- Process control network
A Process Control Network (PCN) is a communications network that is used to transmit instructions and data between control and measurement units and Supervisory Control and Data Acquisition (
SCADA ) equipment.These networks have, over the years, used many of the technologies and topologies utilised in other network applications. However, Process Control Networks (PCNs)have several special requirements that must be met in order for the solution to be acceptable to the industry. These requirements are, in no particular order: Robustness, Determinacy, Compatibility. Robustness includes requirements such as connection redundancy, reduced sensitivity to Electromagnetic Interference (EMI), and good error checking and correction. Determinacy involves assuring that each device is guaranteed access to the network, and in many cases mechanisms to allow priority information (such as alarms) through the system. Compatibility allows SCADA andDistributed Control System s (DCS) from various manufacturers to communicate with control and measurement equipment from others.Many early PCNs were serial based, using low level standards such as EIA
RS-422 and EIARS-485 with proprietary protocols on top. One of thede facto standards (which is now becoming anopen standard ) isModbus , originally from Modicon. Many PCNs usedtoken ring token passing based protocols because they are essentially deterministic. BothAllen Bradley andEurotherm utilised such mechanisms. Many of the measurement and control unit manufacturers signed up to the Fieldbus consortium but rather than one standard emerging each manufacturer promoted their own 'standard' leading to a myriad, and confusing, range of physical andlogical system s.
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