Within the previous few decades, the focus of production in the industry has been on the automation of processes and internal system communication. These two targets will make it possible to increase profits while reducing the costs by avoiding high material expenses and disruption, as well as downtimes of networks. Within the research processes and practical applications, the fieldbus has proven itself as most suitable tool for the realization of these tasks. This is a matter of the link between the actuators and sensors with the control unit in a system. The acquired data can be various information and messages, so that a fieldbus can gather documented data for consumption in addition to the conventional data concerning the status or existing participants of a network. The Meter bus, widely known by the abbreviation M-bus, was developed especially for this.
The M-bus in Detail
The Meter bus has its origin in the research by Horst Ziegler, who started the development of the fieldbus at the University Paderborn, and who worked with the companies Techem and Texas Instruments for optimization of the desired standard. However, this technology still is considered a German development. Originally, the M-bus was defined in the standard EN1434 for heat meters. However, over time, this fieldbus has developed to an independent standard, which has been established in the standard EN13757. This standard legitimizes the technology for use via a two-wire bus and the capacity for use with wireless transmission. When the two-wire line becomes effective, the transmission of the acquired consumption data takes place as serial transmission from the masters and the connected measuring devices, which are called slaves in the technical jargon. The two-wire line is considered protected against reverse polarity. Basically, the internationally recognized OSI model is called on. This is composed originally of seven different layers between which internal network communication takes place. However, a derived version of the conventional model is found with the M-bus, which at this point is constructed of only three layers. The data link layer is structured on the standard IEC 870-5.
Data Transmission with the M-bus
First, a device must be declared as master in the fieldbus-capable system. This is also responsible for collecting the data from the slave and storing or processing them as required. The initialization of the communication is then done by polling the counter reading, sent from the master to the bus. Accordingly, there may be only one master in a network, as otherwise there is the risk of wrong addressing. When there is then communication between the components, the fieldbus can be supplied with power via the slaves. The position of the master is taken either by a PC having a level converter or by an independent device. As a rule, transmission of the consumption data reaches a speed in the range of 300 to 9600 baud. When the M-bus is used, the omission of a fixed topology proves to be advantageous, as the participants can then be formed as desired as a string or can take the form of a star structure. The number of usable counters is limited to 250 in a segment, but the sum can be increased without problems to a multiple by the use of repeaters, which is used especially for large and complex installations. When the master has transmitted an enquiry to the slave, the slave answers by modulation of the power consumption. With this form of data transmission, it is important that the value of the quiescent current always remains constant within limits.
Strengths and Weaknesses of the M-bus
Use of the fieldbus technology has proven very efficient, with the result that the M-bus also has some advantages.
It is inexpensive, easy to realize, and has an installation protected against reversed polarity.
- The terminals can all be provided with current via the bus, and
- simple, integrated interface circuits are available through the partnership with Texas Instruments.
- Thanks to the digital communication via the Meter bus, it becomes possible to determine exact measuring values and to transfer them to the downstream installations for evaluation.
- furthermore, many slaves can be operated on a single segment, and
- the network can be expanded as desired by means of repeaters.
- Ultimately, the M-bus makes it possible to register the consumption of installations at great distances by use of a modem master.
As with every technology where formulation of an international standard is tried, certain weaknesses can also be found with the M-bus.
- For example, the standardization within the protocol level must be considered as incomplete, so that the general compatibility with the evaluating unit must be assured before the use of a new slave.
- The plugs used for connection are not standardized and
- the data transmission is not suitable for process control.
Still, the M-bus has established itself with the manufacturers of measuring devices, with the result that now it is used in many measuring tasks like the heat meter, the water meter, the electricity meter, and the gas meter.