SCADA as a System
There are many parts of a working SCADA system. A SCADA system usually includes signal hardware (input and output), controllers, networks, user interface (HMI), communications equipment and software. All together, the term SCADA refers to the entire central system. The central system usually monitors data from various sensors that are either in close proximity or off site (sometimes miles away).
The core of a SCADA system are the Remote Terminal Units (sometimes referred to as the RTU). The Remote Terminal Units consists of a programmable logic controller PLC and input and output terminations. The RTU are usually set to specific requirements, however, most RTU allow human intervention, for instance, in a office setting, the RTU might control the setting of an air conditioning plant, and the temperature can be changed or overridden at any time by human intervention. In addition, any changes or alarms are automatically logged and displayed. Usually a SCADA system will monitor the system and only make changes to the system when required. SCADA systems are considered closed loop systems and run with little or no human intervention.
The main advantage of a SCADA system is its ability to monitor an entire system in real time. This is facilitated by data acquisitions including meter reading, checking statuses of sensors, etc that are communicated at regular intervals. Besides the data being used by the RTU, it can also be displayed to a human that is able to interface with the system to override settings or make changes when necessary.
SCADA can be seen as a system with many inputs called points. Usually each point is a monitor or sensor. Usually points can be either hard or soft. A hard data point can be an actual monitor; a soft point can be seen as an application or software calculation. Data elements from hard and soft points are usually always recorded and logged to create a time stamped alarm or history.
SCADA systems are an extremely advantageous way to run and monitor processes. They are great for small applications such as climate control or can be effectively used in large applications such as monitoring and controlling a large Data Centre.
User Interface (HMI)
A SCADA system includes a user interface, usually called Human Machine Interface (HMI). The HMI of a SCADA system is where data is presented to be viewed and monitored by a human operator. This interface usually includes controls where the individual can interface with the SCADA system.
HMI's can also be linked to a database, which can use the data gathered from RTU's to provide graphs on trends, logistic info, schematics for a specific sensor or machine. Nowadays , practically all SCADA systems include an integrated HMI and PLC device making it extremely easy to run and monitor a SCADA system.
