M-Bus (Meter Bus) is the meter bus. The purpose of M-Bus bus development is to meet the needs of network systems and remote meter readings. M-Bus is the product of Dr. Paderborn University. Horst Ziegler and TI’s bluetooth low energy Module Deutschland GmbH and Techem GmbH jointly proposed a bus structure specially used for public utility instruments. The full name is Meter-Bus, referred to as M-Bus.
In the OSI seven-layer network model, M-Bus only defines the functions of the physical layer, link layer, network layer, and application layer. Because the ISO-OSI reference model does not allow changes in the upper layer such as baud rate and parameters such as addresses, so M-Bus defines a management layer outside the seven-layer model, which can manage any layer without complying with the OSI model.
M-Bus has the characteristics of simple wiring, topology independence, and strong anti-interference. Its applicability in non-power instruments is obviously better than RS485, Lonwork and other transmission methods. It has been widely used in building and industrial energy consumption data collection.
The M-Bus system adopts half-duplex asynchronous communication, the transmission rate is 300~9600bps, and the transmission distance is relatively long. The bus connection method adopts the bus topology, which can connect hundreds of slave devices at a distance of several kilometers. SOC module M-Bus is a hierarchical system consisting of a master device, several slave devices, and a pair of connecting cables. All slave devices are connected to the bus in parallel, and the master device controls all serial communication processes on the bus. A typical M-Bus bus framework, as shown in the figure below, where the Master represents the M-Bus bus controller, and the Slave represents the instrumentation device hanging on the bus, which is the master-slave response mode.
M-Bus bus characteristics
(1) The two-wire bus system has no positive and negative poles, and the construction is simple;
(2) Using unique level characteristics to transmit digital signals, strong anti-interference ability, and long transmission distance;
(3) Bus power supply can be selected to reduce maintenance costs;
(4) Bus topology, easy expansion, low networking cost;
(5) The failure of any slave station does not affect the function of the entire bus;
(6) The specially designed message format meets the needs of networking and remote reading of energy consumption metering instruments.
How M-Bus works
M-Bus two-wire cables usually use standard twisted-pair wires, with no positive and negative poles. There is no topology requirement for wiring, the bus is self-powered, and it has the advantages of strong anti-interference ability, multiple relay series, and many terminal series. It is one of the best bus standards for automatic meter reading systems at present.
M-Bus physical layer stream transmission has unique level characteristics. The code stream transfer from the master device to the slave device is realized by level shift, and the code stream transfer from the slave device to the master device is realized by modulating the current consumed by the slave device. Define logic “1” as MARK and logic “0” as SPACE.
When the M-Bus master device sends a logic “1” (MARK) to the slave device, the bus voltage is Vmark (≤42V); when sending a logic “0” (SPACE), the voltage drops by more than 10V to Vspace (≥12V); When the slave station sends logic “1” to the master station, the current taken by the slave station is Imark (≤1.5mA), when sending logic “0”, the M-Bus interface of the slave station will add a pulse current of 11-20mA to Imark, forming Ispace.
The master device determines whether to receive “0” or “1” by detecting whether there is an 11-20mA pulse current on the bus; when the slave device receives data since the absolute voltage of the bus will change with the distance and the bus current Whether the dynamic reference voltage differs by more than 10V determines whether to receive “0” or “1”. Therefore, the data transmission of M-Bus at any time is a one-way transmission, from the master device to the slave device or from the slave device to the master device. This communication method not only realizes the remote power supply to the slave device but also obtains strong anti-interference Ability to resist external interference.
The M-Bus protocol stipulates that the idle state of the bus is represented by logic “1”, that is, the bus voltage is maintained at Vmark, and each slave station draws a current Imark≈1.5mA, that is, the total current on the two-wire bus is equal to Imark*slave station total. In this way, regardless of whether the bus is in an idle state or a data transmission state, the bus voltage is not lower than Vspace, and the current obtained by each slave station is not less than Imark, and this current can be used as a power supply for the slave station. In the normal operation state of M-Bus, the bus can continuously transmit signals and provide power, so that the battery used by the terminal instrument becomes a backup power supply, reducing the regular maintenance of the instrument, replacing the battery, etc., and the installation position of the instrument can also be more random.
The M-Bus bus is widely used in various controllers, concentrators, and other equipment, which is convenient for the control system to collect real-time data of related instruments, and then transmit the data to the server for analysis and processing through various communication methods so that remote control can be realized. Real-time data collection, inspection, and monitoring functions. The M-Bus bus is widely used in the fields of centralized reading of three meters, intelligent home control networks, fire alarm and linkage networks, residential intelligent control networks, and central air-conditioning control systems.