Products/ EMU Professional II/ M-Bus
Interface · EMU Professional II
Two wires, polarity-independent, up to 250 meters and 5000 m of cable. M-Bus is unspectacular and for exactly that reason the standard for remote meter reading inside buildings. This page explains how it works, how to size it and what the EMU M-Center takes care of.
The principle
M-Bus – short for Meter-Bus – is the European standard for remote reading of consumption meters, defined in EN 13757-2 for the physical layer and EN 13757-3 for the application layer. It was designed for exactly the case that is normal in a building: many meters, long distances, small amounts of data, no real-time requirement.
The topology is deliberately undemanding. A master feeds an unscreened two-wire line with around 40 volts. All meters hang off it in parallel – there is no stub length restriction as with RS-485, and no polarity: anyone who swaps the two wires will not notice, because it makes no difference. That is precisely what makes the bus robust on site.
Communication is asymmetric. The master transmits by briefly lowering the bus voltage. The meter answers by increasing its current draw – from its quiescent load to a multiple of it. A meter’s quiescent load is called a unit load and amounts to around 1.5 mA. The entire sizing of the bus follows from it.
Unit load
Quiescent current draw of a meter, around 1.5 mA. One device can draw several unit loads.
Primary address
Short address from 1 to 250, set on the meter. Fast, but awkward if assigned twice.
Secondary address
Eight-digit identification number of the device, combined with manufacturer, version and medium. Unique from the factory.
Baud rate
300, 2400 or 9600 Bd. In practice 2400 Bd, because that gives the best range at a usable speed.
Telegram
A request-and-answer sequence. The master asks, the meter delivers its complete data record.
Step by step
Using an EMU M-Center with an EMU Professional II and the Helvatron H2O and DELTA III meters as an example: from powering the bus and selecting meters by secondary address to passing on the readings.
Sizing
The figure of 250 appears on every data sheet, but it is rarely the real limit. Three things set the limit, in this order:
Every meter permanently draws its unit load. The total must stay below what the master can deliver – and that depends on how the master itself is powered. On the EMU M-Center this is 450 mA continuous and 600 mA peak at 230 V AC, but 1.6 A at 24 V DC. Anyone planning 250 unit loads needs to calculate with 24 V DC, not 230 V AC.
The bus tolerates up to 5000 m at 100 nF/km and a maximum loop resistance of 90 Ω. The capacitance smears the signal edges; the more cable, the lower the baud rate has to be. The recommended cable is J-Y(ST)Y 2×2×0.8 mm, screened where possible.
At 2400 Bd a telegram takes well under a second, depending on the data record. With 250 devices that still adds up. For quarter-hour values this is uncritical; for polling every device every minute it is not.
| 120 meters × 1.5 mA | 180 mA |
|---|---|
| Reserve for inrush currents | approx. 20 % |
| Requirement | around 215 mA |
| Master at 230 V AC | 450 mA continuous – fits |
| Master at 24 V DC | 1600 mA – ample |
Devices with more than one unit load and long stub lines increase the requirement. When in doubt we work it out with you.
Star, tree and line are all permitted and may be mixed. The bus is monitored against short circuit and overcurrent and is galvanically isolated – an accidental short while connecting does not bring the installation down.
Use case
An M-Bus meter on its own is of little use – it needs a master to power it, poll it and pass the values on. That is the role of the EMU M-Center. It has a built-in level converter, so no separate power supply unit is needed.
The practical gain is in commissioning. The M-Center searches for meters automatically by primary and secondary address; adding them manually is possible but rarely necessary. Plug-and-play drivers make sure the data records arrive with sensible names instead of as numbered raw values. Everything is operated in a web browser, with no local PC software.
After that the M-Bus becomes invisible. The M-Center reads cyclically, stores the values in its 16 GB and passes them on to whatever you actually use: MQTT for cloud and dashboards, BACnet IP or OPC UA for the building management system, Modbus TCP for the control system, CSV, JSON or Excel over (s)FTP and email for billing – and Joulio-Web for analysis and utility cost statements.
Failed readings are detected and logged; error flags, communication faults and outliers go into monitoring around the clock. With 120 meters, nobody would otherwise notice that one has been silent for three weeks.
Field
Professional II M-Bus, plus water and heat meters from other manufacturers on the same bus
Collect
EMU M-Center as master, up to 250 devices, cyclic reading
Forward
MQTT, BACnet IP, OPC UA, Modbus TCP, (s)FTP, email
Analyse
Joulio-Web: load profile, reports, utility cost statements
Where it fits
Documents
This page explains the principle. Register lists, telegram structure, terminal assignment and the exact parameters of each version are in the interface manual.
PDFM-Bus interface manual1480 · Professional II · English PDFProfessional II data sheet1441 · English PDFM-Center manual2151 · master side · English PDFM-Center data sheet2141 · bus load and limits
Frequently asked questions
The limit is not the number of devices but the supply current. Each meter draws around 1.5 mA, one unit load. On the EMU M-Center, 450 mA continuous and 600 mA peak are available at 230 V AC, and 1.6 A at 24 V DC. The figure of 250 devices is therefore a 24 V DC calculation.
No. M-Bus is polarity-independent. Swapped wires work just the same – one of the reasons the bus has proven itself on site.
J-Y(ST)Y 2×2×0.8 mm is recommended, screened where possible. The limit is 5000 m at 100 nF/km and a maximum loop resistance of 90 Ω. Star, tree and line topologies are permitted and may be mixed.
Secondary addressing is the safe choice in mixed installations, because the eight-digit identification number is unique from the factory. Primary addresses from 1 to 250 are faster, but they have to be set on the device and documented; otherwise duplicates only show up in operation. The M-Center searches for both automatically.
In practice 2400 Bd. The options are 300, 2400 and 9600 Bd; 2400 Bd is the usual compromise between range and speed. For quarter-hour values that is ample, even in large installations.
You need an M-Bus master in any case, because the bus has to be powered and polled. The M-Center has the level converter built in and also handles meter discovery, storage and forwarding to your target systems.
Send us the number of measuring points, the distances and the target system. We will tell you which version makes sense – and when another interface would be the better choice.
Phone
+41 (0)41 545 03 00
Technical support
helpdesk@emuag.ch
Orders Switzerland
orders@emuag.ch