Fieldbus is simply a means of communicating with input devices (sensors, switches, etc.) and output devices (valves, drives, indication lamps etc.) without the need to connect each individual device back to the controller (PLC, Industrial PC etc.). Therefore, overall, Fieldbus can reduce costs.
The Fieldbus protocols have been standardized as IEC61158.
Basically, Fieldbus works on a network that permits various topologies such as the ring, branch, star, and daisy chain.
Prior to Fieldbus protocols, industrial controller systems were connected using RS232 serial communications.
As you know, serial communications allowed only two devices to communicate.
Whereas today, the Fieldbus connections are more closely compared to the typical Ethernet connections where you can connect multiple field devices to a single connection point that would then connect to the controller.
There are three types of network architectures for FOUNDATION Fieldbus: star, line, and tree (or trunk) and spur topologies. Each FOUNDATION Fieldbus architecture offers advantages and disadvantages, so let’s break them down.
Star: This structure connects all devices directly to the control room without spurs or trunk. Typically, you’ll use this application for devices close to the control room. This topology has become rare because it needs the most cable of the three.
Line: This structure uses a single pair of cables to connect all devices. Many companies use the line topology because of its clear structure and low wiring cost.
Trunk (or Tree) and Spur: This structure is based on a line. The trunk line runs from the control room to the junction box in the field, where each device connects to the spur on the device coupler.
In a distributed architecture, the trunk goes out to a single junction box, and all spurs come out of this box. The box contains one or several device couplers to support the desired number of spurs. In a highly distributed architecture, the trunk goes to multiplejunction boxes. Each box will be smaller and closer to the field device, and each segment contains one segment protector or field barrier.
Fieldbus Host and Instrument interface
First, a particular vendor could not provide all the parts/components needed for a plant and that a particular manufacturer cannot make all the devices better than others. This led to either choosing the less-than-the-best devices from a single manufacturer or else settling for choosing the best devices from different manufacturers.
The latter option would give rise to an
integrability problem and lead to isolated islands of automation and consequent
inter operability difficulty with devices from different manufacturers.
Initially when fieldbus was introduced, it suffered
from numerous problems such as proprietary protocols, slow transmission speed,
and different data formats. Improvements in field signal transmission
technology resulted in increasing levels of decentralization.
In 1985, industry experts in the field sat together
to work out a vendor independent fieldbus standard—i.e., it would be
interoperable. The bus standard would provide bus power, intrinsic safety, and
the ability to communicate long distances over existing wires—the basic
requirements for a process plant automation system.
Partly due to the complexities of instrumentation
automation systems and mostly due to the reluctance on the part of the
manufacturers, a single standard protocol architecture is yet to be
established. Foundation Fieldbus and PROFIBUS are now the two most dominant
fieldbus technologies that are ruling the process automation field. Devices
embracing these two technologies cannot communicate with each other because of
protocol mismatch and thus seamless interoperability is yet to be achieved.
There are many types of fieldbuses in use today;
the particular type to be used depends on the type of industry—discrete or
manufacturing automation. Different types of fieldbuses include: Foundation
Fieldbus, PROFIBUS, DeviceNet, ControlNet, InterBus, HART, AS-i, MODBUS, CAN
Bus, Ethernet, LonWorks, and WorldFIP.