What Is M-Bus? A Guide to Wired and Wireless M-Bus

In short: M-Bus (Meter-Bus) is a communication protocol defined by the European standard EN 13757 for reading water, heat, gas and electricity meters remotely. Wired M-Bus uses a two-wire, polarity-independent bus that can power the meters from the line; Wireless M-Bus carries the same data model over licence-free radio bands such as 868 MHz.

What is M-Bus and which standards define it?

M-Bus was developed by Horst Ziegler of the University of Paderborn in cooperation with Texas Instruments Deutschland and Techem, as a master–slave bus designed to connect consumption meters to a central point at low cost. A master (a level converter or gateway) polls the meters on the line one by one; the meters only answer when they are asked.

Today the protocol is split into parts under the EN 13757 series:

• EN 13757-2: the physical and link layer of wired (twisted pair) M-Bus.
• EN 13757-3: the application layer, which defines how a meter encodes values such as index, flow and temperature. It can be used with different physical layers.
• EN 13757-4: Wireless M-Bus (wM-Bus), the physical and link layer for radio transmission.

In practice, "M-Bus meter reading" usually means connecting the water and heat meters of a building to a collector over a two-wire line. For installation details on the device side, see our M-Bus water meter page; this article explains the protocol itself.

How does the wired M-Bus physical layer work?

The most distinctive feature of wired M-Bus is that the two directions use different electrical methods:

• Master → meter (voltage modulation): when the line is idle ("mark", logical 1) the master output is at a nominal +36 V. For a logical 0 ("space") the voltage is reduced by 12 V to a nominal +24 V.
• Meter → master (current modulation): an idle meter draws a constant current of at most 1.5 mA. To send a logical 0 it increases its current draw by 11–20 mA; the master detects this change in current.
• Bus powering: the idle 1.5 mA current can be used to power the meter's communication interface from the line, so many M-Bus meters need no separate supply cable.
• Polarity-independent wiring: meter interfaces are polarity independent; swapping the two wires does not affect communication.

Characters are sent asynchronously: each character has one start bit, 8 data bits, an even parity bit and one stop bit. Standard speeds range from 300 to 9600 baud; in the field 300, 2400 and 9600 baud are the most common. If no answer arrives, the master waits roughly "330 bit periods + 50 ms" and then repeats the request at most two more times.

Primary and secondary addressing

On M-Bus every meter can be reached in two different ways:

• Primary address: a single-byte address. Meters are assigned addresses 1–250. Address 0 denotes unconfigured devices; 253 is the "pseudo primary address" used for secondary addressing, 254 is a test address to which every meter replies with its own address, and 255 is the broadcast address to which no meter replies.
• Secondary address: an 8-byte identity: a 4-byte serial number (BCD), a 2-byte manufacturer code, a 1-byte version and a 1-byte medium field (water, heat, gas, electricity…). The master first sends a selection telegram to address 253 to select the meter, then requests the data. Because digits of the serial number and the other fields can be given as wildcards, the master can scan the line and discover the meters automatically.

Which method to use depends on the site: primary addressing is fast and simple but requires giving every meter a unique address by hand. Secondary addressing uses the factory-assigned serial number, which removes the risk of address collisions, and is preferred for systems with more than 250 meters.

Cable length, device count and unit loads

The standard configuration in the M-Bus documentation is: 300–9600 baud, at most 250 meters and at most 350 m between a meter and the repeater (master), corresponding to about 29 Ω of cable resistance. To keep the cable capacitance under 180 nF, the total cable length should not exceed 1000 m; the reference cable is a standard two-wire telephone cable (JYStY N×2×0.8 mm). Distance can be increased by lowering the baud rate and using fewer meters, but because the meters are powered from the line, the voltage in the "space" state must not fall below 12 V anywhere in the segment.

Master capacity is expressed in unit loads (UL): 1 UL = at most 1.5 mA. Most meters draw 1 UL, but some devices need more than one unit load, which reduces the number of meters the line can carry.

