Communication · wired
M-Bus for water and heat meters
The Meter-Bus is the standard in consumption metering for connecting many meters to a single line. This page shows how the H2O and DELTA III connect to M-Bus, what to consider at the planning stage and how the values reach the control system.
How it works
One master, two wires, many meters
- M-Bus: one cable, many metersThe master supplies the two-wire line with about 40 V. All meters are connected in parallel; polarity does not matter.
- M-Bus: readingThe master selects a meter by its secondary address. Only that meter replies, by modulating its current draw.
- RS-485: line with terminationA twisted pair A/B runs as a line from device to device, with 120 Ω at both ends. RS-485 is the physical interface.
- Modbus RTU: request and responseThe protocol layer: the master polls a slave address (1–247) with a function code; the meter replies with a CRC-16 checksum.
- LoRa: radio to the gatewayThe meter sends short uplinks on 868 MHz. Every gateway in range receives them, with no cable to the metering point.
- LoRa: server and applicationGateways forward the data to the network server, which passes it on to the application. Range and transmission interval limit data volume and timeliness.
How is an M-Bus system structured?
An M-Bus master, for example a data logger, supplies a two-wire line. All meters are connected in parallel; polarity does not matter. Line, star and tree topologies are possible; rings should be avoided.
In the idle state, each meter draws a standard load of about 1.5 mA from the bus. The master must be able to supply the total of all loads. When a meter replies, it briefly increases its current draw, and the master detects this change.
Addressing
In addition to the primary address (1 to 250), every meter has a unique secondary address assigned at the factory. The master uses it to select a specific meter, with no need to assign addresses on site. Helvatron meters identify themselves with the manufacturer code HVT.
Bus power
The H2O can optionally be powered via the M-Bus. The DELTA III is available with M-Bus power and a backup battery. Without this option, both meters run on battery power and the M-Bus is used for communication only.
Planning notes
- Add up the number of meters and their standard loads for the master
- Choose cable length and cross-section according to the master manufacturer’s specifications
- Typical baud rate 2400 Bd; a telegram takes well under a second
- Document secondary addresses and meter locations during installation
- Heat meters and water meters can run on the same bus
System design
From the M-Bus to the building management system
A typical set-up: a building’s meters are connected to an EMU M-Center, which reads the meters, stores the values and passes them on to higher-level systems.
Step by step: commissioning the EMU M-Center with the H2O and DELTA III. Our sister company EMU covers M-Bus basics in detail, including bus load calculation, on emuag.com.
Versions
H2O and DELTA III with M-Bus
H2O with M-Bus
DN15 to DN500, part number suffix ABM, for example WABG.20C.ABM for DN20. Also available with wireless M-Bus (S1, T1 and OMS).
DELTA III with M-Bus
DN15 to DN1000, suffix ABM. Combined with RS-485 (ABMM) or LoRa (ABML) for two communication paths.
Questions about M-Bus
Do I need to observe polarity when connecting?
No. M-Bus is polarity-independent, so the two wires can be swapped.
How many meters can an EMU M-Center read?
Up to 250 M-Bus devices. Its two RS-485 interfaces add up to 64 Modbus devices.
Which manufacturer code do Helvatron meters use?
HVT. The master uses this code to recognise Helvatron devices when scanning the bus.