The T-bus protocol

Especially in Germany, agriculture is a key branch of the economy, so that in most cases much research is done here in order to increase sales by a larger yield and to simultaneously lower the costs. However, this requires a proven technology, which is able to establish stable communication between the individual network components and to create a clear structure. Often, this is done with the aid of protocols. The T-bus protocol is the main protocol used in agriculture. This generally means an open protocol which enables communication with sensor and actuator applications. In addition to the already mentioned use in agriculture, this protocol is also used in horticulture and in other sectors, as it does not specify a transfer medium and as the conditions for an electrical interface are not specified in advance by the T-bus protocol.

The potential data frames of the T-bus protocol

Thanks to dropping of certain conditions and limitations, up to eight different variants of the data frame can be found with the T-bus protocol.

  • SyncByte
  • Destination Device Family
  • Destination Address
  • Source Device Family
  • Source Address
  • Data Length
  • Data
  • CRC

The first option, the SyncByte, has the task of synchronization and automatic baud rate recognition. Here, the least significant bit must always have the value 1, so that the structure with this data frame corresponds to the format 0B xxxx xxx1. The remaining bits are then provided for the other versions.

Each destination device family is one byte long, as this can be used to directly address device classes, which enables multicast, meaning that several components can receive a signal during one control process. The maximal number of device classes is 254. This structure is also found with the T-bus protocol data frame of the source device family.

The destination address always has a length of three bytes and is used to assign an individual address to each participant. Here, the maximal number of participants is 16,777,214. This structure is also the same as that of the source address.

The data length is actually a field range indicating the length of the payload. Here, the payload is the data frame “Data”, which has an optional content, where the interpretation of the data depends on the respective device class and the application protocols. In this way, commands as well as measuring data can be transferred. The data length always covers two bytes. The maximal value that can be reached for the data length is 65535 bytes.

The CRC is the data frame with the T-bus protocol that handles the determination of the checksum.
Data Transfer with the T-bus Protocol according to RS-485

When many manufacturers market an application that is intended to fulfill the same purpose, as time goes on the main objective will be the specification of a standard which enables all individual components of the different versions to form a single version. The result can then be regarded as an international standard, as it is supported by all sides. A standard can also be found with the T-bus protocol, namely the RS-485, which here defines the transmission. Thus, the RS-485 is initially an interface standard, which is used with the following data transfers:

  • Digital line-bound transfer
  • Differential transfer
  • Serial transfer

With this standard, the transfer always takes place via the half-duplex procedure and a 4-wire line. With the RA-485, the speed with which a data packet is sent can be recognized automatically and differs according to the line length. On average, speeds from 1200 baud to 230 kbaud are possible.

The Advantages of the T-bus protocol

The positive aspects of the use of this protocol are clearly structured, but not less important. Although up to now the T-bus protocol has shown itself as productive nearly exclusively in horticulture, it can still be used in other applications because of its properties. because the specific advantage of this open communication protocol is the fact that it provides a specific process for data transfer. Thus it is left up to the user to equip the process with commands according to his own needs, so that the
T-bus protocol never contains an unneeded or unused function, but is designed only for those areas where it is needed. Furthermore, this communication protocol is independent of physical transfer media. This also provides the application with more flexibility, offering the user a wider margin, as he can use different transfer media, which can again be connected more easily via a converter, so that a network is created again.

To sum up ISOBUS IV

The remarkable fact in regard to this communication protocol seems to be that it has only one detectable disadvantage, and that this only refers to the fact that up to now this application has been used mainly in agriculture and horticulture. The T-bus protocol is a protocol that can be used flexibly by omitting specified conditions and offers the user flexibility in the execution, thus guaranteeing optimal use.