Foundation Fieldbus and Profibus

Before we look at Foundation Fieldbus (FF) and Profibus let’s find out why digital signals are commonly used in industrial data transmission.

Digital signals can be transmitted without loss of integrity, via a hardwired parallel or serial bus, radio transmitter or fiber optics.

Digital data transmission speeds are higher than with analog data transmission.

Digital signals can be transmitted without loss of accuracy and can contain codes for limited automatic error correction or for automatic requests of data retransmission.

Digital transmitters consume less power as compared to analog transmission devices hence they are preferred for use in industrial communication systems over analog devices.

Foundation Fieldbus and Profibus

The Foundation Fieldbus (FF) and Profibus are the two most universal serial data bus formats that have been developed for interfacing between central processor and smart sensing devices in a process control system.

The FF is primarily used in the USA and the Profibus is primarily used in Europe.

Process control equipment is presently manufactured to accept either of these formats.

A serial data bus is a single pair of twisted copper wires, which enables communication between a central processing computer and many monitoring points and actuators when Smart Sensors are used.

Although initially more expensive than direct lead connections, the advantages of the serial bus include: Minimal bus cost and installation labour. The system replaces the leads to all the monitoring points by one pair of leads. New units can be added to the bus with no extra wiring i.e. plug and play feature, giving faster control. Programming is also the same for all the systems.

The accuracies achieved are higher than from using analog, and more powerful diagnostics are available.

The bus system uses time division multiplexing, in which the serial data word from the central processor contains the address of the peripheral unit being addressed in a given time slot, and the data being sent.

In the FF, current from a constant current supply is digitally modulated. Information on the FF is given in the ISA 50.02 standards.

One drawback of the FF is that a failure of the bus, such as a broken wire, can shut down the entire process, where with the direct connection method, only one sensor is disabled. This disadvantage can be overcome by the use of a redundant or backup bus in parallel to the first bus, so that if one bus malfunctions, then the backup bus can be used

You can also read: 

• Basics of Instrumentation and Control Systems
• How Digital Communication Technology is applied in measurement and control systems

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