MULTIFUNCTION VEHICLE BUS
The Multifunction Vehicle Bus (abbreviated: MVB) is a bus used mostly for the transmission of data in the sector of railroads. It is used mostly in the Train Communication Network or with the Wire Train Bus. With the second potential use, data can be transmitted even over the entire train.
Increasingly modern railroad vehicles increased demand for a uniform diagnosis system. Amongst other things, the toilet installations or the air-conditioning installations were now operated electronically. However, initially all of these components were not linked to each other, which meant that in case of a failure the exact cause of the trouble could be read only at the control itself. This, however, involved a lot of work, like the opening of covers of control boxes to gain access to the control. Because of this problem, the Multifunction Vehicle Bus was developed in the 1990s by a committee composed of the companies ABB, AEG, Firema, and Siemens.
New Possibilities through the development of the Multifunction Vehicle Bus
Until today, a data network enabling secure and fast data transmission in the entire rail vehicle has been standardized by the international railroad union and the International Electrical Commission. Because of this, railroads and trains are compatible worldwide, so that even the exchange of parts of the systems is now standardized worldwide.
Areas of Application
The Multifunction Vehicle Bus is naturally used mainly for trains and rail vehicles, as it has been developed for them. However, the bus also is used for controlling in high-voltage installations. However, the Multifunction Vehicle Bus must not be confused with the train bus: The train bus connects the cars of a train, while the Multifunction Vehicle Bus (vehicle bus) connects the equipment onboard of each individual wagon.
The MVB has been installed as a standard since around 1995 in all models of the manufacturers who participated in the development.
The Multifunction Vehicle Bus operates with 1.5 Mbps via twisted wire pairs and via optical fibers. It is structured with two channels to guarantee a higher reliability of transmission. These two channels are separated inpassages from one wagon to another. The transmission of the data on the bus is controlled by several bus managers or only by one bus manager. With this, the data transfer is asynchronous. For the system, this means that each bus manager has its own clock.
The Multifunction Vehicle Bus is based on the master-slave principle. With this, the master can be coupled to the bus at any location. For higher availability, it is recommended to connect several masters. However, these do not operate at the same time, but only individually.
Furthermore, the function method is based on the transmission of protocols, which are transported synchronously and in cyclic data transmission. These data are source-addressed.
The Multifunction Vehicle Bus principally transfers two types of data:
1. Process Variables – periodic data
Process variables are short data (16 to 256 bits). They provide information about the status of the train, for example its velocity. The process variables are transported in cycles, so as to minimize the transmission times, namely below 16 ms within a car and below 100 ms within a train.
This data is called periodic.
2. Messages – sporadic data
Messages are longer information. They enable analysis, for example, of the network management. With this, the message size can vary in range from a few bytes up to megabytes. The messages are transferred according to demand, without time pressure.
This then are called sporadic data.
Periodic and sporadic data is on the same bus in the devices, but they are transmitted alternately and never together. With this, the transmission time of the bus is divided into basic periods, in which it is specified which data is sent when. The periods for sporadic data transmission can also remain unused, while the periodic data is transmitted in each period assigned to them.
Source-addressed Process Data Transmission
Basically, process data is transmitted to all devices on the bus. In principle, the master sends a telegram with a specific address of the process data. Each participant device checks whether the address is subscribed or not. When this is the case, the data is received. This transmission type is called source-addressed broadcasting.
Broadcasting of Process Variables and Traffic Memories
Data transfer is again coordinated and implemented by the bus controller. The process data to be transferred is saved in the traffic memory. This functions as interface between bus and application. The applications then read the respective process data from the traffic memory. This is mostly a cyclic process. The process data is repeated by the cyclic transfer at a specific time and thus is overwritten.
Meaning of the Telegrams with the Multifunction Vehicle Bus
The data of the telegrams are encoded according to the principle of the Manchester code. A 9-bit long header is at the beginning of each telegram, and an 8-bit long check sequence is at the end. The header provides synchronization.
The Multifunction Vehicle Bus uses two types of telegrams:
a. Master frame
The master frame is sent by the bus master.
b. Slave frame
The slave frame is sent by one of the slave devices.