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The system consists of central PLC (Programmable Logic Controller) which is connected to several remote I/O units through Ethernet Network or other networks (Modbus,Canbus etc).
The central PLC can optionally be a redundant system consisting of two central PLCs , one in operation and the other in standby mode. This ensures system continuous availability in case of one CPU failure.
The network can also be optionally redundant (through redundant loop architecture).
This architecture assures the continuous availability of the network in case of broken network cable.
Distances of LANs are often limited and there is often a need to increase this range. There
is a number of interconnecting devices, which can be used to achieve this ranging from
repeaters to routers to gateways. It may also be necessary to partition an existing network
into separate networks for reasons of security or traffic overload.
These components to be discussed separately are:

  • Repeaters: A repeater operates at the physical layer of the OSI model and simply retransmits an incoming electrical signal.
  • Bridges: Used to connect two separate networks to form a logical network.
  • Routers: Used to transfer data between two networks that have the same network layer protocols (such as TCP/IP) but not necessarily the same physical or data link protocols.
  • Gateways: A gateway is designed to connect dissimilar networks.
  • Hubs: Used to implement physical star networks for 10BaseT and token ring systems in such a way that electrical problems on individual node-to-hub links would not affect the entire network.
  • Switches: enables direct communications between multiple pairs of devices in full duplex mode; thus eliminating the limitations imposed by the classical Ethernet architecture.

Sensors are connected locally to Remote I/O Units or other local PLCs.
Sensors can be of the following types :

  • Analog inputs : Range of 4–20 mA (other possibilities are 0–20 mA/±10 volts/0–10 volts)
  • Analog outputs : Range from 4–20 mA/± 10 volts/0 to 10 volts
  • Counter inputs
  • Digital inputs
  • Digital outputs

All measurements are transferred to the central PLC over the available network.
This architecture reduces the length of cabling from local sensors to central PLC.
A SCADA (Supervisory Control and Data Acquisition) system is a system
consisting of a number of remote terminal units (RTUs) collecting field data connected
to a master station via a communications system. The master station displays the
acquired data and 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. This results to a more efficient, reliable and most
importantly, safer operations.


  • 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 locally or remotely via Internet or satellite
  • Historical archiving
  • System consideration
  • Reliability/availability
  • Speed of communications/update time/system scan rates
  • System redundancy
  • Expansion capability
  • Application software and modeling
  • User interface
  • Graphics displays
  • Alarms
  • Trends
  • RTU (and PLC) interface
  • Scalability
  • Access to data
  • Database
  • Networking
  • Fault tolerance and redundancy
  • Client/server distributed processing


  • Keyboard
  • Mouse
  • Trackball
  • Touch screen
  • Graphics displays HMI panels can be installed anywhere in the network for local operations


  • Client server architecture
  • Time stamped alarms to 1 millisecond precision (or better)
  • Single network acknowledgment and control of alarms
  • Alarms are shared to all clients
  • Alarms displayed in chronological order
  • Dynamic allocation of alarm pages
  • User-defined formats and colors
  • Up to four adjustable trip points for each analog alarm
  • Deviation and rate of change monitoring for analog alarms
  • Selective display of alarms by category (256 categories)
  • Historical alarm and event logging
  • Context-sensitive help
  • On-line alarm disable and threshold modification
  • Event-triggered alarms
  • Alarm-triggered reports
  • Operator comments can be attached to alarms


  • Client server architecture
  • True trend printouts not screen dumps
  • Rubber band trend zooming
  • Export data to DBF, CSV files
  • X/Y plot capability
  • Event based trends
  • Pop-up trend display
  • Trend gridlines or profiles
  • Background trend graphics
  • Real-time multi-pen trending
  • Short and long term trend display
  • Length of data storage and frequency of monitoring can be specified on a per-point basis
  • Archiving of historical trend data
  • On-line change of time-base without loss of data
  • On-line retrieval of archived historical trend data
  • Exact value and time can be displayed
  • Trend data can be graphically represented in real-time