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Fieldbus High Speed Digital Communications

Profibus communications are supported by a number of products from Eurotherm. This ‘fieldbus’ system allows very high speed digital communications using an enhanced RS485 wiring technology, and has become a de facto standard in factory.

Profibus Variants

Profibus DP

This is designed for fast, cyclic, transfer of time critical data from intelligent devices such as temperature controllers, I/O units, drive, etc to a PLC or PC based controller, with a scan time of around 10mS. Applications are typically in industrial automation, such as extrusion, bottling, and baking, amongst many others.

Profibus PA

Profibus PA is an enhanced variant of Profibus DP aimed at the process industry. PA adds an intrinsically safe wiring system which also provides power for loop powered devices, as well as a protocol enhancements designed to provide the global access to plant data required by large scale DCS installations.

Profibus FMS

Profibus FMS provides a plant wide communications network, designed for factory integration and PLC to PLC communications. Network scan times are of the order of 100mS. Eurotherm currently supports the Profibus DP variant in its products.

Why Use Profibus DP?

Traditional PLC based control systems use a central processor unit fitted in a rack, connected to I/O modules via an internal PLC ‘backplane’. Terminals on the I/O modules are connected to the real world using point to point cabling, ending at field terminal units. The PLC scans the input modules cyclically and places the values from them into registers in the central processor, where they are operated on by ladder logic. Similarly, values from PLC registers are provided to output modules and applied to physical outputs.

‘Traditional’ PLC Control System

In a fieldbus based system, the internal backplane is replaced by a single wire, which is used to pass data between devices using high speed digital communications. The modules are replaced with devices which may be physically located close to their point of application, allowing enormous savings in wiring cost, and dispensing with the need for separate field terminal units. Typical savings in terms of wiring cost reduction are 35 – 40% of total installation cost. However there are additional benefits.

The simplicity of the wiring scheme makes for easier maintenance, and provides flexibility to add new devices when and where needed. Furthermore, instead of being tied to the offerings of a single vendor, ‘best of breed’, specialised, devices may be used, which reduce the processing load on the PLC. With devices located close to their point of application, and closely coupled to it, they may be used to provide feedback on their operation, reducing the need for planned maintenance and allowing quality improvement using statistical process
control techniques.

Profibus DP was chosen by Eurotherm for initial fieldbus product offerings because it has proven to be very easy to apply and understand, even for non specialists, and is supported by a very large and growing number of device and PLC vendors. Profibus DP systems
are now being constructed on a very routine basis without problems.

For Eurotherm Profibus configuration software click here.

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