Meter-Bus

Key takeaways

Meter-Bus belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Meter-Bus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Meter-Bus from memory before moving on to harder problems.

Reference excerpt

M-Bus or Meter-Bus is a European standard (EN 13757-2 physical and link layer, EN 13757-3 application layer) for the remote reading of water, gas or electricity meters. M-Bus is also usable for other types of consumption meters, such as heat meters or water meters. The M-Bus interface is made for communication on two wires, making it cost-effective. A radio variant of M-Bus Wireless M-Bus is also specified in EN 13757–4. The M-Bus was developed to fill the need for a system for the networking and remote reading of utility meters, for example to measure the consumption of gas or water in the home. This bus fulfills the special requirements of remotely powered or battery-driven systems, including consumer utility meters. When interrogated, the meters deliver the data they have collected to a common master, such as a hand-held computer, connected at periodic intervals to read all utility meters of a building. An alternative method of collecting data centrally is to transmit meter readings via a modem. Other applications for the M-Bus such as alarm systems, flexible illumination installations, heating control, etc. are suitable.

Relation to the OSI model Since no bus system was available for the requirements of meter reading, the M-Bus was developed by Horst Ziegler of the University of Paderborn in cooperation with Texas Instruments Deutschland GmbH and Techem GmbH. The concept was based on the ISO-OSI Reference Model, in order to realize an open system which could use almost any desired protocol. Since the M-Bus is not a network, and therefore does not – among other things – need a transport or session layer, the levels four to six of the OSI model are empty. Therefore, only the physical, the data link, the network and the application layer are provided with functions.

Physical wire and connectors M-Bus connection is called M-Bus or HAN (Home Area Network) consumer connection. M-Bus uses two-wire telephone cable (JYStY 1x2x0.8 mm or similar, 73 ohm/km, 120 nF/km) maximum length of 350 meters when using nominal transfer speeds 300 and 9600 baud. Lowering the speed up to 1000 meter cable can be used. There is no standardized connector, but RJ11 and RJ12 Modular connectors are used by meter manufacturers. The master communication uses voltage signaling, where 1 (idle state, mark) is the bus nominal 36 volts, 0 (space) drops the voltage to 24 volts (V). As bus voltage can vary with length and load, the signal is specified as 1 for bus voltage drop less than 5.5 V, 0 for drop higher than 8.2 V. Slaves communicate by current consumption, where 1 (idle state, mark) is less than 1.5 milliamperes (mA), 0 (space) raises current to 11–20 mA. The signal is specified as the at least 11 mA current increase. The slaves are connected via diode bridge and can use either polarity of the wires. To protect the bus against shortcircuited slaves, a 430 ohm is connected in series at each slave (or, two 215 ohm resistors, one for each wire). A M-bus load unit is 1.5 mA. Most slaves use at most this, some can need two units (3 mA). Masters can provide type-dependent number of load units, and usually visually indicate overload.

Data link protocol The data link protocol is described by IEC 870-5, or its updated version, IEC 60870-5. The data are sent in serial form, at speed between 300 and 9600 bit/s (some variants may operate up to 19200 or 38400 bit/s), using one start bit, one stop bit, and even parity (8e1). The least significant bit is sent first. When sending packets ("telegrams"), there is no pause between stop and subsequent start bit. Suggested speeds are 300, 2400, 9600, and with newer hardware 38400 bit/s, while 2400 bit/s is most common. Devices with different baudrates can coexist on the same bus. Some devices use Automatic Baud Rate Detection (ABR) or autobauding. There are four kinds of packets:

C-field is the control/function field. The sequence, from bit 7, is:

bit 7: 0 bit 6: 1 for master-to-slave, 0 for slave reply bit 5: from master: FCB, frame count bit – indicates request to repeat message when reply was not received from slave: ACD, access demand – 1 when slave wishes to transmit class-1 data, priority data (class-2 data is ordinary non-priority) – the master then should request the class-1 data transfer bit 4: from master: FCV, frame count valid – when 0, slave should ignore FCB from slave: DFC, data flow control – when 1, slave can not accept further data bit 3,2,1,0: F3,F2,F1,F0, function code – e.g. for short frame, 0 is for initialization of slave, xA is for class-1 (priority) data read, xB is for class-2 (normal) read. For long/control frame, x3 is sending data to slave, x8 is data reply from slave. A-field is the address field. It is a 8-bit number:

0x00 – unset address, assigned at manufacture time, some meters fixed at this 0x01..0xFA – slave addresses 0xFB, 0xFC – reserved 0xFD – "broadcast" for secondary addressing, addressing done on network layer instead of on data link layer 0xFE – test broadcast, all slaves reply (collisions will happen, use for testing with a single slave; slave replies with its own address in A-field), also possible to use when there is only one slave on the bus 0xFF – broadcast, no reply from slaves CI-field is the control information field. Defined at application layer. Length field in control/long frame is sent twice. Both bytes have to be equal. Minimum value is 0x03, as C-field, A-field and CI-field are mandatory parts of the payload. Slaves respond only to correctly formed packets that match their address. Any fault is indicated by lack of response. Absence of response is defined as no response for 330 bit periods (35 ms for 9600 bit/s, 1.1 s for 300 bit/s) plus 50 ms. Numerical values are usually sent in BCD format.

Worked examples

Example 1 — a first encounter with Meter-Bus

Start with the simplest possible case. Write down what Meter-Bus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Meter-Bus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Meter-Bus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Meter-Bus

In research: Meter-Bus appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Meter-Bus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Meter-Bus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer buses, EN standards, so understanding it makes those chapters shorter.
In everyday life: Look for Meter-Bus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Meter-Bus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Meter-Bus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Meter-Bus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Meter-Bus in simple terms?

M-Bus or Meter-Bus is a European standard (EN 13757-2 physical and link layer, EN 13757-3 application layer) for the remote reading of water, gas or electricity meters. M-Bus is also usable for other types of consumption meters, such as heat meters or water meters.

Why does Meter-Bus matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Meter-Bus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Meter-Bus.