ISOBUS II – Success: Layer by Layer

The term ISOBUS II stands for the second version of an ISO standard for a fieldbus system that is used in agriculture for automation technology and for communication between machines. The basis for the development of this technology, which today plays a decisive role in the fulfillment of important demands for this branch of the economy, is the so-called OSI reference model. This is a layer model that has been designed by the International Standardization Organization, the ISO. It provides the basis for the design of protocols serving for interaction between the electronic components in computer networks. This reference model was introduced in the year 1983. The construction of the model provides for a division into seven layers, which build on top of each other:

  • Layer 1 - Bit transmission layer
  • Layer 2 - Security layer
  • Layer 3 - Exchange layer
  • Layer 4 - Transport layer
  • Layer 5 - Session layer
  • Layer 6 - Representation layer
  • Layer 7 - Application layer

Within this model, the representation is understandably reversed, so that layer 7 defines the top layer and layer 1 the lowest layer. There are requirements for each layer, but these can be realized in different ways, so that it cannot be specified in advance how the realization will look. The consequence of this is that nowadays a single layer has many communication protocols.

Difficulties with the conventional Bus Systems

The simplicity of the OSI reference model should have been that although there are seven layers, only the first, second, and seventh layer had to be taken into consideration. In the past, the CAN bus was used for this, as it was able to cover large parts of the first two layers As such, it performed the main part of the communicative work by itself. However, the remaining problem for farmers was the question concerning the actual possibility of a common standard. As such, the LBS was promptly standardized as DIN 9684, but the disagreement about the practical implementation of the idea remained. Additionally, the difficulty of limited expansion of such a system soon proved itself. The decision was made for a CAN bus with an 11-bit identifier, and the available address space was notably more limited than with a 29-bit identifier. The consequence of this was that hardly any reference was made to the subscribers, as opposed to the parameter group numbers (PGNs). The data rate of just 125 kbps did not offer a basis for an expansion of the network. However, the necessity for a bus-capable system was generally proven, so that there was a merger of the basic ideas of the LBS. This was developed further and finally advanced to form the ISOBUS. Now, the ISOBUS II also stands for the more powerful and expansion-capable version of LBS.

The Construction of an ISOBUS II System

When ISOBUS II is completely extended, many devices play a role in the network. They have the characteristic that by themselves, they are small computers, and as such also function this way. Sometimes such devices, depending on their type, are combined in one device. This can also extend to the CPU. Furthermore, it is sometimes usual within ISOBUS II that higher numbers of logical attachment controls are also located on such a CPU. The main components of such a complete ISOBUS II system are:

  • ISOBUS plug - this is used to connect the attachments so that data lines are available for communication. Connections via the electrical power also can be made at this point. A circuit is integrated in the ISOBUS II plug. This has the purpose of actively terminating the CANBUS when this process is required.
  • Virtual terminal - this is the so-called man-machine interface of ISOBUS II. It is a display device which can be used to control the system. It also has a screen and several push buttons. Rotary knobs are rather rare.
  • Tractor control unit - this is a job computer located on the tractor or the carrier vehicle. It serves to provide information that is converted to messages. The tractor control unit provides a connection between the ISOBUS II and the tractor bus. When the system is expanded, the tractor can be controlled.
  • Job computer - this component of the ISOBUS II system is normally found on the attachment. It takes on the tasks of machine control and the displaying of data. The functions of this component also include the implementation of operator input.
  • Task controller - this is the interface between a “farm management system” and the device control. Basically it serves the task of documenting the work processes, however in the future it should also take on the controlling of attachments and in the best case, the tractor.
  • File server - the devices coupled to the ISOBUS II get the storage space required for the storage of data concerning the configuration as well as information from the file server.
  • GPS receiver - this component can provide position data and as such, it serves the task of navigation and documentation. A type FPP transport protocol is used for this.