial to ensure it is properly rated for the specific convention being utilized, whether normally open or normally closed In the case of the normally open convention, the digital input power supply can be de-rated.
Optical isolation is a good idea to cope with surges induced in the field wiring A typical circuit and its operation are indicated in Figure 2.12
Digital input circuit with flow chart of operation
The two main approaches of setting the input module up as a sink or source module are as indicated in the Figure 2.13
Configuring the input module as a sink or source
Typically the following would be expected of a digital input module:
• Associated LED indicator for each input to indicate current states
• Digital input voltages vary from 110/240 VAC and 12/24/48 VDC
• Optical isolation provided for each digital input
Counter or accumulator digital inputs 29 2.3.7 Digital output module 31
Pulse-input modules are essential in various applications, such as those involving metering panels These modules can process signals from contact closures or, when the pulse frequency is sufficiently high, from solid-state relays.
Pulse input signals are normally ‘dry contacts’ (i.e the power is provided from the RTU power supply rather than the actual pulse source)
The counter digital input system utilizes optical isolation to reduce the impact of external noise The accumulator's size is crucial for determining the number of pulses it can count before data transfer to another memory location A 12-bit register can handle 4,096 counts, while a 16-bit register accommodates 65,536 pulses, equivalent to 48 minutes at a rate of 20,000 barrels per hour Ignoring these limits may lead to the common issue of the accumulator cycling through zero when it reaches full capacity.
• The accumulator contents can be transferred to RAM memory at regular intervals where the old and current value difference can be stored in a register
The second approach involves precise accounting of liquid flows in and out of a designated area An instantaneous freeze accumulator command is sent to all relevant RTUs, causing the pulse accumulator to capture and store the current values in memory Following this, the accumulator resets to allow for continued counting.
The typical specifications here are:
• Four 16 bit counters (65 536 counts per counter input)
• Count frequency up to 20 kHz range
• Duty cycle preferably 50% (ratio of mark to space) for the upper count frequency limits
The duty rating is crucial because the counter input requires a specific duration to activate and deactivate If the on pulse duration is too brief, it may be overlooked, even if the counting frequency remains within the acceptable range.
A Schmitt trigger gives the preferred input conditioning although a resistor capacitor combination across the counter input can be a cheap way to spread the pulses out
A digital output module drives an output voltage at each of the appropriate output channels with three approaches possible:
The TRIAC is commonly used for AC switching A varistor is often connected across the output of the TRIAC to reduce the damaging effect of electrical transients
Three practical issues should also be observed:
A TRIAC output switching device operates by exhibiting both low and high resistance values rather than fully turning on or off Consequently, even when the TRIAC is in the off state, it allows a certain amount of leakage current to pass through the output.
• Surge currents should be of short duration (half a cycle) Any longer will damage the module
It is essential to adhere to the manufacturer's continuous current rating, which typically applies to individual channels and their total count In some cases, all output channels of a module can operate at their full rated current capacity; however, this may lead to exceeding the maximum allowable power dissipation for the entire module.
• 240 V AC/24 V DC (0.5 amp to 2.0 amp) outputs
• Associated LED indicator for each output to indicate current status
• Optical isolation or dry relay contact for each output
Dry relay contacts, such as reed relay outputs, are commonly used without voltage applied by the output module It's crucial to adhere to the current rating limitations, particularly for inductive loads While individual digital outputs may be rated for 2 Amps, the entire module typically cannot exceed a total current of 60% of the number of outputs multiplied by the maximum output current Exceeding this limit can lead to overheating and ultimately result in module failure.
When discussing I/O modules, it's important to understand the distinction between sinking and sourcing current A module that sinks current draws it from an external source, while a module that sources current provides this current as an output.
Source and sink of current
To effectively manage inductive loads, it is advisable to install a flywheel diode across the relay in DC systems and a capacitor-resistor combination in AC systems This approach helps to mitigate back EMF effects and reduces voltage spikes that occur when devices are turned off.
