M-Bus communication bus installation, part 2 - Converters

This article follows up on the previous section Installing the M-Bus communication bus. We will complete a few tips for working with communication converters, which connect the M-Bus to a PLC or PC with visualization, as well as tips for connecting pulse converters.


The bus ends (or starts ... depending on the angle of view) with an M-Bus converter to another interface, most often an RS232 serial line, Ethernet also appears. Here we must realize how it is necessary to use the correct terminology, because behind the converter it is no longer a physical M-Bus layer (ie voltage and current ratios on the wires), but "only" the M-Bus protocol (ie the way of encoding logic signals, construction of communication telegram, addressing, meaning of individual variables, etc.). We say that there is, for example, an RS232 interface with the M-Bus protocol between the converter and another member, eg a computer or a controller - a PLC.

There are also interesting devices, such as the Schrack MGDIZ205 electricity meter, which communicate via the M-Bus protocol on the RS485 interface. Therefore, the rules for the RS485 bus (line topology, terminations, twisted wires, etc.) apply to bus design, while in terms of protocol, conventional M-Bus drivers are used. Working with these meters is easier, because the M-Bus to RS232 or RS485 converter is not required - process stations are usually already equipped with an RS485 line, but of course these meters cannot be connected to a classic M-Bus, and therefore it is necessary to use another PLC communication port.

Another important feature of the converter is the galvanic separation of the M-Bus from other circuits. Correctly, the M-Bus should be "in the air", ie galvanically separated from all other circuits, such as supply voltage, ground and housing of the RS232 interface. If this is not the case, one of the devices on the RS232 side can be damaged due to incorrect wiring or field problems (bus-to-ground connection). However, for transducers for a small number of meters, which are likely to be in the same room and near the switchboard, galvanic separation is not critical. In several cases, a notebook has proven to be a critical element, the housing (and thus the housing of the Cannon RS232 connector) is usually connected to a protective (yellow-green) power supply wire in the socket. A short circuit occurred when the M-Bus converter was connected to the notebook. Therefore, for galvanically non-isolated converters, it has become a good habit to set the bus when running the laptop on batteries.

Let's now look at several types of converters supplied by Czech companies and some of their features:

Type Manufacturer / distributor Max. number of meters Galvanic separation Interface
M-Bus 810 PiiGAB / PaPouch 5, 20, 60 (by type) no Ethernet
DM-MB2ET/A AMiT 3 no Ethernet
AD01 Westermo / FCC PS 120 yes RS232
SLC-31 Elsaco 20 yes RS232
R095, R096 Domat 25, 60 yes RS232
IMP5001L SV-Tech 250 yes RS232
M007 Domat 10 yes Ethernet
HWg-PWR HW Group 3 yes Ethernet

The last two devices are not just M-Bus converters, but communicators that actively send the read data via the Internet to special servers for further processing. So we will no longer find the M-Bus protocol on the Ethernet interface, but probably HTTP. The description of the function of communicators is beyond the scope of this text, but it is certain that such "smart converters", which allow in addition to energy readouts and sensing of several temperatures or operating conditions, are a great solution for remote reading of values from a large number of remote locations. applications, installation of industrial technologies at customers, etc.).

The overview is only brief, the interested party will find details in the catalog sheets of individual products. There are differences between the types, which will be reflected in the price, eg AD01 has 230 V power supply, two serial interfaces, M-Bus repeater function and really extreme immunity to interference, M095 and M096 use digital detection of response current levels and so read without problems even meters that do not fully comply with the standard, etc.

For converters with Ethernet interface, the M-Bus protocol is either transported directly in a TCP packet or encapsulated in a proprietary protocol (APE for DM-MB2ET / A), or the so-called virtual serial port is used on the PC side, which is software that emulates classic serial line (COM port) and transmits data from the converter to it, so the visualization or reading program communicates with this virtual port as if the M-Bus was connected via the RS232 converter directly to the computer port. This solution has the advantage of greater flexibility - you can get to the other side of the world, for example, over the IP network, but it is necessary to pay attention to possible packet delays in the network, which does not occur on direct serial lines and for which the meter driver may not be ready. A particularly bad idea is to try to communicate M-Bus converters over the network via GPRS routers, because mobile data transmissions are characterized by high latency (in the order of seconds), which can completely prevent communication. It helps to switch to another protocol, which is suitable for transmission over networks, or to modify the communication driver (not always possible). Here it is therefore necessary to have the considered solution confirmed not only by the supplier of the converter, but also by the reading program.

Pulse converters

If the meter clearly does not have a direct M-Bus interface, but a pulse output on which pulses appear whose number is proportional to the measured energy, it is possible to use pulse to M-Bus converters. These converters (Relay PadPuls, Siemens AEW 310.2 and others) tend to have an adjustable constant, ie how many and what units one pulse represents - eg 10 liters / imp., So we are already reading the physically correct value on the M-Bus. Again, you need to verify

  • whether the type of pulse input corresponds to the meter output: meters usually have a reed relay contact, an open collector or a so-called S0 output (contact treated with two resistors, so we measure a larger impedance at rest and the pulse means a short-term decrease in impedance),

  • whether the maximum pulse frequency is able to be processed by the converter so that distorted values are not read,

  • whether the voltage at the terminals of the converter is high enough to be able to open the transistor (the transistors are protected by resistors, which together with the internal resistance of the converter act as a voltage divider).

Pulse converters are addressed on the M-Bus side in the same way as the meters themselves. Let's just be careful with a converter with multiple pulse inputs, these sometimes behave like two independent meters on the bus with two consecutive addresses (only the lower one is set). Here it could happen that we install a bus with too many converters and it will not be possible to assign unique primary addresses to all of them. Of course, direct connection of pulse meters to the bus is not possible.

Example: Pulse meters on an M-Bus

At one foreign event, electricity meters were supplied by a company installing high current. Devices with both pulse and M-Bus output were delivered to the construction site, and the installers connected all pulse outputs instead of M-Bus terminals in parallel to the M-Bus converter. When the meters sent pulses and thus short-circuited the bus, an alarm LED flashing on the converter signaling a short circuit - but the installers reported "we have it connected well, it communicates, it flashes on the converter and on the meters!". The connection to the correct M-Bus terminals helped.

In conclusion

Without a functioning remote metering system, we would not be able to imagine any heating network, a larger complex of rental apartments or a shopping center or office building with dozens of tenants. For larger systems (eg the Na Slupi residential complex in Prague, where there are over 1200 meters in total on several buses and the long-term probability of failure of at least one meter of this number already limits to 1), the bus system is a necessary condition for fair cost allocation - it is not possible to wait until an error occurs during regular manual readings. Only if the network is properly designed, properly put into operation and regularly diagnosed and managed, can it work to the full satisfaction of the administrator and energy consumers.