Worked example: take a 40-apartment block with one M-Bus water meter per apartment and 4 heat meters in the building. If the water meters draw 1 UL and the heat meters 2 UL, the total load is 40 × 1 + 4 × 2 = 48 UL. A 20-unit-load level converter cannot drive this line; you need a master that covers at least 48 UL, preferably with room for expansion, or two segments that split the line.

Wireless M-Bus (EN 13757-4): modes and encryption

Wireless M-Bus is preferred in existing buildings where pulling cable is difficult and for battery-powered meters. The meter broadcasts telegrams at set intervals; a receiver (gateway, data concentrator or handheld terminal) collects them. Common modes:

The "1" variant of each mode (S1, T1, C1) only transmits, while the "2" variant (S2, T2, C2) is bidirectional. AES-128-based encryption and authentication protect the data; the Open Metering System (OMS) specification defines these security profiles so that devices from different manufacturers interoperate. In the field this means that to read an encrypted wM-Bus meter you need the per-meter AES key from the manufacturer or the company that supplied the meters.

Differences between M-Bus and Modbus

Although the names are similar, the two protocols were designed for different needs. For the TCP variant of Modbus, see our What is Modbus TCP article.

Thanks to M-Bus's self-describing data structure, the unit of the value a meter sends (m³, kWh, °C) travels inside the telegram. In Modbus, the meaning of each register is taken from the manufacturer's register map.

Typical meters and M-Bus gateways

M-Bus is most common in: cold and hot water meters, heat meters (calorimeters), gas meters and some electricity meters. It is the basis of per-apartment or per-unit consumption tracking in residential complexes, hospitals and commercial buildings.

To connect meters to software there is always a master on the line side:

• Level converter: converts the M-Bus voltage/current signals to RS-232, RS-485 or USB and powers the line. Its capacity is stated in unit loads, and it includes over-current and short-circuit protection.
• M-Bus to TCP gateway: brings the meter line onto Ethernet; the upper system polls the meters over IP. It simplifies cabling on sites with several buildings or blocks.
• Data concentrator: reads the meters periodically itself, stores the values in memory and forwards them to the upper system; for example the ZENNER D1205 EDC-MBus.

Common commissioning errors and a checklist

1. Address collision: with primary addressing, if two meters keep the same address (for example both at the factory default), their answers collide and readings arrive corrupted. Give every meter a unique address or switch to secondary addressing.
2. Exceeding unit loads: if the master is chosen without adding up the number of meters and each meter's UL value, the line trips the over-current protection or distant meters answer unreliably.
3. Cable length and capacitance: polarity independence does not mean the line can be pulled carelessly. On long runs with thin cable, the voltage drop can push the space-state voltage below the 12 V limit; error rates rise especially at higher baud rates.
4. Wrong baud rate: if the meter and master run at different speeds, the meter never shows up. Check the default speed in the meter documentation before scanning.
5. Short circuits and loose terminals: a short circuit on a single meter cable can affect the whole segment; test segments one by one after installation.
6. wM-Bus keys: encrypted radio meters cannot be read until the AES keys are handed over; store the key list matched to the meter serial numbers.

Reading M-Bus meters with Argus EMS

Argus EMS reads M-Bus meters and Modbus TCP/RTU devices through a Field Agent installed on site, and sends the data to the server over TLS-encrypted MQTT. When the connection drops, data is buffered locally on site and sent when the connection returns, so no data is lost.

• Monitors water, electricity and natural gas consumption, UPS units and generators on one platform.
• Provides hourly, daily and monthly consumption dashboards and data export.
• Sends e-mail alarms on threshold breaches and supports water leak detection work with indicators such as night-time flow.
• Provides month-end consumption/cost projection and budget tracking, and shows multiple sites on one screen.

To monitor your M-Bus meters centrally, explore our water consumption monitoring solution.