Talking the differences between SCADA (Supervisory Control and Data Acquisition) and DCS (Distributed Control System), they tend to involve a lot of details.
Some of differences point will be described as below:
SCADA does monitoring processes across multiple sites involve the collection of data to a single location. SCADA could be described by taking “DA” portion that is “Data Acquisition. Some control usually present but not a significant amount. SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system.
DCS is a single unit, or a group of local units. DCS system controls the process as a stand alone system. It has the control loops built into its own controller. The communication path will be through something like a LAN high speed Ethernet, or other communications network.
In data security and database maintenance you can see the differences between SCADA and DCS as below:
SCADA needs to get secure data and control over a potentially slow, unreliable communications medium, and needs to maintain a database for prompt operator display. It frequently needs to do event processing and data quality validation. Redundancy is usually handled in distributed manner.
DCS is always connected to its data source, so it does not need to maintain a database of current values. Redundancy is usually handled by parallel equipment, not by diffusion of information around a distributed database.