The terms seems to have changed, but historically, this is where the names came from.
DCS - Distributed Control System. A control system that distributed the controls across multiple processors. They could be in different locations throughout the plant close to I/O, but were typically in a central location, and the I/O was remote, connected by a network. DCS systems were used mostly for the control of analog systems. They were slow and expensive. Most Analog systems could be controlled with slow control systems, with update times usually running from 1 to 10 seconds per update. They could also do discrete control, but due to the costs of the systems, most discrete control ran on less expensive, and faster PLC systems.
They had HMI integrated in the system. The tags/blocks used to set up the control loops had corresponding graphics/faceplates. The graphics was well integrated with the controls configuration. They also may have included a historian for trending.
PLC - Programmable logic controller. A system set up to replace hard wired relays. The were expensive, but not as expensive as DCS systems. They were typically used for discrete. The analog was expensive. They didn't have HMIs integrated with them, so analog controls were hard to monitor. Typically, controls were monitored via lights. Inputs were made through pushbuttons and thumbwheels. Later, MMIs, Man Machine Interphases became available. These were usually in the form of a panel based display. This was a huge improvement over Lights and thumbwheels. This allowed the PLCs to start doing some of the analog controls that used to be done primarily with DCS. However, the panels were not well integrated like the DCS. The tags needed to be exported from the PLC, and imported into the panel configuration software.
HMI - Human Machine Interphase. Same thing as an MMI. As time moved on, software became available that allowed personal computers to be used for operator interphases. Multiple PLCs and HMIs could be set up to provide the same control functions of a DCS for less equipment costs, but the integration of the HMI and the controls was not as easy as using a DCS.
RTU - Remote Terminal Unit. These were controls put out in remote locations, ie, off site, often hundreds of miles away from the control rooms. They control the off site equipment. They are set up so that if communications is lost, the process continues on, but is set up to shut the equipment down due to abnormal conditions. The control could be PLCs, but are often a less general controller set up for the specific equipment and industry.
SCADA - Supervisory Control And Data Acquisition. A broad term for a control system, with an HMI or Historian. This term is typically used by new engineers and IT staff that do not understand the control systems they are talking about.
PAC - Programmable Automation Controllers. PLC that is integrated to an HMI. These look a lot like DCS systems.
