Interbus - a Fieldbus System for the Automobile Industry

The term Interbus characterizes a fieldbus system that is designed for broad base use in companies that want to profit from use of automation technology. Here, the application covers various application areas from the sensor and actuator level in process automation through to the monitoring computer. Interbus was developed in 1983 by Phoenix Contag as specifications for a fieldbus that was suitable for industry.

Advantages of the Use of Fieldbuses

Companies engaged in manufacturing are confronted with the need for automated processes, just like other sectors of the economy. It is necessary to create data exchange between the machines so that the processes in a production facility can run by themselves. However, this form of communication places high requirements on the technologies, and these have to be met with fieldbuses. Fieldbuses have the advantage that the costs of wiring all equipment are kept as low as possible. Here, in comparison with conventional wiring which is characterized by a large number of input and output points, the work comfort is increased. On the other hand, serial networking via the fieldbuses has the appeal of flexibility and cost reduction. When Interbus is used, there is only a single fieldbus cable that can be used for transmitting all kinds of data in any formats and quantities. At the same time, the type of automation equipment does not play an important role.

The Characteristics of Interbus

The Interbus is generally considered to be an optimal technology in the production installations of the automobile industry. This opinion about the benefits of using the fieldbus system is based on several recognized advantages. In this way,

  • start-up can take place quickly and without major problems,
  • There are high-quality, yet still simple diagnosis tools in the system.
  • The data throughput is high in spite of a low clock speed, and
  • there are many possibilities for connections via a light conductor, including functions such as the diagnosis of the route, error messages and overlaid controls.

The Interbus also includes

  • cost reduction using an integrated bus coupler which operates via light conductors, 
  • the galvanic isolation of individual installation parts as well as the absolute insensitivity of the route with regard to coupling / EMV interference.

The Technology employed within the Interbus

From the point of topology, the operation of the fieldbus system is based on an active ring-shaped structure. However, the forward and the back channel are joined in a connection cable. Furthermore, each subscriber has two connection terminals, so that a tree-like structure of the physical wiring results.
When a bus participant wants to close the ring structure of Interbus, it can bridge the respective outputs internally. However, this only works when no other subscriber follows which would be cut off by this bridging. When branching occurs at the bus terminal, the next new branch is bound in the forward channel. This expands the ring topology. This arrangement has the advantage that when faults occur in the system (for example if a subscriber fails) the network still does not crash as the preceding subscriber simply corrects this gap by bridging his output. This way, the system can be kept operating until the failed subscriber is once again in the position to participate in the communication. The tree structure of the Interbus is composed of three hierarchical levels occurring in four types:

  • Remote bus - from the electrical point of view, this is an RS-485-specified interface where the energy supply is performed locally at the subscriber level. 400 m of cable (max.) can exist between the two participants. However, when copper lines are used, the possible distance increases to 13 km.• Installation remote bus - this is basically specified like the remote bus, but has a central energy supply.
  • Local bus - this branches via the bus terminals, i.e. the bus coupler, from the level of the remote bus. No further branching is possible. Here as well, the energy supply takes place centrally and the local bus can separated from the remote bus separately so that it is then switched off.
  • Interbus loop - Here, the distance between two participants is 20 cm to 20 m as a rule, and the max. distance can be 200 m. The Interbus loop has a two-wire interface, via which the energy is supplied centrally. The data for the bus here are modulated so that at this point, the subscribers can be disbanded the physical ring structure.

Within the arrangement of the subscribers, each one takes the position of a repeater. Then the wiring of the remote bus and the local bus can be used via electrical conductors or light conductors. Additionally, all subscribers act as slaves under a master connected to the remote bus. This is called a connection assembly group.