M-Bus (fieldbus)

Scheme for integrating remote meter reading into telecontrol systems

The M-Bus , short for Meter-Bus, is a technical standard , through the application of its rules, for example in electricity meters , the consumption of electricity can be transmitted as measurement data. The consumption of gas, heat or water can also be measured and transmitted by meters with M-Bus. As a so-called bus system , it is a technique of electrical data transmission of measured values of used energy. It is a " communication system for the transmission of meter data for various sensors and actuators". Sensors are technical sensors for recording measured values, actuators are elements that can actively intervene in the flow of energy by remote control , e.g. B. can turn off or limit the electricity or gas supply. The European guideline for this technology goes back to the development of the M-Bus in 1992 by Horst Ziegler from the University of Paderborn in cooperation with Techem and Texas Instruments . As part of intelligent energy meters , they play a role under the catchphrase smart metering . The special feature is the remote reading, in which the collected data can be transmitted by other connected devices, including over the Internet or the mobile radio network, and can thus replace human reading.

A European standard

The M-Bus was originally described in the European standard EN1434 ( heat meter ). In the meantime, the M-Bus has become an independent standard in the EN13757 series of standards . This standard describes the M-Bus both for use via the two-wire bus (Part 2) and for radio transmission (Part 4). The description of the communication of the M-Bus is only partially compatible with the international OSI model . The selected layer model corresponds to the 3-layer model ( EN 61334-4-1 ), which was derived from the 7-layer model OSI model. The data link layer was implemented in accordance with the international standard IEC 60870-5 .

Applications

There are M-Bus devices for the following measuring tasks:

The M-Bus has gained some market importance in the field of measuring device manufacturers. It is particularly interesting in connection with inexpensive consumption recording measurement technology with building management systems. It is also popular with compact heat transfer stations. There, the temperature and volume sensors of a heat meter are used for control purposes. In the area of the originally planned application of "remote meter reading for consumption-dependent billing", the M-Bus has been replaced in many places by inexpensive radio systems.

Transmission and consideration in the building automation via DDC-GA is possible via gateways . Some DDC-BA manufacturers have also integrated M-Bus masters into their DDC-BA components. When integrating via DDC-BA components, the compatibility lists of the DDC-BA manufacturers must be consulted.

Technical details

The M-Bus is a field bus for recording consumption data. The transmission takes place serially on a reverse polarity protected two-wire line from the connected measuring devices (slaves) to a master. The master queries the counters via the bus. The slaves can be supplied with power via the bus. The master can be a stand-alone device or a PC with a level converter . The data is usually transmitted at speeds of 300 to 9600 baud . No specific topology (string or star) is prescribed for the cabling. Normal telephone cables of the type JY (St) Y Nx2x0.8 mm can be used. A maximum of 250 meters are allowed per segment - larger systems are interconnected with the help of repeaters. The data transfer from the master to the slave takes place via the modulation of the supply voltage (1 = 36 V / 0 = 24 V). The slave responds to the master by modulating its power consumption by 11–20 mA. The quiescent current (mark state or logical "1") must be constant within narrow limits. A "unit load" is set to 1.5 mA. Slaves with higher power requirements can load the bus with up to 4 unit loads.

All manufacturers of M-Bus meters offer the download of the specification of the transmitted M-Bus data from their meters. This is necessary because neither the order of the values nor their scope is described in the M-Bus standard. In order to ensure interchangeability, the Open Metering Specification was developed and introduced to the responsible committees to revise EN 13757.

Manufacturer-specific data and information can be transmitted in a freely available segment in the data telegram. These manufacturer-specific data are mostly configuration data and are of no interest for normal data exchange.

The initial commissioning of the meters is usually carried out using software supplied by the manufacturer with the aid of a laptop. To do this, the M-Bus slave / device must be connected directly to the laptop via an M-Bus master. A so-called "level converter" is used for this. Then the address and the time can be set.

functionality

In an M-Bus system, a master is required that simultaneously supplies the bus with power. This is responsible for collecting the data from the M-Bus slaves and, if necessary, storing and / or processing them. There may only be one master in the bus system, otherwise voltage and packet collisions will occur. Intelligent routers or so-called splitters allow time-shifted access from different masters. The address space for primary bus addresses extends from addresses 1 to 250. This means that 250 slaves can be connected to a network. If bus users also support 8-digit secondary addressing, a great many more devices can be connected. Reading out via the secondary address takes about twice as long as via the primary address.

