Meter-Bus

The M-Bus, short for Meter-Bus is a technical standard that can be transmitted as data by the application of its rules, such as current meters, the consumption of electricity. Also, the consumption of gas, heat or water can be measured and transmitted by meters with M- Bus. As it is a technique known as bus data transmission of measured values ​​of electric energy used. It is a " communication system for meter data transmission for various sensors and actuators ". Technical sensors are sensors for measuring value recording, actuators are elements that can actively intervene in the flow of energy remotely, eg can disable or limit the electricity or gas supply. The European guideline of this technology is the development of the M-Bus in 1992 by Prof. Dr. Horst Ziegler of the University of Paderborn in cooperation with the company Techem and Texas Instruments back. As part of intelligent energy meters to play a role under the heading of smart metering. The special feature is the remote readout, in which transmitted the data collected by other connected device including through the Internet or the mobile radio network and thus can replace the reading by humans.

A European standard

The M-Bus was originally in the European Standard EN1434 ( heat meters ) above. Meanwhile, the M-Bus has established itself as a separate standard in the standards EN13757. This standard describes the M-bus, both for use on the two- wire bus (part 2) and for the radio transmission ( part 4). The description of the communication of the M-Bus is only approach compatible with the international OSI model. The selected layer model corresponds to the three -layer model ( EN 61334-4-1 ), which was derived from the 7-layer model OSI model. The data link layer has been implemented in accordance with the international standard IEC 60870-5.

Applications

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

  • Gas meters
  • Electric meter
  • Heat Meter
  • Water meter
  • Pulse converter
  • Analog to Digital Converter
  • Humidity / temperature sensors

The M-Bus has gained in the field of measuring instruments manufacturers, some market importance. Interestingly, it is particularly in the context of cheaper consumption sensing measurement technology with the building control system. Even with heat transfer - compact stations it is often used. There the temperature and volume sensors of a heat meter for control purposes. In the area of the originally planned application " Remote Meter Reading for usage-based billing " of the M-Bus was in many places replaced by inexpensive radio systems.

Gateways and the transfer into account in the building automation via DDC GA is possible. Some DDC GA manufacturers have also incorporated into their DDC GA -component M -Bus master. When integrating over DDC GA components, the compatibility lists of DDC - GA- manufacturer shall be consulted.

Technical details

The M- bus is a fieldbus for consumption data acquisition. The transmission is made serially on a polarized two-wire cable from the connected instruments (slaves) to a master. The master requests via the bus from the counter. The power supply of the slaves can be effected via the bus. The master can be a stand-alone device or a PC with a level converter. The data is normally transmitted at speeds from 300 up to 9600 Bd. For the wiring, no specific topology (phase or star ) is prescribed. It can be used standard telephone cable type JY ( St) Y Nx2x0, 8 mm. Maximum of 250 meters are allowed per segment - larger systems are interconnected using repeaters. The data transfer from master to slave is carried out by modulating the supply voltage ( V = 36 1 / 0 = 24 V). The slave responds to the master via the modulation of its electricity consumption by 11-20 mA. The quiescent current (Mark -state or logic " 1") must be constant within narrow limits. A " Unit Load" is set to 1.5 mA. Slaves with higher power requirements may burden the bus with up to 4 Unit Loads.

All manufacturer of M- Bus meters offer the download of the specification of the transmitted M-Bus data of their counter. This is necessary, because neither the sequence of the values ​​or the periphery thereof is described in the M- bus standard. To ensure the interchangeability of the Open Metering Specification was developed and introduced to the revision of EN 13757 in the competent bodies.

In a freely available segment in the data telegram manufacturer-specific data and information can be transferred. This data is generally vendor-specific configuration data, and for normal data exchange is not of interest.

The initial commissioning of the meter is usually a manufacturer-supplied software with the help of a laptop. For this, the M-Bus-Slave/-Gerät via an M -Bus master is to connect directly to the laptop. For this purpose, a so-called " level converter " used. Then, the address and the time can be set.

Operation

In an M- bus system, a master is required, which also supplies the bus with power. This is responsible for ensuring that the data is collected from the M - bus slaves and optionally stored and / or processed further. There must be only one master in the bus system, since this will cause voltage and packet collisions. Intelligent router or so-called splitter permit from the time- offset access from different masters. The address space for primary bus addresses extends the addresses 1 to 250 can thus be 250 slaves connected to a network. If all nodes also support the 8-digit secondary addressing, and consequently a lot more devices can be connected. The reading on the secondary address takes about twice as long as on the primary address.