Flywheel diode or RC circuit for digital outputs
Mixed analog and digital modules 33 2.3.9 Communication interfaces 33
Many RTUs have modest requirements for analog and digital signals, making a mixed analog and digital module a typical solution.
The modern RTU should be flexible enough to handle multiple communication media such as:
• Dialup telephone lines/dedicated landlines
• Radio via trunked/VHF/UHF/900 MHz
The design of radio communication interfaces for Remote Terminal Units (RTUs) presents greater challenges compared to the relatively straightforward landline interfaces Detailed discussions on these standards will be provided in a subsequent section.
Power supply module for RTU 33
The RTU is designed to operate on a power supply of 110/240 V AC ± 10% at 50 Hz or 12/24/48 V DC ± 10%, utilizing lead acid or nickel cadmium batteries It typically requires a 20-hour standby operation and a 12-hour recharging time for a fully discharged battery at 25°C The power supply, battery, and charger are generally housed within the RTU unit.
Other important monitoring parameters, which should be transmitted back to the central site/master station, are:
• Alarm for battery voltage outside normal range
Cabinets for batteries are normally rated to IP 52 for internal mounting and IP 56 for external mounting.
RTU environmental enclosures 33 2.3.12 Testing and maintenance 34
Printed circuit boards are commonly inserted into a backplane within the RTU cabinet, which is housed in an environmental enclosure designed to shield it from extreme temperatures and weather conditions.
Typical considerations in the installations are:
To prevent heat accumulation in the RTU enclosure, it is essential to install circulating air fans and filters at the base Ensuring uniform air circulation helps avoid hot spots on electronic circuitry, which can lead to malfunctions Additionally, conducting a heat soak test is crucial for optimal performance and reliability.
• Hazardous areas: RTUs must be installed in explosion proof enclosures (e.g oil and gas environment)
The typical operating temperatures of rooftop units (RTUs) can vary significantly when the unit is installed outside in a weatherproof enclosure However, these temperature specifications can be less stringent if the RTU is located indoors, where temperature fluctuations are generally milder, as long as potential failures of ventilators or air-conditioning systems are taken into account.
Humidity levels typically range from 10% to 95% To prevent condensation on circuit boards at high humidity levels, which can lead to contact corrosion or short-circuiting, it's crucial to manage moisture effectively In such cases, applying a lacquer to the printed circuit boards can be a viable solution.
Low humidity levels, particularly around 5%, can lead to static electricity buildup on circuit boards, posing risks for CMOS-based electronics To mitigate static voltages, it is essential to screen and ground affected electronic areas Additionally, maintenance personnel should always wear wrist ground straps to minimize the risk of generating and transferring static electricity.
To ensure optimal performance of Remote Terminal Units (RTUs) in environments with high electromagnetic interference (EMI) and radio frequency interference (RFI), it is crucial to implement special screening and earthing measures Manufacturers often advise against using handheld transceivers near their RTUs due to potential interference Additionally, continuous vibration from nearby machinery can adversely affect RTU functionality, making the use of vibration shock mounts essential Other critical considerations for RTUs include protection against lightning and electrical surges, as well as the impact of earthquakes, which can produce vibrations at frequencies ranging from 0.1 to 10 Hz.
Many manufacturers offer test boxes designed to evaluate communication between Remote Terminal Units (RTUs) and master stations, as well as to simulate either component within the system Figure 2.18 illustrates three typical configurations for these setups.
SCADA test box operating mode
The typical functions provided on a test box are:
• Message switches: The simulated messages that the user wants to send to the RTU or master station is input here
• Message indicators: Display of transmit and receiver data
The device operates in three distinct modes: eavesdropping mode between the RTU and master station, testing from the test box to the RTU, and testing from the test box to the master station Additionally, a self-test mode is typically included to ensure optimal functionality.
There are other features provided such as continuous transmissions of preset messages Often the test box is interfaced to a PC for easier display and control of actions.
