2.  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.

4.  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

6.  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.

7.  PLCs have replaced hardwired relays with a combination of ladder–logic software and solid state electronic input and output modules.

8.  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.

9.  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  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.

11.  Typical RTU hardware modules include:  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)  Analog input modules  There are five main components making up an analog input module. They are:  The input multiplexer  The input signal amplifier  The sample and hold circuit  The A/D converter  The bus interface and board timing system

13.  These are used to indicate items such as status and alarm signals.

14.  Digital output module  A digital output module drives an output voltage at each of the appropriate output channels with three approaches possible:  Triac switching  Reed relay switching  TTL voltage outputs

15.  The modern RTU should be flexible enough to handle multiple communication media such as:  RS-232/RS-442/RS-485  Dialup telephone lines/dedicated landlines  Microwave/MUX  Satellite  X.25 packet protocols  Radio via trunked/VHF/UHF/900 MHz

16.  The advantage of a PLC over the RTU offerings from various manufacturers is that it can be used in a general-purpose role and can easily be set up for a variety of different functions.  PLCs are popular for the following reasons:  Economic solution  Versatility and flexibility  Ease of design and insta  More reliable  Sophisticated control  Physically compact  Easier troubleshooting and diagno

17.  The central site/master station can be pictured as having one or more operator stations connected to a communication system consisting of modem and radio receiver/transmitter.  The features that should be available are:  Operator interface to display status of the RTUs and enable operator control  Logging of the data from the RTUs  Alarming of data from the RTU  master station has two main functions:  Obtain field data periodically from RTUs and submaster stations  Control remote devices through the operator station

19.  The master station has the following typical functions:  Establishment of communications  Configure each RTU  Initialize each RTU with input/output parameters  Download control and data acquisition programs to the RTU  Operation of the communications link  If a master slave arrangement, poll each RTU for data and write to RTU  Log alarms and events to hard disk (and operator display if necessary)  Link inputs and outputs at different RTUs automatically  Diagnostics  Provide accurate diagnostic information on failure of RTU and possible problems  Predict potential problems such as data overloads

21.  It may also be necessary to set up a submaster station. This is to control sites within a specific region.  The submaster station has the following functions:  Acquire data from RTUs within the region  Log and display this data on a local operator station  Pass data back to the master station  Pass on control requests from the master station to the RTUs in its region

23.  Point-to-point (two stations)  Multipoint (or multiple stations)

24.  Relay Stations

25.  Communication philosophies  There are two main communication philosophies possible. These are; polled (or masterslave) and carrier sense multiple access/collision detection (CSMA/CD).