Thursday, October 20, 2011

The Functions of Fieldbus in Programmable Logic Controller System

A fieldbus is the medium of communication utilized in automation systems industry and in process control to interrelate subsystems that are spatially separated. Dispersed control permits local, hierarchical control. There is a significant benefit of such a non centralized control approach: it prevents extensive cabling and high processing power. The subsystems control can be placed close to the actuators and sensors in the site. An instance of a network of fieldbus is an assembly line of automobile, where the fieldbus interrelates controllers situated at every stage of assembly.

A fieldbus basic composition
A Programmable Logic Controller system has a hierarchical organization in which the upper levels of the network of fieldbus utilize Ethernet based networking. This ladder joins with the other IT systems of corporate management. The Fieldbuses are digital, serial networks, bidirectional. CANopen, Ethernet, Interbus, Modbus, CCLINK, ControlNet, PROFIBUS, and DeviceNet are fieldbus networks examples. PROFIBUS DP has grown to be one of the most frequently used fieldbuses for automation of factory. PROFIBUS DP is mainly utilized to multiple controller network in decentralized locations.

The fieldbus substantial layer is generally based on CAN, Ethernet, and RS-485. The fieldbus joins to a subsystem of PLC with a module of fieldbus. The module of fieldbus connects the system backplane of PLC to the fieldbus. The backplane which is general to all modules in the system of PLC can be based on full-duplex or half- RS-485. The RS-485 is perfectly matched to backplane interrelate in applications of industrial because of its tolerance of high-EMI, hot-plug, and high speed capability.

In the module of fieldbus the controller translates the protocol of backplane to the protocol of fieldbus. UARTs or Universal asynchronous receiver-transmitters describe the data rates of fieldbus, make sure data integrity, and interface to either the PROFIBUS or RS-485 transceivers. Callous conditions characteristic of industrial applications can create defending devices a challenge and the interface cabling. It is critical, hence, that both the system and device(s) withstand callous conditions. Only in this method can one make sure that the system signal of PLC reliability and integrity are maintained.

To guarantee that the system is secured in environments of harsh industrial, the designers of PLC need to integrate fairly detailed safeguards:
• Protection of Fault: up to ±80V
• Line termination to decrease indications on the cables
• Isolation to let great common mode ground differentials
• High ESD Protection: up to ±20kV (Air Gap) and ±35kV (HBM)
• Temperature grade of Automotive -40°C to +125°C.


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