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PROFIBUS: EVOLUTION OF CONTROL SYSTEMS INTO FIELD-BUS TECHNOLOGY

PROFIBUS: EVOLUTION OF CONTROL SYSTEMS INTO FIELD-BUS TECHNOLOGY

Industrial automation technology has evolved over the years from control of field devices using I/O signal modules to bus network communication systems called FIELDBUS. In this article, we would highlight the advantages of FIELDBUS control systems over the traditional voltage and current control methods, with PROFIBUS as the case study.

A case in point would be the control of simple Variable Frequency Drive(VFD). A typical VFD has at least the following digital parameters: Start/Run, Stop, Forward, Reverse. This would require digital input and output signal modules and and several lengths of running wires. The analogue signal parameters are the frequency command from the PLC and frequency feedback. These would be connected the PLC analogue output and input cards respectively with reasonable lengths of running wires. For extra control or data acquisition even more wires are required. A Fieldbus system such as PROFIBUS would be able to transmit all these parameters over a single line of communication cable in addition to giving access to more parameter control/monitor and diagnostic data with no extra installation or wiring costs and hassles.

This is made possible because the PLC directly accesses the memory of the field devices to modify or read parameters in place of running control wires externally which does Digital to Analogue(DA) or Analogue to Digital(AD) conversions.

The Fieldbus communication protocol used for communication on the bus must be supported by both the PLC(s) and devices on the bus. This is because the PLC connects to and communicates with each field I/O device according to the bus’s protocol.

Each device on the bus has a unique address because the PLC uses an I/O bus network scanner which reads and writes to each field device using this address. Multiple devices can be looped to create a network of fieldbus devices that communicate with each other building a more intelligent automation system. Devices can be added and removed from the network with very little hassle.

Fieldbus cables can also transmit data over long distances (1200m in the case of PROFIBUS) making them suitable for use in remote i/o situations. There is also a variant called "intrinsically safe" which are used in hazardous environments.

There are different fieldbus variants available from different manufacturers, some of the most popular being PROFIBUS, MODBUS, INTERBUS, Foundation Fieldbus, DeviceNet, ControlNet, CanOpen and a host of others. Manufacturers are adding fieldbus capabilities to most of their new devices and are creating special modules for old devices without inherent fieldbus capability.

PROFIBUS

PROFIBUS is the serial fieldbus implementation created by SIEMENS. Although it is majorly used by Siemens devices, over the years it has also become an open protocol adopted by other manufacturers as well. PROFIBUS stands for PROcess FIeld BUS. It is based on an RS-485 network with slight modifications. It uses standardized shielded cables which are easily recognized by their purple color and standardized connectors. This means the wiring is the same for any device that implements PROFIBUS unlike some of its other counterparts such as Modbus.

PROFIBUS operates a Master-Slave system and can accommodate up to 126 devices (32 devices per segment) on a network with a maximum distance of 4000ft. Devices can either be masters or slaves but not both.

In the video on the page you'll find a deeper explanation on PROFIBUS along with a demonstration of controlling a drive using PROFIBUS.

There is a growing need for automation engineers that are well versed in the installation, configuration and troubleshooting of PROFIBUS systems since SIEMENS is arguably the leading industrial automation company worldwide.