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Home / Technical Articles / Five Terms You MUST Be Familiar With: SCADA, DCS, PLC, RTU and Smart Instrument

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This technical article shed light on terminology used in the field of SCADA and industrial automation. The terms SCADA, distributed control system (DCS), programmable logic controller (PLC), remote terminal unit (RTU) and smart instrument are very important when we’re talking about concept of a industrial telemetry system.

Five Terms You MUST Be Familiar With: SCADA, DCS, PLC, RTU and Smart Instrument
Five Terms You MUST Be Familiar With: SCADA, DCS, PLC, RTU and Smart Instrument (on photo: PLC panel)

Let’s shed a light on these five terms:

  1. SCADA
  2. Distributed control system (DCS)
  3. Programmable logic controller (PLC)
  4. Remote terminal unit (RTU) and
  5. Smart instrument

1. 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 below.

Diagram of a typical SCADA system
Figure 1 – Diagram of a typical SCADA system

On a more complex SCADA system there are essentially five levels or hierarchies:

  1. Field level instrumentation and control devices
  2. Marshalling terminals and RTUs
  3. Communications system
  4. The master station(s)
  5. 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.

SCADA system
Figure 2 – SCADA system

Considerations 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

Benefits of a SCADA system

Obviously, a SCADA system’s initial cost has to be justified. A few typical reasons for implementing a SCADA system are:

  1. Improved operation of the plant or process resulting in savings due to optimization of the system
  2. Increased productivity of the personnel
  3. Improved safety of the system due to better information and improved control
  4. Protection of the plant equipment
  5. Safeguarding the environment from a failure of the system
  6. Improved energy savings due to optimization of the plant
  7. Improved and quicker receipt of data so that clients can be invoiced more quickly and accurately
  8. Government regulations for safety and metering of gas (for royalties & tax etc)

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2. Distributed control system (DCS)

Definition – 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).

Distributed control system (DCS)
Figure 3 – Distributed control system (DCS)

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3. 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.

Programmable logic controller (PLC) system
Figure 4 – Programmable logic controller (PLC) system

How Are Field Devices Wired to PLCs (VIDEO)

Part I


Part II

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4. 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 5:

Typical RTU hardware structure
Figure 5 – Typical RTU hardware structure

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

Typical requirements for an RTU system:

In the writing of a specification, the following issues should be considered:


Hardware:

Individual RTU expandability (typically up to 200 analog and digital points)

  • Off the shelf modules
  • 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)
  • Heat generation minimized
  • Rugged and of robust physical construction
  • Maximization of noise immunity (due to harsh environment)
  • Temperature of –10 to 65°C (operational conditions)
  • Relative humidity up to 90%
  • Clear indication of diagnostics
  • Visible status LEDs
  • Local fault diagnosis possible
  • Remote fault diagnostics option
  • 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
  • Ease of module replacement
  • Removable screw terminals for disconnection and reconnection of wiring

Environmental considerations

The RTU is normally installed in a remote location with fairly harsh environmental conditions.

Typically it 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.

Software (and firmware)
  • 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:
    • System setup
    • Hardware and software setup
    • Application code development/management/operation
    • Error logs
    • Remote and local operation

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5. Smart instrument

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

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.

Typical example of a smart instrument
Figure 6 – Typical example of a smart instrument

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Reference // Practical SCADA for Industry by David Bailey, Edwin Wright

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About Author

author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

10 Comments


  1. Erric Ravi
    Sep 13, 2018

    Supervisory Control and Data Acquisition mechanism plays an important role in determining the processes and control system. It further sends processed information to supervising unit for analytics. Schneider Electric India has helped businesses have a clear vision of machinery operations with SCADA. https://www.schneider-electric.co.in/en/product-category/6000-telemetry-and-remote-scada-systems/


  2. Rajesh Yadav
    Jul 31, 2018

    Thanks a lots sir it is great information regarding automation filed it’s helped me too much


  3. Srinivas
    May 27, 2018

    Thanks a lot for a well-written article. Keep up the good work.


  4. RAMARAJAN VAITHYNATHAN
    Apr 27, 2018

    In figure 6, ‘typical example for for smart instrument’ and in “modular /demodular block diagram, the first A/D(receiving signal from field device) is supposed to be mentioned as analog to digital converter and the last D/A is to be mentioned as digital to analog converter instead.


  5. AQIL NOOR KHAN
    Apr 26, 2018

    Instrumentation & Control in EHV Electrical Network or in Industrial activities Scenario are Back Bone and playing a Key Role for Safe,Reliable & Uninterrupted Operation & Maintenance
    The article is reflecting the better concept of Industrial Telemetry system for an engineer working in the Field. The efforts of the Author is appreciated to comprised all Terminology.


  6. Cristi Negrut
    Apr 26, 2018

    Consider professional useful all your technical articles, even though my most activity area is in Europe, US standard of electrical and automations wich I’ve met in many projects and your engineering literature
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  8. FRANCE MPONDA
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  9. Suresh Babu TS
    Apr 25, 2018

    From my experience the main reasons for failure of SCADA are due to too much of complexity and lack of awareness among Engineers at the lower end in the system.

    The write up was useful as an introductory narrative!


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    Apr 25, 2018

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