What is scada? Definition and Details paessler com/it-explained/scada it explained: scada back to index Content

SCADA components
Field devices
Field devices in SCADA systems, also called field instrumentation hardware, include
sensors, samplers, actuators, relays, control units, transmitters, and tranducers. Sensors
are the field devices that detect or measure physical properties like whether a machine is
on or off, or the level of fuel in a tank. Actuators are field devices that control a
component, for example switching off a valve. They are managed through RTUs, IEDs,
and PLCs.
RTUs, IEDs, and PLCs
RTUs, IEDs, and PLCs are physical, microprocessor-based field device controllers. Field
device controllers in SCADA systems monitor and collect data in real time from field
devices, transmit information from the devices’ location to a supervisory controller called
an MTU, and compile it for optimal presentation to system operators at a centralized
control hub manned by human operators. They are usually panel mounted and connect
via interfaces with field devices through I/O modules as well as with the central control
hub using serial or network communications.

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An RTU is a microprocessor-based electronic device. It consists of I/O hardware and a
communication interface and supports a wireless connection. RTUs are responsible for
transmitting data collected from field devices to the control hub and issuing commands
from the control center to field devices.
A PLC is a physically robust microcontroller that can withstand harsh physical conditions
as may be found in an industrial environment. A microcontroller is an integrated circuit
(IC) used to control specific functions of an electronic device. A microprocessor has a
central processing unit (CPU), whereas a microcontroller has a CPU, memory, and I/O
capabilities on one chip. A PLC provides the information that a SCADA system requires
to automate processes or respond to alerts according to coded instructions in SCADA
system programs and functions. PLCs replace the relays and timers of early SCADA
systems. Unlike relays, PLCs are able to monitor as well as control circuits and can be
programmed. Traditional SCADA systems almost always have PLCs. There are some
proprietary I/O modules that allow for simple monitoring and automations without a PLC.
PLCs that can communicate through radio waves can be used as a substitute for RTUs.
In SCADA systems, an IED is a microprocessor-based device used to transmit and
receive data from field devices. They are sometimes described as an upgrade to a
traditional RTU, the main differences being that an IED is integrated with the device it
monitors and controls and has built-in metering, data transmission, and computational
capabilities. Examples of IEDs are protective relaying devices, circuit breaker controllers,
and voltage regulators. IEDs are sometimes compared to PLCs. The main difference
between PLCs and IEDs is that PLCs are usually used for centralized (local) automation
tasks whereas IEDs are usually used for substation (remote) automation tasks.
The functionalities of RTUs, IEDs, and PLCs overlap and it can be confusing to
differentiate between them but they have distinct uses. RTUs are considered more
suitable than PLCs for remote geographical telemetry as they support wireless
communication, whereas PLCs are more suitable for local control tasks, for example in
factories and warehouses. In some cases, an IED may communicate directly with the
controller or the system may be set up so that an RTU polls the IED for the data and
passes it to the controller.
MTU
An MTU is also called a control server, master controller, or supervisory controller. The
MTU hosts the software that provides instructions about what must be done with data
gathered from RTUs and PLCs.