Supervisory control and data acquisition (SCADA) is a system for remote monitoring
and control that operates with coded signals over communication channels (using
typically one communication channel per remote station). The control system may
be combined with a data acquisition system by adding the use of coded signals over
communication channels to acquire information about the status of the remote equipment
for display or for recording functions.
The term SCADA usually refers to centralized systems which monitor and control entire
sites, or complexes of systems spread out over large areas (anything from an industrial
plant to a nation). Most control actions are performed automatically by RTUs or
by PLCs. Host control functions are usually restricted to basic overriding or supervisory
level intervention. For example, a PLC may control the flow of cooling water through
part of an industrial process, but the SCADA system may allow operators to change
the set points for the flow, and enable alarm conditions, such as loss of flow and
high temperature, to be displayed and recorded. The feedback control loop passes
through the RTU or PLC, while the SCADA system monitors the overall performance
of the loop.
Hardware solutions SCADA solutions often have distributed control system (DCS) components.
Use of "smart" RTUs or PLCs, which are capable of autonomously executing simple
logic processes without involving the master computer, is increasing. A standardized
control programming language, IEC 61131-3 (a suite of 5 programming languages including
function block, ladder, structured text, sequence function charts and instruction
list), is frequently used to create programs which run on these RTUs and PLCs. Unlike
a procedural language such as the C programming language or FORTRAN, IEC 61131-3
has minimal training requirements by virtue of resembling historic physical control
The term supervisory station refers to the servers and software responsible for
communicating with the field equipment (RTUs, PLCs, SENSORS etc.), and then to the
HMI software running on workstations in the control room, or elsewhere. In smaller
SCADA systems, the master station may be composed of a single PC. In larger SCADA
systems, the master station may include multiple servers, distributed software applications,
and disaster recovery sites. To increase the integrity of the system the multiple
servers will often be configured in a dual-redundant or hot-standby formation providing
continuous control and monitoring in the event of a server malfunction or breakdown.