Typical requirements for an RTU system 35
In the writing of a specification, the following issues should be considered:
Individual RTU expandability (typically up to 200 analog and digital points)
• Maximum number of RTU sites in a system shall be expandable to 255
• Modular system – no particular order or position in installation (of modules in a rack)
• Robust operation – failure of one module will not affect the performance of other modules
• Minimization of power consumption (CMOS can be an advantage)
• Rugged and of robust physical construction
• Maximization of noise immunity (due to harsh environment)
• Temperature of –10 to 65°C (operational conditions)
• Status of each I/O module and channel (program running/failed/ communications OK/failed)
• Modules all connected to one common bus
• Physical interconnection of modules to the bus shall be robust and suitable for use in harsh environments
• Ease of installation of field wiring
• Removable screw terminals for disconnection and reconnection of wiring
The RTU is normally installed in a remote location with fairly harsh environmental conditions It typically is specified for the following conditions:
• Ambient temperature range of 0 to +60°C (but specifications of –30°C to 60°C are not uncommon)
• Storage temperature range of –20°C to +70°C
• Relative humidity of 0 to 95% non condensing
• Surge withstand capability to withstand power surges typically 2.5 kV, 1 MHz for 2 seconds with 150 ohm source impedance
• Static discharge test where 1.5 cm sparks are discharged at a distance of 30 cm from the unit
• Other requirements include dust, vibration, rain, salt and fog protection
• Compatibility checks of software configuration of hardware against actual hardware available
• Log kept of all errors that occur in the system both from external events and internal faults
• Remote access of all error logs and status registers
• Software operates continuously despite powering down or up of the system due to loss of power supply or other faults
• Hardware filtering provided on all analog input channels
• Application program resides in non volatile RAM
• Configuration and diagnostic tools for:
• Application code development/management/operation
Each module must include internal software that continuously tests the system's I/O and hardware Additionally, diagnostic LEDs should be implemented to identify faults and diagnose component failures It is crucial for all these conditions to be communicated back to the central station for operator awareness.
Application programs 36
Advancements in processing power and memory storage capabilities have enabled many applications, once executed at the master station, to now be performed directly at the Remote Terminal Unit (RTU) Additionally, numerous RTUs are equipped with a local operator interface, allowing for enhanced functionality Typical application programs that can be effectively run on the RTU include various monitoring and control tasks.
PLCs used as RTUs 36
PLC software 37
The ladder-logic programming method is favored for its resemblance to conventional electrical circuits, featuring two vertical power supply lines on either side of the diagram, with horizontal logic lines connecting them.
The example below shows the ‘real world’ circuit with PLC acting as the control device and the internal ladder-logic within the PLC
The concept of PLC ladder-logic
Basic rules of ladder-logic 38
The basic rules of ladder-logic can be stated to be:
• The vertical lines indicate the power supply for the control system (12 V DC to
240 V AC) The ‘power flow’ is visualized to move from left to right
• Read the ladder diagram from left to right and top to bottom (as in the normal Western convention of reading a book)
• Electrical devices are normally indicated in their normal de-energized condition This can sometimes be confusing and special care needs to be taken to ensure consistency
• The contacts associated with coils, timers, counters and other instructions have the same numbering convention as their control device
Devices designed to initiate operations for specific items are typically wired in parallel, allowing any of them to activate or switch on the item.
Ladder-logic start operation (& logic diagram)
Devices designed to halt operations for specific items are typically connected in series, allowing any one of them to stop or switch off the items effectively.
Ladder-logic stop operation (& logic diagram)
Latching operations utilize a momentary start input signal to maintain the start signal in an ON state, even after the start input is turned OFF This process, often called holding or maintaining a sealing contact, ensures that the system remains energized For visual examples of latching, refer to the previous diagrams.
Interactive logic in ladder programming allows later rungs to interact with earlier ones, creating a feedback mechanism that can indicate the successful completion of a sequence of operations or safeguard the overall system against potential failures.