Strengthen

The M-Bus is inexpensive and easy to implement on the device side and is reverse-polarity protected during installation.
End devices can be supplied with power via the bus.
Simple, integrated interface circuits are available from, for example, Texas Instruments TSS721, ON Semiconductor NCN5150, and NCN5151.
The digital communication via the bus enables precise measured values to be transmitted to downstream evaluation devices.
Many slaves can be operated on one bus (segment).
Large networks can be set up with the help of repeaters.
By using M-Bus modem masters, it is possible to record the consumption of systems that are far away.
There are no license costs.
No special cables are necessary. Unshielded and untwisted cables are also possible.
The transmitted data are self-explanatory in terms of their binary type, unit and resolution.

weaknesses

The connection elements (plugs) are not standardized.
The data transfer is comparatively slow (2400 baud) and unsuitable for process control.
The standardization at the protocol level is incomplete. Before using new slaves, ensure compatibility with the evaluation unit.
There is no certification body that ensures the proper functioning of the master and slaves.
The self-explanatory type definition in each data packet leads to larger amounts of data.
The evaluation electronics (master) become more complex and expensive as the number of devices on the bus increases.

Problems

The simplicity of the installation leads to errors. The cable lengths are not only dependent on the ability of the master, but also on the cable cross-sections and the device types.

Another problem arises from the original idea of remote meter reading. Since reading is done a maximum of once a day for billing purposes, many devices use their internal battery instead of using the standard 1.5 mA unit load for power supply. However, since energy optimization tasks have to be read at least every 15 minutes, the built-in battery in such devices is quickly read empty (Panasonic BR-2 / 3A 3V / 1200mAh for 10 years) . Not every cheap device is suitable for every task.

Application in water supply companies

In utility companies, remote meter reading systems can become part of the systems integrated in telecontrol technology. The advantage of this lies in the fact that no separate transmission structures need to be set up for remote meter reading. The data and telecontrol telegram traffic on the existing structures is only slightly increased. The data from the remote meter reading can then be displayed directly in the control system and also evaluated and further processed using the control system's mechanisms. This data can also be transferred to systems for further processing of the data via the common interfaces, or this data can be accessed by external systems.

Since the meters are usually placed in shafts or other structures and these are equipped with a connection with an underground telecommunication cable, a pair of wires is usually used for remote meter reading. The M-Bus protocol is used as the protocol for communication, as it enables bidirectional communication on one wire pair and the active components can also be supplied with voltage at the same time with this wire pair. The electrotechnical components for the M-Bus structures are partly distributed by the manufacturers of the water meters or can be obtained from various manufacturers of this technology.

An M-Bus route can have a length of up to 10 km with good quality underground telecommunication cables. The range of the M-Bus is physically limited by the looping of the square-wave signals used and is very much influenced by the cable quality. The number of meters on a route depends on the signal quality and the required reading interval of the meters. In addition, the M-Bus master units to be used are only ever dimensioned for a certain number of meters to be connected.

The water meter data transmitted via the M-Bus are transferred to an M-Bus master unit. This component implements the M-Bus's own voltage level and the special timing of the M-Bus. The M-Bus protocol becomes a serial protocol and the data is then available at an RS 232 data interface for transfer to other systems in the usual data format. Here the coupling can take place with any common telecontrol or automation system. The expansion of the telecontrol or automation systems due to remote meter reading is therefore usually limited to an additional communication module.

The software of this module has to hold and organize a lot of information. All counters from all connected M-Bus master units must be stored in this module and the way in which they can be reached. The individual time behavior must also be set individually for each connected route. The interpretation of the often very different response telegrams of the M-Bus meters and the transfer in a standardized telecontrol protocol also take place in this module. A meter is read out by addressing it, usually via the secondary address, reading and subsequent de-addressing of the meter in accordance with the M-Bus standard. The procedure for the next counter and all other counters is the same until all counters have delivered their data. Over the existing telecontrol or transmission technology, only the current meter readings are recorded in a standard protocol, e.g. E.g .: IEC 60870-5-101 or -104 passed.

