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PLC
Programmable Logic Controllers are devices which are widely used to perform data acquisition and control functions in industrial applications. They can be programmed with special purpose languages to perform acquisition cycles, to react to the changing of status lines, to provide alarms and to control various kinds of hardware devices.
Modbus RTU
Modbus RTU is a very simple and robust protocol to exchange data between two programmable devices. It was originally developed by Modicon for its Programmable Logic Controllers, but has been adopted by many other vendors because its specifications have been made publicly available.
In its original form, it allows a master unit to issue data exchange commands to a number of slave units cascaded on a single serial line. Commands are essentially read or write requests of blocks of data. Only two data types are supported: word (16 bits) and bit. Each command includes a unit identification number so that only the addressed slave unit of the many which receive the request will reply to it.
Recently the specification has been updated to allow the protocol to be transported by a TCP/IP connection.
SCADA
SCADA is the acronym for ``System Control and Data Acquisition'' and usually refers to software systems used to centralize the gathering of data coming from a variety of sources (many subsystems of a plant or even many plants) in order to perform global coordination of the production processes.
In a typical production plant a SCADA system (based on data related to the availability of raw materials and storage space, production rates, status of the individual machines, customer orders, trucks' movement, waste disposal and so on) could program production, maintenance, purchasing etc. optimizing the whole process.