The different ladder-logic instructions 40
Ladder-logic instruction can be typically broken up into the following different categories:
A few of these instructions will be discussed in the following sections
There are three main instructions in this category These are:
(Sometimes also referred to as ‘examine if closed’ or ‘examine on’) The symbol is indicated in Figure 2.22
This instruction checks the memory address to determine its state If the memory location is set to ON (1), the instruction is activated as 'ON', 'TRUE', or '1' Conversely, if the memory location is OFF (0), the instruction is deactivated as 'OFF', 'FALSE', or '0'.
Symbol for normally open contact
The instruction, often called 'examine if open' or 'examine if off,' checks a specific memory address for an 'OFF' condition If the memory location is marked as 'OFF' (1), the instruction is set to 'OFF' (0) Conversely, if the memory location is 'ON' (0), the instruction changes to 'ON' (TRUE or 1), as illustrated in Figure 2.23.
Symbol for normally closed contact
When a ladder-logic rung is in a 'TRUE' or 'ON' state, the output energize instruction activates its memory location to 'ON'; conversely, if the rung is 'FALSE' or 'OFF', the output energize coil changes its memory location to 'OFF'.
The symbol is indicated in Figure 2.24
Symbol for output energize coil
Chapter 3 illustrates a practical circuit example, highlighting the use of coils and contacts These components can represent either external inputs and outputs from the 'real world' or internal signals within the system.
There are two types of timers:
• Timer OFF delay There are three parameters associated with each timer:
(Which is the constant number of seconds the timer times to, before being energized or de-energized)
(Which is the number of seconds which records how long the timer has been actively timing)
• The time base (Which indicated the accuracy in seconds to which the timer operates e.g 1 second, 0.1 seconds and even 0.01 seconds)
The 'timer ON' operation, illustrated in Figure 2.25, activates the timer output coil when the accumulated time reaches a predetermined value, provided that the rung remains energized during this duration If the rung conditions change to false before the accumulator equals the preset value, the accumulator resets to zero.
Operations of timer on with timing diagram
The 'timer OFF' timer operates by energizing the timer coil when the rung is active Once the rung becomes inactive, the timer counts down until the accumulated value matches the preset value, at which point the timer coil de-energizes If the rung conditions drop again before reaching the preset value, the accumulator resets to zero This sequence is illustrated in Figure 2.26.
Operation of the timer off with timing diagram
There are two types of counters, Count up and Count down The operation of these counters is very similar to the timer ON and timer OFF timers
There are two values associated with counters:
The counter increases the accumulator by one with each transition of the input contact from false to true Once the accumulated value reaches the preset threshold, the counter output activates Additionally, issuing a reset instruction at the same address as the counter will reset it, bringing the accumulated value back to zero.
This counter decrements the accumulator value (which started off at the preset value) by
When the input contact transitions from false to true, the counter output is activated if the accumulator value is zero Notably, these counters maintain their accumulated count even during a power failure or after being programmed following an MCR instruction.
The arithmetic instructions primarily focus on integer and floating-point arithmetic, with some allowance for manipulating ASCII or BCD values For a comprehensive guide on binary number manipulation and conversion to integers, please refer to Appendix C The typical instructions provided include a range of arithmetic operations.
For accurate arithmetic operations in a rung, it is essential that the rung is correctly configured, particularly at the location of the rung coil An illustrative example of this can be seen in Figure 2.27, which demonstrates an addition operation.
When monitoring control bits like carry, overflow, zero, and sign bits, caution is essential to avoid potential issues Additionally, it's crucial to use floating point registers as destination registers for floating point source values to maintain accuracy during arithmetic operations.
Besides the logical operations that can be performed with relay contacts and coils, which have been discussed earlier, there may be a need to do logical or boolean operations on a
In the examples provided, the corresponding bits of the source words are manipulated individually to produce the final destination value The logical operations utilized in this process include various types of bitwise functions.
For a logical operation to function correctly, the corresponding rung must be true, typically found at the usual location of a rung coil Detailed definitions of the logical operations can be found in Appendix E.