Strengthen

  • The M-bus is inexpensive on the device side and easy to achieve and reverse polarity protection during installation.
  • Terminals can be supplied via the bus with power.
  • There are simple, integrated interface circuits, for example, Texas Instruments TSS721, On Semiconductor NCP5150 available.
  • Digital communication over the bus possible to transmit precise measurements to subsequent data evaluation.
  • Many slaves can be operated on one bus (segment).
  • With the help of repeaters large networks can be established.
  • Through the use of M-bus masters modem it is possible to detect the consumption of remote units.
  • There are no special cable required. Also unshielded and untwisted cables are possible.
  • The transmitted data are self-explanatory in terms of their binary type, unit and resolution.

Weaken

  • The connecting elements ( plugs) are not standardized.
  • Data transmission is comparatively slow (2400 baud) and unsuitable for process control.
  • The standardization at the protocol level is patchy. Prior to the use of new slaves compatibility must be ensured to the evaluation.
  • There is no certification body which ensures the ordnungsgemäse function of master and slaves.
  • The self-explanatory type definition in each data packet leads to larger amounts of data.
  • The transmitter ( master) with increasing number of devices on the bus always more complex and expensive.

Problems

The simplicity of installation led to errors. Thus, the line lengths of not only the ability of the master, but also on the cable cross sections and types of devices are subject.

Another problem comes from the original idea of the meter reading forth. As is read for billing purposes at most once per day, many devices provide from its internal battery. Since energy optimization tasks but at least every 15 minutes to read, in such devices, the permanently installed battery is read quickly. Thus, not every cheap device suitable for every task.

Application in water utilities

In utility meter reading systems can become an integral part of the integrated in telecontrol systems. The advantage lies in the fact that no separate transmission structures are set up for remote meter reading. The data or telecontrol telegram traffic on the existing structures is thus only slightly increased. The data from the meter reading can then be displayed in the control systems directly and also evaluated the mechanisms that control system and further processed. Similarly, these data can then be passed to the system for further processing of the data or to access external systems on these data on the common interfaces.

Since the counters are usually placed in manholes or other structures, and they are equipped with a fitting of a telecommunications underground cable, one pair for the meter reading is most often used. The protocol used for communication, the M -Bus protocol is used because it allows two-way communication on a pair of wires and the active components with this pair can be powered simultaneously. The electrical components for the M- bus structures are partially driven by the manufacturers of water meters or can be obtained from different manufacturers of this technology.

An M- bus route can take up to 10 km with good quality of the telecommunications underground cable used a length. The reach of the M- bus is physically limited by the smoothing of the square signals used and very influenced by the cable quality. The number of counters on a line is dependent on the signal quality and the desired read-out interval of the meter. In addition, to use M -Bus master units are always dimensioned only for a certain number to be connected counter.

The transmitted via the M -Bus data of the water meter to be passed to an M -Bus Master unit. This component is the M-Bus own power level and the particular timing of the M- bus. From the M-Bus protocol is a serial protocol and on an RS 232 interface, the data is then to pass on to other systems in standard data format ready. Here, the coupling may be by any conventional remote control or automation system. Therefore, the extension of the remote control or automation systems for a meter reading is limited usually to only one additional communication module.

The software of this module Group has to a lot of information and organize. All counters from all connected M -Bus master units must be stored in this module and the way how to reach them. Likewise, the individual timing for each connected section must be set individually. Also the interpretation of often very different response messages of the M- Bus meter and disclosure in a standardized telecontrol protocol is carried out in this module. The sequence of the reading of a counter is made by its address, usually on the secondary address, the read out and the subsequent de-addressing of the counter in accordance with the M- bus standard. With the next counter and all other counters The same procedure until all counter their data have supplied. Using existing telecontrol or transfer technology only the current counter readings in a standard protocol, eg be: IEC 60870-5-101 or -104 passed.

Any solution for remote meter reading systems has advantages and disadvantages. A disadvantage of this solution lies in the fact that, in a counter, a new exchange counter is always installed with the same secondary address and the same generation. Failure to do so necessitate an adjustment to the existing programming of the communication module to the new meter.

The preparations for a meter reading via M-Bus is to check the cable properties belongs. By measurement of the insulation resistance and loop interruptions and insulation faults can be detected. Poor cable sleeves, wet cable cuts and consequences of surge foothills by storm action must be identified and eliminated. Since overvoltages on telecommunication underground cables can always occur, it is very important to provide the cable wires to the meter reading with overvoltage protection modules. This risk minimization with respect is achieved in damage to the electrical components.

The commissioning of a Zählerfernauslesungsstrecke done after connecting all counter 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 reveals whether all counters can be read. The tested there as successfully time parameters for the reading can then be set to the communication module and simplify the subsequent commissioning of the remote control unit or the programmable logic controller.

Energy flow (ecology) Comparator#Level shifter Open Metering System
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