Field data interface devices as eyes and ears of a SCADA system

Field Data Interface Devices – Eyes and Ears of a SCADA System
Field Data Interface Devices – Eyes and Ears of a SCADA System (photo credit: Festo)

It’s all about information //

Field data interface devices form the “eyes and ears” of a SCADA system. Devices such as reservoir level meters, water flow meters, valve position transmitters, temperature transmitters, power consumption meters, and pressure meters all provide information that can tell an experienced operator how well a water distribution system is performing.

In addition, equipment such as electric valve actuators, motor control switchboards, and electronic chemical dosing facilities can be used to form the “hands” of the SCADA system and assist in automating the process of distributing water.

However, before any automation or remote monitoring can be achieved, the information that is passed to and from the field data interface devices must be converted to a form that is compatible with the language of the SCADA system.

To achieve this, some form of electronic field data interface is required //

  1. Remote Terminal Units (RTUs)
  2. Programmable Logic Controllers (PLCs)

Remote Terminal Units (RTUs)

Remote Terminal Units (RTUs), also know as Remote Telemetry Units, provide this interface. RTUs are primarily used to convert electronic signals received from (or required by) field devices into (or from) the language (known as the communication protocol) used to transmit the data over a communication channel.

RTUs appear in the field as a box in a switchboard with electrical signal wires running to field devices and a cable link to a communication channel interface, such as a radio (see Figure 1 below).

RTU cabinet usually equipped with RTU, radio, and field wiring terminations
Figure 1 – RTU cabinet usually equipped with RTU, radio, and field wiring terminations

The instructions for the automation of field data interface devices, such as pump control logic, are usually stored locally. This is largely due to the limited bandwidth typical of communications links between the SCADA central host computer and the field data interface devices.

Such instructions are traditionally held within local electronic devices known as Programmable Logic Controllers (PLCs), which have in the past been physically separate from RTUs (see Figure 2 below).

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Programmable Logic Controller (PLC)

PLCs connect directly to field data interface devices and incorporate programmed intelligence in the form of logical procedures that will be executed in the event of certain field conditions. However, many water systems with SCADA systems have no PLCs. In this case, the local control logic is held within the RTU or in relay logic in the local switchboard.

PLCs have their origins in the automation industry and therefore are often used in manufacturing and process plant applications.

The need for PLCs to connect to communication channels was not great in these applications, as they often were only required to replace traditional relay logic systems or pneumatic controllers.

SCADA systems, on the other hand, have origins in early telemetry applications, where it was only necessary to know basic information from a remote source. The RTUs connected to these systems had no need for control programming because the local control algorithm was held in the relay switching logic.

Programmable Logic Controller (PLC) performing local control functions, physically separated, but wired to a nearby Remote Terminal Unit (RTU)
Figure 2 – Programmable Logic Controller (PLC) performing local control functions, physically separated, but wired to a nearby Remote Terminal Unit (RTU) – (on photo PLC control cabinet with SIEMENS PLC SIMATIC S7-400; photo credit:

As PLCs were used more often to replace relay switching logic control systems, telemetry was used more and more with PLCs at the remote sites. It became desirable to influence the program within the PLC through the use of a remote signal. This is in effect the “Supervisory Control” part of the acronym SCADA.

Where only a simple local control program was required, it became possible to store this program within the RTU and perform the control within that device.

At the same time, traditional PLCs included communications modules that would allow PLCs to report the state of the control program to a computer plugged into the PLC or to a remote computer via a telephone line. PLC and RTU manufacturers therefore compete for the same market.

As a result of these developments, the line between PLCs and RTUs has blurred and the terminology is virtually interchangeable. For the sake of simplicity, the term RTU will be used to refer to a remote field data interface device; however, such a device could include automation programming that traditionally would have been classified as a PLC.

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Field Data Communications System

The field data communications system is intended to provide the means by which data can be transferred between the central host computer servers and the field-based RTUs.

Bandwidth. An important property of a communications channel is its capacity to carry data. The term bandwidth is used to describe this capacity. Originally, the term bandwidth applied to the width, in Hertz, of an analog channel.

For example, a telephony voice channel that occupies the nominal band 0.3 to 3.4 kHz has a bandwidth of 3.1 kHz and a radio channel that occupies the spectrum from 929.88875 to 929.8875 MHz has a channel bandwidth of 12.5 kHz.

With digital transmission, the term bandwidth has been extended to include the data transmission rate in bits per second (bps).

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SCADA Systems for electric power industry // VIDEO

Here’s an animated diagram of a scada monitored electrical network, providing electricity from a regional substation to a number of other key areas and communities, including a residential area, commercial city district and a local airport. A busy port and a small rural community are also supplied by the primary substation through another circuit.

This demonstration walk through video shows how the scada system is used to manage and maintain the network intelligently following a fault.

When the fault occurs, the scada system receives notification of the circuit breaker trip and uses fault passage indicators placed around the network to determine where the fault has occurred.

Cant see this video? Click here to watch it on Youtube.

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Reference // The Fundamentals of SCADA – Bentley Systems

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About Author //


Edvard Csanyi

Edvard - 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

One Comment

  1. Wasswa Noah
    May 13, 2015

    Hey Edvard
    Thanks for the very useful portal for electrical engineering.
    I am a young electrical engineering student about to graduate in a 1 month from today, and I want to say, I have always been following your articles. I find them so invaluable to me and my future career ahead
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