This instruction moves the source value at the defined address to the destination address every time this instruction is executed
These are useful to compare the contents of words with each other Typical instructions here are to compare two words for:
• Less than or equal to
• Greater than or equal to
When these conditions are true they can be connected in series with a coil which they then drive into the energized state
2.5.3.9 Sub routines and jump instructions
There are two main ways of transferring control of the ladder-logic program from the standard sequential path in which it is normally executed These are:
• Jump to part of the program when a rung condition becomes true (sometimes called jump to a label)
• Jump to a separate block of ladder-logic called a sub routine
Users may encounter issues when entering ladder-logic rungs into PLCs due to the reporting limitations of certain software packages, which often fail to effectively communicate syntax errors.
• Numbering of coils and contacts per rung (or network)
Ladder-logic designs generally permit a single coil per rung, alongside a limited number of parallel branches—typically up to seven—and a maximum of ten series contacts per branch To accommodate more contacts than a single rung or network can manage, additional rungs featuring 'dummy' coils must be added.
Vertical contacts are normally not allowed
Contacts may only be allowed to be nested to a certain level in a PLC In others no nesting is allowed
‘Power flow’ within a network or rung always has to be from left to right Any violation of this principle would be disallowed.
The master station 46
The central site or master station consists of multiple operator stations interconnected through a local area network, linked to a communication system that includes a modem and a radio receiver/transmitter Alternatively, a landline system can replace the radio system, with the modem interfacing directly to the landline Typically, there are no direct input/output modules connected to the master stations, although a Remote Terminal Unit (RTU) may be situated near the master control room Essential features should be readily available for effective operation.
• Operator interface to display status of the RTUs and enable operator control
• Logging of the data from the RTUs
• Alarming of data from the RTU
As discussed earlier, a master station has two main functions:
• Obtain field data periodically from RTUs and submaster stations
• Control remote devices through the operator station There are various combinations of systems possible, as indicated in the diagram below
Various approaches possible for the master station
It may also be necessary to set up a submaster station This is to control sites within a specific region The submaster station has the following functions:
• Acquire data from RTUs within the region
• Log and display this data on a local operator station
• Pass data back to the master station
• Pass on control requests from the master station to the RTUs in its region
Typical structure of the master station
The master station has the following typical functions:
• Initialize each RTU with input/output parameters
• Download control and data acquisition programs to the RTU
Operation of the communications link
• If a master slave arrangement, poll each RTU for data and write to RTU
• Log alarms and events to hard disk (and operator display if necessary)
• Link inputs and outputs at different RTUs automatically
• Provide accurate diagnostic information on failure of RTU and possible problems
• Predict potential problems such as data overloads
There are three components to the master station software:
• The system SCADA software (suitably configured)
Firmware, including BIOS, serves as a crucial interface between the operating system and computer hardware, facilitating communication and functionality Notably, operating systems such as DOS and Windows exemplify this relationship, though they will not be elaborated on in this discussion.
NT and the various UNIX systems
This refers to the software put together by the particular SCADA system vendor and then configured by a particular user Generally, it consists of four main modules:
• The man machine interface (MMI)
The next chapter will provide a detailed discussion of the software, emphasizing the importance of the central site structure in successful SCADA system design This section will assess the central site features, including the use of Local Area Networks (LANs), which will be briefly reviewed here.
A central site structure can utilize a distributed architecture and a high-speed data highway, employing LAN standards such as 802.3 (Ethernet), 802.4 (token bus), or 802.5 (token ring) The most prevalent method is the Ethernet or token bus configuration, which operates without a single master operator station Currently, the token bus approach is gaining popularity in the market, as it uses a token to transfer control between stations, facilitating easy system expansion.
Each of the network options will be discussed in the following paragraphs Specific reference will be made to the three types of LANs:
• Token ring (e.g IBM token ring)
• Token bus (e.g MAP/PLC type industrial systems) Each of these network types is considered in more detail in the following sections
Ethernet commonly utilizes a 10 Mbps baseband coaxial cable network, employing Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as its media access control (MAC) method This approach is widely favored for Local Area Networks (LANs) and will be explored in greater detail compared to alternative methods.
