MG-IP - M-Bus vs Modbus RTU - The advantage of M-Bus in metering systems

The article focuses on the advantages of the M-Bus protocol over Modbus RTU when used in meter buses.

1. The M-Bus protocol is less susceptible to interference (e.g., when running the bus in parallel with power cables - strong current cables) - making it possible to run the bus with a lower quality (i.e., cheaper) cable, e.g., without a shield to collect voltage interference induced by power cables. This lower susceptibility is due to the fact that the transmission is carried out with a current signal, not a differential voltage signal (as in RS485).

2. More devices can be connected on a single bus (depending on the power supply at the M-Bus master) (there are solutions even for 250 devices), the RS485 (Modbus RTU) standard allows up to 32 server devices to be connected (if they are made in the most popular version on the market: 1 device = 1 unit load, but in the best case, as in iSMA modules, it can be 1 device = 1/4 unit load, which still gives a maximum of 128 devices).

3. Admittedly, M-Bus operates at lower speeds than Modbus RTU, but in counter systems there is no need for it to be high, as 1 reading per day is completely sufficient. In special cases, it will not be more often than once every 15 minutes (that's the averaging period for power overruns at energy suppliers in Poland). In contrast, Modbus RTU was created for readings close to the periods: once every 1 / 5 / 30 seconds.

4. M-Bus just like Modbus RTU, depending on the speed, allows to achieve different maximum bus lengths (the lower the speed, the longer the possible bus). In the M-Bus protocol, it is possible to achieve up to 10 km (at a baud rate of 300) of bus length, where in Modbus RTU it is up to 1.2 km (at a baud rate of 9600 - support for lower speeds in Modbus RTU is rather uncommon). Of course, it should be remembered that in both cases the longer bus does not allow to achieve the declared number of devices (instead of 250 in M-Bus it will support, for example, 220 devices with a long bus, and in Modbus RTU instead of 32 devices, it will be, for example, 25 devices with the longest bus).

5. Some meter manufacturers allow their device to be powered from the M-Bus - allowing the additional savings of not having to run a power cable. In the case of devices with the Modbus RTU protocol, an additional power supply must always be provided.

6. A very large proportion of M-Bus meter manufacturers (except for energy meters that power themselves from the measured phase) use battery power, which allows them to operate for 2 to 5 years (depending on the frequency of transmission) without replacing batteries. This means (as in point 5) that there can be savings on the device's power cord, which is impossible to achieve with Modbus RTU meters (the Modbus RTU standard says that the device must be ready even for very frequent queries, e.g., every 1 second, so manufacturers do not offer such solutions).

7. M-Bus has predefined device types (e.g., flow meter, energy/heat/cool meter) which allowed manufacturers to create such a communication driver that automatically defines measurement units, without the need for additional manual setting of facets by the engineer (of course, this is not applied in all solutions), Modbus RTU does not define units, so the engineer always has to manually set facets if the BMS system gives him the possibility.

8. M-Bus allows to remotely change the configuration of units (e.g., communication speed) using a broadcast address. Modbus RTU also has such an address, but there is no way to change the communication speed, unless the manufacturer has made this possible using a holding register (which rarely happens) - but there is no uniform standard for this, and if even each manufacturer of the meters installed on the bus gave this possibility, each of them would put it on a different register, so there would be no way to do it using broadcast.

9. M-Bus, in addition to the device's primary address (based on almost the same rules as the device address in Modbus RTU), which is not automatically assigned, also has a secondary address (the 8-digit unique address of the device - often all or part of the device's serial number). This allows meters to be installed with the serial number written down (to which premises it was installed - and this is how it has to be done even in Modbus RTU, because when billing, the serial number of the meter is given, on the basis of which the invoice is issued), and then the programmer can remotely configure the devices by communicating with them precisely using the secondary address. In Modbus RTU, it is required to pre-program the addresses to all meters or, after installation, approach each one individually and configure them, and sometimes the meters (especially water consumption meters) are in hard-to-reach places.

10. M-Bus allows the discover function for devices (both by primary and secondary address). The difference is that a search by secondary address can take a very long time - if there is no limit to the scope of the search), while there is no such functionality at all in the Modbus RTU standard.