Fieldbus isn’t really one thing
but more of a collection of things.
Fieldbus is a group of protocols
that are used in the industrial arena.
The Fieldbus protocols have been standardized as IEC61158.
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Basically, Fieldbus works on a network
that permits various topologies
such as the ring, branch, star, and daisy chain.
Prior to Fieldbus, 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.
However, Fieldbus is not a connection type, per se,
but instead, a description used
to indicate a group of protocols.
There are several protocols in the group
such as ControlNet in the Allen Bradley family,
Modbus, Profibus, EtherCAT, HART, CIP, and many more.
A primary benefit of Fieldbus is for field connected devices.
For instance, consider you have several field devices
that are quite a distance from your computer.
With Fieldbus devices such as sensors, motors,
lamps, switches, these devices are connected to an I/O data block,
which is then connected to a field distribution device,
which is connected to a Fieldbus power supply,
then finally connected back to the controller.
The connections to the Fieldbus components,
with the exception of the field devices themselves,
are single cable connections.
This connection scheme may vary depending on the
Fieldbus protocol that you are using.
This type of field device connection saves on wiring runs
that would have to go all the way back to the controller.
As you can see, this type of device wiring
would have many benefits when the distance is an issue.
As you can imagine, your controller system
will need to have a communications module
that will be able to establish communications with the field devices.
In the case of ControlNet, you would need a ControlNet module,
Profibus would need a Profibus module, etc.
This is not to say that every protocol is dedicated
to a particular manufacturer
and must connect to the proprietary controller.
For instance, Profibus is a widely used protocol
that can connect to Allen Bradley, Siemens, Modicon, etc.
The same applies to the universal protocol of Modbus.
However, some protocols do not play well with others.
ControlNet is an example that cannot integrate into a Siemens processor.
With this information, as you can see,
there should be great care taken when designing your plant control system,
including the communications network,
and specifically the device layer if considering Fieldbus devices.
Do your research and make sure that all family members
like each other and are willing to communicate together.
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