M-Bus [Meter-Bus] modulok

The M-Bus ("Meter-Bus") is a European standard for remote reading of gas-meter, heat-meter or other electricity meters. This bus fulfills the special requirements of remotely powered or battery driven systems, including consumer utility meters, various sensors and actuators. When interrogated into a network system, the meters deliver the measured data to a common master, which can, for example, be a hand-held computer, connected at periodic intervals to read all utility meters of a building. An alternative method of collecting data centrally is to transmit meter readings via a modem.

With its standardization as a galvanic interface for remote readout of heat meters this bus wins a great importance for the energy industry as relevant users.

The remote reading of utility meters can take place in different ways, beginning with the classical method - manual reading by the personnel of the providers - up to the remotely controlled collection of all the meter values for a complete housing unit. The latter is a logical continuation/extension of the technical development of consumption meters and is realizable with the help of the M-Bus.

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.

M-Bus Cable Length and Meter Number List:

A two-wire standard telephone cable 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 Ω. 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.