The philosophy of Ethernet stems from radio transmission experiments where multiple stations aimed to communicate randomly Before a node sends a message over the shared cable, it first listens for any ongoing bus activity If no transmissions are detected, it proceeds to send its message However, there is a chance that another station may transmit simultaneously, leading to a collision In such cases, both nodes will back off for a random period before attempting to transmit again, ideally at different times due to the random delay.
A typical hardware layout for a CSMA/CD system
The integrated tap and transceiver unit (referred to as the MAC unit) has the following components:
• A transceiver unit to transmit and receive data, detect collisions, provide electrical isolation and protect the bus from malfunctions
• A tap to make a non intrusive physical connection of the coaxial cable
The controller card, linked to the transceiver via shielded cable, houses a medium access control unit for message framing and error detection, along with a microprocessor to execute network-dependent protocols Ethernet cabling comes in three varieties: standard Ethernet, coaxial Ethernet (10BASE2), and the 10BASE-T standard.
Standard Ethernet, defined by the ISO 8802.3 standard as 10BASE5, operates at a speed of 10 Mb/sec and utilizes baseband transmission Each segment can extend up to 500 meters and support up to 100 Media Access Units (MAUs), with a total of five segments permitted in an entire Ethernet system.
The 10BASE5 standard utilizes 50-ohm coaxial cable, measuring 10.28 mm, and is commonly known as thickwire Ethernet For connections, male N-connectors are employed for splicing, along with a female-female N-type connector barrel The attachment unit interface (AUI) consists of a 15-conductor shielded cable, featuring five individually shielded pairs To ensure proper functioning, each segment must be terminated with a 50-ohm N-connector terminator.
The medium attachment unit (or MAU) is available in two forms:
The vampire tap is an innovative device that simplifies the connection to a coaxial cable by utilizing a unique design where one pin is drilled into the center conductor, while the other pin connects to the shield, ensuring a secure and efficient link.
The N-type connector features two female connectors, necessitating the cutting of the cable to attach male N-type connectors for connecting to the trunk line This method is especially advantageous in dirty factory environments, as it is more reliable than using vampire connections Additionally, it is important to maintain a minimum distance of 2.5 meters between MAUs for optimal performance.
Physical layout for 10BASE5 Ethernet
Thin coaxial Ethernet, also known as 10BASE2, was developed to lower installation costs following the 10BASE5 standard It supports a maximum segment length of 185 meters and utilizes RG-58 A/U or C/U coaxial cables with a characteristic impedance of 50 ohms Commonly referred to as Cheapernet or thin wire Ethernet, this technology provides a cost-effective solution for network connectivity.
Thin Ethernet trunk coaxial cables should not be spliced, and MAUs must be connected at consistent intervals of 0.5 meters Each 185-meter segment of 10BASE2 can support up to 30 MAUs, including those for repeaters Installation guidelines for 10BASE2 are similar to those for 10BASE5, with the exception that thin coaxial cable cannot serve as a link segment between two 10BASE5 segments.
The 10BASE-T Ethernet standard utilizes a star topology, connecting multiple terminals to a central hub via twisted pair cables Each terminal can be located up to 100 meters from the hub, making this standard ideal for small networks Additionally, 10BASE-T networks are often integrated with 10BASE5 networks for expanded connectivity.
A few suggestions on reducing collisions in an Ethernet network are:
• Keep all cables as short as possible
• Keep all high activity sources and their destinations as close as possible Possibly isolate these nodes from the main network backbone with bridges/routers to reduce backbone traffic
• Use buffered repeaters rather than bit repeaters
• Check for unnecessary broadcast packets which are aimed at non existent nodes
• Remember that the monitoring equipment to check out network traffic can contribute to the traffic (and the collision rate)
• Ensure that earthing of the cable is done at only one point on one of the cable terminators
The token ring system, developed by IBM in the early 1980s, is a type of network commonly used in office environments, though it is less popular in industrial settings This network operates by using a token message that grants control to one node at a time, allowing it to manage the ring network for a limited duration Once the time expires, or if the node has no messages to send, the token is passed to the next node, ensuring efficient communication within the network.