Wired M-Bus Specification


Physical Layer

More and detailed informations about the specifications of the physical layer are listed in the document ‘WG4N85R2.DOC’ § .

4.1 Principles of Operation

The M-Bus is a hierarchical system, with communication controlled by a master (Central Allocation Logic). The M-Bus consists of the master, a number of slaves (end-equipment meters) and a two-wire connecting cable: see Figure 8. The slaves are connected in parallel to the transmission medium - the connecting cable.

Fig. 8 Block diagram showing principle of the M-Bus System Fig. 8 Block diagram showing principle of the M-Bus System

In order to realize an extensive bus network with low cost for the transmission medium, a two-wire cable was used together with serial data transfer. In order to allow remote powering of the slaves, the bits on the bus are represented as follows:

The transfer of bits from master to slave is accomplished by means of voltage level shifts. A logical “1” (Mark) corresponds to a nominal voltage of +36 V at the output of the bus driver (repeater), which is a part of the master; when a logical “0” (Space) is sent, the repeater reduces the bus voltage by 12 V to a nominal +24 V at its output.

Bits sent in the direction from slave to master are coded by modulating the current consumption of the slave. A logical “1” is represented by a constant (versus voltage, temperature and time) current of up to 1.5 mA, and a logical “0” (Space) by an increased current drain requirement by the slave of additional 11-20 mA. The mark state current can be used to power the interface and possibly the meter or sensor itself.

Fig. 9 Representation of bits on the M-Bus Fig. 9 Representation of bits on the M-Bus

The transmission of a space by a slave results in a slight reduction in the bus voltage at the repeater due to output impedance, as can be seen in Figure 9.

The quiescent state on the bus is a logical “1” (Mark), i.e. the bus voltage is 36 V at the repeater, and the slaves require a maximum constant quiescent current of 1.5 mA each.

When no slave is sending a space, a constant current will be drained from the repeater which is driving the bus. As a result of this, and also the resistance of the cable, the actual Mark voltage at the slaves will be less than +36 V, depending on the distance between the slave and the repeater and on the total quiescent current of the slaves. The slave must therefore not detect absolute voltage levels, but instead for a space detect a voltage reduction of 12 V. The repeater must adjust itself to the quiescent current level (Mark), and interpret an increase of the bus current of 11-20 mA as representing a space. This can be realized with acceptable complexity only when the mark state is defined as 36 V. This means that at any instant, transmission is possible in only one direction - either from master to slave, or slave to master (Half Duplex).

As a result of transmission in the master-slave direction with a voltage change of 12 V, and in the answering direction with at least 11 mA, besides remote powering of slaves a high degree of insensitivity to external interference has been achieved.

4.2 Specifications for Bus Installations


An M-Bus system can consist of several so-called zones, each having its own group address, and interconnected via zone controllers and higher level networks. Each zone consists of segments, which in turn are connected by remote repeaters. Normally however, an M-Bus system consists of only a single segment, which is connected via a local repeater to a Personal Computer (PC) acting as master. Such local repeaters convert the M-Bus signals into signals for the RS232 interface. From now on, the local repeater will simply be termed the “repeater”, and the combination of PC and local repeater termed the “master”.


A two-wire standard telephone cable (JYStY N20.8 mm) is used as the transmission medium for the M-Bus. The maximum distance between a slave and the repeater is 350 m; this length corresponds to a cable resistance of up to 29 W . This distance applies for the standard configuration having Baud rates between 300 and 9600 Baud, and a maximum of 250 slaves. The maximum distance can be increased by limiting the Baud rate and using fewer slaves, but the bus voltage in the Space state must at no point in a segment fall below 12 V, because of the remote powering of the slaves. In the standard configuration the total cable length should not exceed 1000 m, in order to meet the requirement of a maximum cable capacitance of 180 nF.


There is so far no standard or recommendation for a M-Bus plug to connect the meters to the bus system, but the Usergroup investigates in defining a proper connector. Three different plugs have to be defined for the connector at a) the installation mode b) meter to fixed installation and c) meter to handheld connection.

4.3 Specifications of the Repeaters

See chapter ‘Electrical Requirements Master’ in the document ‘WG4N85R2.DOC’ § .

4.4 Slave Design

The requirements for slaves are listed in the paper ‘WG4N85R2.DOC’ § . The following characteristics are part of it:

M-Bus Transceiver TSS721

In order to meet the requirements for the slaves mentioned above, an IC was developed by Texas Instruments Deutschland GmbH, namely the Transceiver (i.e. Transmitter and Receiver) TSS721. The use of the TSS721 in M-Bus slaves as the interface to the bus reduces the number of components needed, and therefore the cost of slaves. Apart from the transmission and reception of data in accordance with the M-Bus specification, this IC also provides translation from and to the operating voltage of the microprocessor to which it is connected, in order to be able to communicate with it. The communication can take place at baudrates from 300 to 9600 Baud. Additional features include integrated protection against reversed polarity, a constant 3.3V power supply for the microprocessor, and the prompt indication of failure of the bus voltage.

By referring to Figure 10, the individual functions of the TSS721 will now be explained in more detail:

Fig. 10 Block Diagram of the Transceiver TSS721 Fig. 10 Block Diagram of the Transceiver TSS721 [4]

Figure 11a) shows three alternative operating modes for the TSS721 which can be used to power a microprocessor. It shows that the processor can be supplied exclusively by the transceiver (remote supply), normally from the TSS721 and with bus failure from a battery (remote supply/battery support), or only by the battery. Few external components are needed to build a complete slave with the TSS721, apart from the microprocessor or microcontroller and the components specifically required for the sensing elements. Besides Fig. 11b) shows a basic optocoupler application.

Fig. 11a) Operating Modes of the TSS721 for Powering a Microcontroller Fig. 11a) Operating Modes of the TSS721 for Powering a Microcontroller [4]

Fig. 11b) Basic optocoupler application Fig. 11b) Basic optocoupler application [4]