RTU and CPU Used in SCADA and Benefits Using SCADA System

Remote terminal units

An RTU (sometimes referred to as a remote telemetry unit) as the title implies, is a standalone data acquisition and control unit, generally microprocessor based, which monitors and controls equipment at some remote location from the central station. Its primary task is to control and acquire data from process equipment at the remote location and to transfer this data back to a central station. It generally also has the facility for having its configuration and control programs dynamically downloaded from some central station. There is also a facility to be configured locally by some RTU programming unit. Although traditionally the RTU communicates back to some central station, it is also possible to communicate on a peer-to-peer basis with other RTUs. The RTU can also act as a relay station (sometimes referred to as a store and forward station) to another RTU, which may not be accessible from the central station.



Small sized RTUs generally have less than 10 to 20 analog and digital signals, medium sized RTUs have 100 digital and 30 to 40 analog inputs. RTUs, having a capacity greater than this can be classified as large.

A typical RTU configuration is shown in Figure below:



A short discussion follows on the individual hardware components. Typical RTU hardware modules include:
• Control processor and associated memory
• Analog inputs
• Analog outputs
• Counter inputs
• Digital inputs
• Digital outputs
• Communication interface(s)
• Power supply
• RTU rack and enclosure

Control processor (or CPU)

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
2 RAM = 640 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:
• Interface to diagnostics terminal
• Interface to operator station
• 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).

Another component, which is provided with varying levels of accuracy, is a real-time clock with full calendar (including leap year support). The clock should be updated even during power off periods. The real-time clock is useful for accurate time stamping of events.

A watchdog timer is also required to provide a check that the RTU program is regularly executing. The RTU program regularly resets the watchdog time. If this is not done within a certain time-out period the watchdog timer flags an error condition (and can reset the CPU).

Considerations and benefits of SCADA system

Typical considerations when putting a SCADA system together are:
• Overall control requirements
• Sequence logic
• Analog loop control
• Ratio and number of analog to digital points
• Speed of control and data acquisition
• Master/operator control stations
• Type of displays required
• Historical archiving requirements
• System consideration
• Reliability/availability
• Speed of communications/update time/system scan rates
• System redundancy
• Expansion capability
• Application software and modeling

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)

Comparison of the terms SCADA, DCS, PLC and smart instrument

A. Scada System

A SCADA (or supervisory control and data acquisition) system means a system consisting of a number of remote terminal units (or RTUs) collecting field data connected back to a master station via a communications system. The master station displays the acquired data and also allows the operator to perform remote control tasks. The accurate and timely data (normally real-time) allows for optimization of the operation of the plant and process. A further benefit is more efficient, reliable and most importantly, safer operations. This all results in a lower cost of operation compared to earlier non-automated systems.

There is a fair degree of confusion between the definition of SCADA systems and process control system. SCADA has the connotation of remote or distant operation. The inevitable question is how far ‘remote’ is – typically this means over a distance such that the distance between the controlling location and the controlled location is such that direct-wire control is impractical (i.e. a communication link is a critical component of the system).

A successful SCADA installation depends on utilizing proven and reliable technology, with adequate and comprehensive training of all personnel in the operation of the system. There is a history of unsuccessful SCADA systems – contributing factors to these systems includes inadequate integration of the various components of the system, unnecessary complexity in the system, unreliable hardware and unproven software. Today hardware reliability is less of a problem, but the increasing software complexity is producing new challenges. It should be noted in passing that many operators judge a SCADA system not only by the smooth performance of the RTUs, communication links and the master station (all falling under the umbrella of SCADA system) but also the field devices (both transducers and control devices). The field devices however fall outside the scope of SCADA in this manual and will not be discussed further. A diagram of a typical SCADA system is given opposite.




On a more complex SCADA system there are essentially five levels or hierarchies:
• Field level instrumentation and control devices
• Marshalling terminals and RTUs
• Communications system
• The master station(s)
• The commercial data processing department computer system
The RTU provides an interface to the field analog and digital signals situated at each
remote site.
The communications system provides the pathway for communications between the
master station and the remote sites. This communication system can be radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data. The master station (and submasters) gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites. In large telemetry systems, submaster sites gather information from remote sites and act as a relay back to the control master station.

SCADA technology has existed since the early sixties and there are now two other competing approaches possible – distributed control system (DCS) and programmable logic controller (PLC). In addition there has been a growing trend to use smart instruments as a key component in all these systems. Of course, in the real world, the designer will mix and match the four approaches to produce an effective system matching his/her application.



B. Distributed control system (DCS)

In a DCS, the data acquisition and control functions are performed by a number of distributed microprocessor-based units situated near to the devices being controlled or the instrument from which data is being gathered. DCS systems have evolved into systems providing very sophisticated analog (e.g. loop) control capability. A closely integrated set of operator interfaces (or man machine interfaces) is provided to allow for easy system configurations and operator control. The data highway is normally capable of fairly high speeds (typically 1 Mbps up to 10 Mbps).



C. Programmable logic controller (PLC)

Since the late 1970s, PLCs have replaced hardwired relays with a combination of ladder–logic software and solid state electronic input and output modules. They are often used in the implementation of a SCADA RTU as they offer a standard hardware solution, which is very economically priced.



Another device that should be mentioned for completeness is the smart instrument which both PLCs and DCS systems can interface to.

D. Smart instrument

Although this term is sometimes misused, it typically means an intelligent (microprocessor based) digital measuring sensor (such as a flow meter) with digital data communications provided to some diagnostic panel or computer based system.



This tutorial will henceforth consider DCS, PLC and smart instruments as variations or components of the basic SCADA concept.

SCADA (Hardware and Software)

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 process. Other benefits include more efficient, reliable and most importantly, safer operations. This results in a lower cost of operation compared to earlier non-automated systems.

On a more complex SCADA system there are essentially five levels or hierarchies:
• Field level instrumentation and control devices
• Marshalling terminals and RTUs
• Communications system
• The master station(s)
• 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 provides the pathway for communication between the master station and the remote sites. This communication system can be wire, fiber optic, radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data.

The master station (or sub-masters) gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites. In large telemetry systems, sub-master sites gather information from remote sites and act as a relay back to the control master station.

SCADA software

SCADA software can be divided into two types, proprietary or open. Companies develop proprietary software to communicate to their hardware. These systems are sold as ‘turn key’ solutions. The main problem with this system is the overwhelming reliance on the supplier of the system. Open software systems have gained popularity because of the interoperability they bring to the system. Interoperability is the ability to mix different manufacturers’ equipment on the same system.

Citect and WonderWare are just two of the open software packages available in the market for SCADA systems. Some packages are now including asset management integrated within the SCADA system. The typical components of a SCADA system are indicated in the next diagram.



Key features of SCADA software are:
• User interface
• Graphics displays
• Alarms
• Trends
• RTU (and PLC) interface
• Scalability
• Access to data
• Database
• Networking
• Fault tolerance and redundancy
• Client/server distributed processing

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 information 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.



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

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.



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 on-board. Typically, an IED could combine an analog input sensor, analog output, PID control, communication system and program memory in one device.



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