SCADA (Supervisory contol and Data Acquisition) is essentially a computer based (usually PC nowadays) system for displaying field data. The difference between it and HMI (Human Machine Inerface) software aren’t significant.Most SCADA systems are tag based, and a lot of HMI software is set up to read registers in PLCs directly. SCADA software like Wonderware or the iFix include graphics displays, trending, alarms, data logging, and recipes.They will work with PLC or DCS systems. I like to refer to most SCADA systems as “leg savers” because they collect and display the data without making a guy run around with a clip board. Most SCADA software is capable of control logic and recipe control, but these functions are often unused. SCADA is a software. Used to monitor,control and acquire data from field devices even from remote locations. its  I/p & O/p are represented in images.Each object is defined using name
The SCADA has to get its field process data from field devices, generally either a DCS or PLC based system. It often includes communications over serial link, but many use ethernet, radio telemtery, and leased line or dial-up phone lines. In the last couple of years remote I/O is also used for getting field data into the SCADA screen .
DCS has its roots in the process industry, where control was generally based on independent single loop PID controllers. DCS was a communications link to get the analog process data, and in some cases tuning info and status info, back to a central computer. The control logic resided in the controllers, and any programming was in proprietary and often obscure languages. The computer display evolved into today’s SCADA systems. The communications was almost always serial, and usually employed proprietary protocols. Bristol Babcock, Fisher Porter, and Foxboro were big names in this kind of product.
The original PLCs were relay replacers, communications was limited, and analog data functions were expensive and complicated. Over the years the ability of PLCs to handle math functions, analog data, and communcations has mushroomed, and now equal or exceed those of a DCS. DCS and PLC systems are becoming indistinguishable. Most DCS’s now have open protocols, and are physically starting to look like a PLC (Bristol Babcock’s Control Wave for example).In large plants the DCS is king because most owners want a single source of hardware support and service, and this mentality naturally denies the PLC a foothold. Package vendors are no longer required to provide PLC for their system. Everything is connected to the DCS. At the same time PLCs began to include DCS type functions like PID and multiple processors (Allen Bradley’s ControlLogix for example). PLCs are gaining market share in the process industries, but a lot of engineers still stick with DCS because of unique industry-specific functions. PLCs are generally, but not always, less expensive.
PLC is is a solid state device. It controls the output of the process through the program given in Ladder diagrams.I/p & O/p are represented in NO,NC and coil contacts.Each component involved are defined using Address. 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.  PLCs have their roots in manufacturing, where most control was done through relay logic and based on discrete status info from limit switches etc.
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