How does M-Bus work?
On the M-Bus there is exactly one master, for example a data logger, and many slaves: the meters. The master transmits by changing the bus voltage. A logical 1 corresponds to the full bus voltage, a 0 to a voltage about 12 V lower.
The meters do not answer with voltage but with current. They briefly increase their current draw by 11 to 20 mA, and the master detects this change. Because the M-Bus interface in the meter is powered from the bus line, reading also spares the battery of water and heat meters.
A request always runs the same way: the master sends the command REQ_UD2 to an address, the meter answers with RSP_UD and delivers its data record with meter readings, instantaneous values, serial number and status. Then it is the next meter’s turn.
Primary or secondary address?
Every meter on the bus needs a unique address. M-Bus offers two ways of doing this:
- Primary address: a number from 1 to 250 set on the meter. This is quick, but if an address is assigned twice, two meters answer at the same time and the answer is unusable.
- Secondary address: an eight-digit identification number, usually the serial number, supplemented by manufacturer, version and medium. It is unique from the factory and does not need to be set.
In larger installations, secondary addressing is the safe route. The master finds the meters with a search run and then addresses them directly.
How many meters fit on one M-Bus master?
The limit is set by the current the master can supply. In the idle state each meter draws one unit load of 1.5 mA. A master for 250 meters must therefore supply 375 mA for the idle current alone, plus reserve for the answers.
Then there is the cable: the loop resistance, outgoing and return conductor together, must not exceed 90 Ω. The more meters and the longer the line, the larger the cross-section must be. A common choice is J-Y(St)Y 2×2×0.8 mm; one pair is enough for the bus. How to calculate the bus load is shown in the article Calculating the M-Bus load.
Which topology is allowed with M-Bus?
M-Bus is tolerant when it comes to cabling. Line, star and tree are permitted, the wires are polarity-independent and no terminating resistors are needed. This clearly sets M-Bus apart from RS485, where only a continuous line works reliably.
A plan still pays off: many long branches increase the capacitance of the line, and in large installations that limits the possible baud rate.
M-Bus at a glance
| Feature | M-Bus |
|---|---|
| Standard | EN 13757-2 (transmission), EN 13757-3 (data format) |
| Line | 2 wires, polarity-independent, no terminating resistor |
| Topology | Line, star or tree |
| Power supply | over the bus, from the master |
| Speed | 300, 2400 or 9600 baud, usually 2400 |
| Addressing | Primary address 1–250 or secondary address |
| Typical devices | Electricity, water, heat, cooling and gas meters |
For meters without a cable connection there is the radio variant Wireless M-Bus to EN 13757-4, widespread for water and heat meters.
M-Bus with EMU devices
The M-Bus power meter EMU Professional II is available as EMU Professional II 3/100 for direct connection up to 100 A and as 3/5 for CT connection.
The EMU M-Center serves as master: up to 250 M-Bus devices and 1.6 A M-Bus load, including meters from other manufacturers. It passes the values on via Modbus TCP, BACnet IP or MQTT.