Basically, SCADA represents the entire central system which consists of the following components:
Input/Output (IO) Devices
IO devices are instruments such as sensors, actuators or control devices. These devices are normally installed at field to collect signals and measurements (e.g. temperature transmitter).
The direct signals from these IO devices can be in either analogue or digital. Analogue IO usually refers to measurement from instruments (e.g. flow meter, pressure transmitter, weighing scale, etc). While digital IO could be flow meter’s pulses, device status, command bit, detector switches, etc.
Remote Terminal Units (RTU)
RTU in a SCADA system plays the role to scan and collect signals from IO devices and convert them to digital data. These data will be collected at the determined polling rate, normally set based on both devices by operators.
The converted data will then be sent to supervisory system for data acquisition and processing. As compared to PLC (Programmable Logic Controller), RTUs are more usually suitable for wider area telemetry that uses wireless communications.
Programmable Logic Controller
Similar to RTU, PLC also connects to IO devices to obtain real time signals, then transfer them as digital data to supervisory system. They are more popular among local area control plants, which is different from RTUs.
Another most significant difference between RTU and PLC is the size and capacities, RTU normally has more inputs/outputs than PLC. Besides, the configuration of PLCs usually required extra programming or script writing in order to perform any logical task.
Communication network is the link that connects everything together. This includes the connection between RTU/PLC and the supervisory system, which serves as the transportation infrastructure for all the acquired digital data and commands.
Other than that, different devices within a SCADA system shall require to communicate via the network (read/write) over the required communication protocol. The examples of communication network media are LAN (Local Area Network) and WAN (Wide Area Network).
Supervisory system is the central system which transmits/receives and processes the digital data from RTU/PLC. It acts as the brain to send out commands to the operation.
A supervisory system shall comprise of a database or storage server, in order to store necessary data and retrieve them for information, trend plotting, reporting or other analysis purposes.
Human Machine Interface
HMI is the front-end interface used by operators to interact with the supervisory system to monitor and control the automation process. Basically, HMI screens shall consist of SCADA mimics, alarms, reports, and trends for analyzing purpose.
HMI screens can be equipped with user authentication for security purpose, where different user role shall have different access rights. For example, an operator may only monitor a treatment plant process, while a supervisor is allowed to start or stop the treatment process.
source - integraxor
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