Every solution for remote meter reading systems has advantages and disadvantages. A disadvantage of this solution lies in the fact that a new meter with the same secondary address and the same generation always has to be installed when the meter is replaced. Failure to do this requires the existing programming of the communication module to be adapted to the new meters.

One of the preparations for remote meter reading using M-Bus is to check the cable properties. By measuring the loop and insulation resistance, interruptions and insulation faults can be detected. Bad cable sleeves, damp cable sections and the consequences of surge voltages caused by thunderstorms must be recognized and eliminated. Since overvoltages can always occur on telecommunication cables, it is very important to equip the cable cores for remote meter reading with overvoltage protection modules. This minimizes the risk of damage to the electrotechnical components.

The commissioning of a remote meter reading section takes place after all meters have been connected to the M-Bus master unit. A notebook PC, connected to the serial port of the M-Bus master unit, with a corresponding M-Bus scan and read program, shows whether all meters can be read out. The time parameters for the readout that have been successfully tested there can then be set on the communication module and simplify the subsequent commissioning of the telecontrol device or the programmable logic controller.

Individual evidence

↑ A brief overview of the M-Bus http://www.m-bus.com/info/mbus.php ( accessed on November 26, 2013)
↑ ^a ^b OMS: Appendix to the Open Metering System Specification. Glossary of Terms used in or related to the OMS. (PDF) November 4, 2011, accessed on January 11, 2017 (English / German).
↑ ^a ^b Installation of an intelligent readout unit for consumption meters - Diploma thesis by Carsten Bories, March 1995 (accessed on November 26, 2013)
↑ Meter Communication - Twisted Pair Baseband (M-Bus) - Physical and Link Layer

literature

G. Färber: bus systems. R.Oldenbourg Verlag Munich Vienna, 1987.
E. Gabele, M. Kroll, W. Kreft: Communication in computer networks. Springer Verlag, Heidelberg 1991.
Andreas Steffens: The M-Bus - properties and applications. Thesis. University of Paderborn, Department of Physics, 1992.
Texas Instruments Germany: Data Sheet TSS 721. 1993.
Texas Instruments Germany: Seminar Material, M-Bus Workshop, 1992.
Horst Ziegler: Seminar Material, M-Bus Workshop, 1992.
IEC 870-5-1: Telecontrol Equipment and Systems, Part 5 Transmission Protocols, Section One - Transmission Frame Formats. 1990.
IEC 870-5-2: Telecontrol Equipment and Systems, Part 5 Transmission Protocols, Section Two - Link Transmission Procedures. 1992.
EN1434-3: Heat Meters, Part 3 Data Exchange and Interface. 1997.
Aquametro AG Therwil: M-Bus Automatic Slave Recognition with Wildcard Algorithm. 1992.
Andreas Papenheim: Application examples for the M-Bus. Thesis. University of Paderborn, Department of Physics, 1993.
Texas Instruments Germany: Applications Report Designing Applications for the Meter-Bus. 1994.
Horst Ziegler, Günther Froschermeier: M-Bus: The measuring bus alternative. In: Electronics. Issue 16, 1993.

Web links

M-Bus website
jMBus , Java library for M-Bus (wired and wireless) on openmuc.org, GPL license

[1] A brief overview of the M-Bus http://www.m-bus.com/info/mbus.php ( accessed on November 26, 2013)

[glossar-2] OMS: Appendix to the Open Metering System Specification. Glossary of Terms used in or related to the OMS. (PDF) November 4, 2011, accessed on January 11, 2017 (English / German).

[Bories-3] Installation of an intelligent readout unit for consumption meters - Diploma thesis by Carsten Bories, March 1995 (accessed on November 26, 2013)

[4] Meter Communication - Twisted Pair Baseband (M-Bus) - Physical and Link Layer