A primary differentiator between a SCADA system and other types of control systems such as DCS is the purpose to which the control system will be put:
• In general DCS is focused on the automatic control of a process, usually within a confined area. The DCS is directly connected to the equipment that it controls and is usually designed on the assumption that instantaneous communication with the equipment is always possible.
• A SCADA system is usually supplied to permit the monitoring and control of a geographically dispersed system or process. It relies on communication systems that may transfer data periodically and may also be intermittent. Many SCADA systems for high-integrity applications include capabilities for validating data transmissions, verifying and authenticating controls and identifying suspect data.
DCS often operates with a ‘state’ paradigm: the system relies on the ability to obtain an immediate view of the current state of the system at any time. SCADA systems in many industries (especially electric power) rely on an ‘event reporting’ paradigm where even transitory or ‘fleeting’ changes in the state of the plants are reported.
In view of this different messaging protocols and formats are used in different industries and applications. In the DCS arena, the Bus protocols (Modbus, Fieldbus, Profibus, etc) and the slew proprietary protocols are prevalent. These are suitable for the requirements of DCS I/O. in the SCADA arena, the most commonly used protocols are DNP3, IEC 60870-5-101, Modbus variants and proprietary protocols. Some specific applications such as gas metering also have specific protocols designed to meet their needs.