Choosing the right fieldbus

How do you find the ideal fieldbus for your needs? There is no single fieldbus that can fulfill all requirements for every application. There can be considerable differences in communication requirements for different applications, therefore the fieldbus must be selected according to these requirements.

Most users want to select a simple communication method that fulfils the requirements of their application. The selected standard is supposed to have appropriate devices and tools for the application.

Apart from ensuring that the chosen device can accept multi-vendor products, users should consider the following:

* Topology
* Access to the fieldbus
* Physical features
* Protocol
* Access rights in multimaster systems
* Noise immunity
* Transmission speed.

Other consideration needs to be given to the technical, communications and system safety requirements. For a complete list, visit the Online Features section of www.mro-esource.com and click on "Choosing the right fieldbus."

Below is a checklist to consider:

Number of nodes
This is the number of devices connected to the field bus or bus segment. This varies from a maximum of 64 nodes with DeviceNet to 247 nodes with Modbus. If, for example, you have an application with 100 drives and want to control from DeviceNet then you will need two DeviceNet scanners.

Transmission speed
Also referred to as Baud rate. The higher the transmission speed the shorter the bus length. The maximum bus segment cable length is defined in the fieldbus standard, such as DeviceNet, Profibus etc., for the supported Baud rates.

Extent of user data
The extent of user data can be limited, depending on either hardware (ASICs, defined by node manufacturer) or software (protocol, defined by the fieldbus standard). This is usually defined in the fieldbus node User Manual.

Topology
The topology usually comes with rules defining how the nodes are connected to one another. The topolgy also defines bus termination rules. This is defined by the fieldbus standard and can be bus, ring, star topology etc.

Modularity, expansion possibilities
Because there is a single cable, the system can be easily constructed in a modular way. This allows the system to be easily expanded later. Machine manufacturers can build their systems based on a modular concept (flexibility).

Noise immunity
Due to reduced amount of cabling there is much less chance to pick up disturbances. Digital technology means drift is eliminated.

Cable requirements
These are defined by the fieldbus standard. Each fieldbus type has its own cable requirements, for example, cross-sectional area, impedance, stray capacitance, etc

Connection cost
Connecting a single cable is much easier than connecting a variety of analogue signal cables. The method of connection is usually standardised.

Device replacement without bus interruption
In daisy-chain bus topology, the bus cable can be removed from the device without interrupting the bus communication. With point-to-point or ring connection, bus communication will always be lost.

Requirements of the application
Type of fieldbus is selected according to the response time required by the application and/or the amount of nodes to be connected to the system

Communication
Required data transfer capacity
Usually the amount of I/O data to be exchanged via the bus

Guaranteed response time
Depending on the bus cycle time, determined by the amount of connected nodes, selected transmission speed and extent of data frame


SYSTEM SAFETY

System safety in a fault situation
Usually the standard fieldbus topology is not redundant but can be made so in many cases. In none redundant bus systems, the action in case of a communication loss can be configured within the nodes, for example fault or remain in operation etc

Safety of data transfer (error recognition)
The safety of the data transfer differs from fieldbus to fieldbus and is defined within the fieldbus protocol

Special attention towards power supply solutions
Some fieldbus systems supply the nodes via the fieldbus cable (DeviceNet) and others require an external power supply (Profibus) to feed the nodes

Readiness of diagnostic tools
For the open fieldbus standards a large variety of diagnostic tools from several manufacturers are available