1:-PLC is a programmable Logic controller which is used mainly for interlocking different equipments.
DCS is Distributed Control Systems which is used to control very big plants by using simple GUI screens.PLCs are interfaced with DCS for interlocking.
SCADA is Supervisory Control and Data Aquisition which is used to control and Monitor small number of Equipments in a field.SCADA systems are typically used to perform data collection and control at the supervisory level. Some systems are called SCADA despite only performing data acquisition and not control.The supervisory control system is a system that is placed on top of a real-time control system to control a process that is external to the SCADA system (i.e. a computer, by itself, is not a SCADA system even though it controls its own power consumption and cooling). This implies that the system is not critical to control the process in real time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time constants of the process. The process can be industrial, infrastructure or facility based as described below
2:-PLC is programmable logic controller. Its inputs are the open or closed position of a switch.(It coulde a a manual switch, a relay contact, a limit/proximity switch etc). Its outputs are the presence(usually 24V dc) or the absence (0V)of a voltage. A program is written which relates the inputs to the outputs. ()The output can activate motors solenoid valves etc. The program could be reasonably complicated. Earlier its task was done by electromagnetic relays with a lot of messy wiring. SCADA is Supervisory Control and Data Acquisition which is more complicated and involves one or more PCs constantly monitoring a system providing information to the operator who can take control action wherever it is not automatic. DCS is digital control systems normally used to automate control of large process plants.
Some other differences between the two are the following:
Scan cycle - DCS' have much slower scan cycles than PLC's. This is because it is not normally needed to scan the software that quickly as the process would not be changing that fast.
Number resolution - The DCS number is made up different to the PLC number. Usually the number has a higher resolution i.e. more decimal points. This gives greater accuracy for mathematical operations releating to the control algorythms incorperated.
Cost - DCS are way more expensive than PLC.
In recent years PLC has started to replace DCS in some fields. This is part because of the huge cost of DCS as well as the fact that PLC has started to incorperate some of the DCS functions. Traditionally PLC was only used for ESD and/or logic control. They were not used for analogue signals. This has changed with many PLC ranges being used for traditional DCS control as well as logic control.
DCS VS PLC
Re: DCS VS PLC
