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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How Digital Communication Technology is applied in Measurement and Control Systems

The continued advancement in digital technology has revolutionized how data is acquired in measurement and control instruments. Data acquisition in instrumentation systems encompasses the measurement and recording of process data.

The ability of being able to relay or communicate large amounts of data over a limited number of channels gives digital technology an upper hand over Analog technology. For example in Analogy technology where  4-20 mA or 3-15 PSI signals are used, each pair of wires can communicate only one variable whereas in digital networks, one pair of wires can communicate a limitless number of variables, with the only limit to it being the speed of that data communication.

From the above, 4-20 mA analog signals can be expensive to utilize in especially for an instrument generating multiple variables of measurement like Coriolis mass flowmeters that measures Flow rate, density, and temperature at the same time. In this case you will need dedicated wire pair for each process variable. Digital Technology overcomes these shortcomings of 4-20 mA analog signals.

One solution to this problem is using HART digital signals superimposed on 4-20 mA signals; we normally call this 4-20 mA plus HART. With this, you retain the analog signals while at the same time enjoying the multi-variable communication benefits that comes with digital technology however wired-HART communication is rather slow by any standard, and this restricts its use to maintenance {range changes, diagnostic data polling} and used only for slow process control processes.

Examples of digital communication standards include:

• Modbus
•  HART
•  FOUNDATION Fieldbus
• Profibus PA
• Profibus DP
• Profibus FMS
• AS-I
•  ControlNET
•  DeviceNet
• BACnet
• LonWorks
• CANbus

Some of the digital communication instruments find common use in distributed control systems (DCS) applications.

SCADA (Supervisory, Control and Data Acquisition) systems use digital communication technology. SCADA is similar to DCS, but it is spread over a large geographical area whereas DCS may cover only a plant floor. You will find SCADA systems applied in areas like:

•  Gas and oil exploration and distribution (pipeline) systems
• Electric power generation and distribution (power line, substation) systems.
• Water and wastewater treatment and distribution lines (water line, pumping stations) systems.
• Large irrigation or harvesting systems.

The process data in a SCADA system is sensed by various measurement devices (transmitters) converted to digital form by an RTU (Remote Terminal Unit), and communicated to one or more MTUs (Master Terminal Units) at a central location where we have human operators monitoring the data and at the same time make command decisions.

In a system where we have the flow of information just in one way (simplex) from the measurement devices to human operators, the system may be referred to as a Telemetry system rather than a SCADA system.

SCADA implies two-way (Duplex) information flow, where human operators not only monitor the process data but also issue commands back to the remote terminal units to effect change.

Actually the need of remote monitoring and control of electric power distribution systems lead to the development of power line carrier analog telemetry systems. These systems superimposed high-frequency (50 kHz to 150 kHz) carrier signals on a low-frequency (50 Hz and 60 Hz power line conductors to communicate basic information like human voice (telephone network dedicated to power system operators), power flow (MVAR meter, Wattmeter) monitoring and protective relay (automatic trip) controls. These are examples of telemetry systems that were among the first to benefit from digital technology.

Large scale power systems cannot be operated safely and with efficiency without the use of remote data monitoring and control systems.

You can also read: How to integrate PLC into a Control System

The bottom line

From the above discussion, you can see that digital communication technology forms an essential part of any modern measurement and control system, and as more research and development is being done in the digital field, industries will continue to use this technology to improve efficiency in their industrial production and processing systems.

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