Definition
M-Bus (Meter-Bus, meter bus) is a meter communication protocol specified by the European EN 1434 standard, using a two-wire bus, supporting master-slave topology, and can connect hundreds of meters on a single bus. It is widely used for remote reading of water meters, gas meters, and heat meters, and is a common downstream protocol for meter collection devices along with Modbus and DL/T 645.
Technical Specifications
M-Bus uses a two-wire system, with the master station providing a 36–42V bus voltage. The slave devices (meters) communicate by modulating the bus current, and the bus can extend for several kilometers. Communication is master-slave polling with a baud rate of 300–9600 bps. It supports both secondary and primary addresses, facilitating batch installation. The data format uses identifiers such as DIF and VIF. The master station can be a meter reading collector or concentrator. After polling the meters via M-Bus, the collector reports to the AMR platform through 4G, LoRa, Ethernet, etc.
Use Cases
Remote reading of water, gas, and heat meters; AMR for residential communities, commercial buildings, and industrial parks; EMS energy management sub-metering. Meter reading collectors usually support M-Bus, DL/T 645, and Modbus simultaneously, enabling unified collection of various types of meters. M-Bus is widely used in Europe and in some projects in China.
Comparison with DL/T 645
DL/T 645 is specifically for electric meters and is a domestic electricity standard; M-Bus is mostly used for water, gas, and heat meters and is a European standard. Both are downlink protocols for meter reading, with different physical layers and data formats. Industrial gateways need to select the corresponding protocol according to the meter type.
Current Modulation on a Powered Pair
The master supplies power; the slave changes current
Electrically, a meter bus is a two-wire loop powered by the master. The master holds the bus at a relatively high DC voltage, which both feeds the line and carries communication from the slaves. A slave does not drive the voltage backward. In its reply window it changes how much current it draws from the bus, and the master detects that change and recovers the bits. Because energy and data share the same pair, a meter can do without a strong local supply and keep only the small power the measurement needs. That is the difference from an ordinary serial instrument bus.
A frame uses data records and value records to say what the quantity is, what the unit is, and what the multiplier is. One readout is often not a bare integer but a record that carries its dimensions. If the master splits the frame only by byte offset, a heat meter's energy, flow, and inlet temperature, packed into the same frame, will be read wrong. A primary address suits small numbering schemes. A secondary address addresses the meter by its serial number. On a large install the two addresses are scanned in a different order. The collector has to follow the meter vendor's instructions. Plugging the bus in does not guarantee that every meter will be enumerated.
What a meter pit or a building bus assumes
This fits meters run together along a corridor, a riser, or a floor, read on an hourly or daily cycle, with very little data per meter. Bus length, meter count, and wire gauge together decide whether the far end still sees enough voltage. Casual T-taps, wire that is too thin, or meters with a high standby current drop the end voltage until a slave can no longer modulate. The symptom is random missed reads on the far meters. During polling the bus must stay powered continuously. Do not insert a supply that cycles off in the middle of the loop.
When the collector forwards a stored reading onto the uplink, that uplink may be cellular, Ethernet, or another radio. It is a separate link. An uplink outage should not erase a reading already taken, or after recovery you cannot tell "the meter did not answer" from "it answered and the reading was never forwarded." If electricity meters are on the same site, they usually use a different national protocol. Keep them on a separate channel. Do not feed meter-bus frames into an electricity-meter parser as they are.
Address clashes and reversed polarity
The easiest construction mistake is polarity. Some transceivers claim to correct it automatically, but older meters and some masters still require the positive and negative conductors not to be swapped. Reversed, the whole bus stays silent. Next is a duplicate address. Replacing a meter and keeping the old address makes two meters answer in the same window. The master sees intermittent garbage, not a clear "address conflict" alarm. Third is treating this two-wire current loop as a differential serial bus and fitting a termination resistor. The extra resistor changes the current the master sees. At best it cuts how many meters you can hang; at worst the master reads a short circuit.
Rate is often set to the maximum in one step. On a long, thin line the edges of a high-rate current pulse slow down, the master's sample point shifts, near meters look fine, and far meters are garbled. Lower the rate to match the waveform at the farthest meter before thinking about encryption and retries. Retries with no limit let one poorly connected meter stall the whole round, so later meters never get a turn inside the collection period. Hanging RS485 instruments on the same pair of terminals, or asking an RTU to "emulate" current modulation with a voltage-mode serial port, will not produce a stable read.