SIST EN 13757-2:2018+A1:2024

Communication systems for meters - Part 2: Wired M-Bus communication

This draft European standard is applicable to the physical and link layer parameters of baseband communication over twisted pair (M Bus) for meter communication systems. It is especially applicable to thermal energy meters, heat cost allocators, water meters and gas meters.
NOTE    It is usable also for other meters (like electricity meters) and for sensors and actuators. For generic descriptions concerning communication systems for meters and remote reading of meters see EN 13757-1.

Kommunikationssysteme für Zähler - Teil 2: Drahtgebundene M-Bus-Kommunikation

Dieses Dokument ist anwendbar für die Parameter der Bitübertragungsschicht und der Sicherungsschicht der Basisbandkommunikation über Zweidrahtleitungen (M-Bus) für Zählerkommunikationssysteme. Es ist insbesondere für thermische Energiemessgeräte, Heizkostenverteiler, Wasserzähler und Gaszähler anwendbar.
ANMERKUNG   Es ist auch auf andere Zähler (wie Stromzähler) und für Sensoren und Stellglieder anwendbar. Für allgemeine Beschreibungen hinsichtlich Kommunikationssystemen für Zähler und der Fernablesung von Zählern siehe EN 13757 1.

Systèmes de communication pour compteurs - Partie 2 : Communication M-Bus filaire

Le présent document est applicable aux paramètres de la couche physique et de liaison de la communication en bande de base sur paire torsadée (M-Bus) pour les systèmes de communication des compteurs. Il s'applique tout particulièrement aux compteurs d'énergie thermique, aux répartiteurs de frais de chauffage, aux compteurs d'eau et aux compteurs de gaz.
NOTE Il est également applicable à d'autres compteurs (tels que les compteurs électriques) ainsi qu'à d'autres capteurs et organes de commande. Pour les descriptions génériques concernant les systèmes de communication et de télérelevé de compteurs, voir l'EN 13757–1.

Komunikacijski sistemi za števce - 2. del: Žične komunikacije po M-vodilu (vključno z dopolnilom A1)

Ta osnutek evropskega standarda se uporablja za parametre fizične in povezovalne plasti komunikacije v osnovnem pasu prek sukane parice (M-vodilo) za komunikacijske sisteme za merilnike. Posebej se uporablja za merilnike toplotne energije, delilnike stroškov, merilnike vode in merilnike plina.
OPOMBA: Lahko se uporablja tudi za druge merilnike (npr. merilnike električne energije) ter za tipala in pogone. Za generične opise v zvezi s komunikacijskimi sistemi za merilnike in oddaljeno odbiranje merilnikov glej standard EN 13757-1.

General Information

• Status: Published
• Publication Date: 17-Mar-2024
• ICS: 33.200 - Telecontrol. Telemetering
• ICS: 35.100.10 - Physical layer
• ICS: 35.240.99 - IT applications in other fields
• ICS: 91.140.50 - Electricity supply systems
• Technical Committee: I13 - Imaginarni 13
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 03-Jan-2024
• Due Date: 09-Mar-2024
• Completion Date: 18-Mar-2024
• Mandate: M/441 - Standardisation mandate to CEN, CENELEC and ETSI in the field of measuring instruments for the development of an open architecture for utility meters involving communication protocols enabling interoperability
• Ref Project: EN 13757-2:2018+A1:2023 - Communication systems for meters - Part 2: Wired M-Bus communication

Relations

• Merged From: SIST EN 13757-2:2018/kprA1:2023 - Communication systems for meters - Part 2: Wired M-Bus communication
• Effective Date: 11-Oct-2023
• Merged From: SIST EN 13757-2:2018 - Communication systems for meters - Part 2: Wired M-Bus communication
• Effective Date: 11-Oct-2023

Overview - EN 13757-2:2018+A1:2023 (Wired M‑Bus)

EN 13757-2:2018+A1:2023 is the CEN European standard that defines the physical layer and link layer parameters for Wired M‑Bus (M‑Bus) communication over twisted pair for meter communication systems. It covers baseband M‑Bus signalling for thermal energy meters, heat cost allocators, water and gas meters and is also usable for electricity meters, sensors and actuators. The document includes Amendment A1 (approved Oct 2023) and supersedes EN 13757‑2:2018.

Key Topics and Technical Requirements

This part of EN 13757 focuses on low‑level communication requirements and testable parameters, including:

• Physical layer specifications

  • Master/slave electrical requirements (voltages, currents, dynamic behaviour)
  • Slave powering options and multiple unit loads
  • Definition and requirements for mini‑masters and repeaters
  • Burst, surge and protection considerations (informative guidance and examples)

• Link layer behaviour and framing

  • Baud rates (required, optional and auto‑speed modes) and transmit accuracy
  • Bit, byte and block formats, interbyte gaps and datagram sequencing
  • Collision handling, datagram abort rules and more precise collision state definitions introduced in A1
  • Addressing, datagram types (including new SND‑UD2) and use of ACK/REQ‑SKE mechanisms

• Implementation and deployment guidance (informative annexes)

  • Schematic slave transceiver example, surge protection techniques, wiring installation types (small in‑house to wide‑area nets), and protocol examples for startup and readout

Practical Applications and Users

EN 13757‑2 is intended for professionals who design, build, test, install or certify wired M‑Bus meter networks:

• Meter manufacturers (heat, water, gas, electricity) for compliant transceivers and slave devices
• Utility and energy service providers specifying remote reading infrastructures
• System integrators and installers planning M‑Bus wiring topologies and surge protection
• Test laboratories and certification bodies validating physical/link layer conformance
• IoT/sensor vendors adapting M‑Bus for actuators or building automation

Using EN 13757‑2 ensures interoperability, predictable performance (baud rates, collision handling) and compliance with European M‑Bus deployments.

Related Standards

• EN 13757‑1 - Communication systems for meters: Data exchange (higher‑layer protocols and generic descriptions)
• Other parts of EN 13757 cover wireless M‑Bus and application profiles; consult the EN 13757 series for end‑to‑end metering communication solutions.

Keywords: EN 13757‑2, Wired M‑Bus, M‑Bus, meter communication, physical layer, link layer, meter networking, metering standards.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2024
Komunikacijski sistemi za števce - 2. del: Žične komunikacije po M-vodilu
(vključno z dopolnilom A1)
Communication systems for meters - Part 2: Wired M-Bus communication
Kommunikationssysteme für Zähler - Teil 2: Drahtgebundene M-Bus-Kommunikation
Systèmes de communication pour compteurs - Partie 2 : Communication M-Bus filaire
Ta slovenski standard je istoveten z: EN 13757-2:2018+A1:2023
ICS:
33.200 Daljinsko krmiljenje, daljinske Telecontrol. Telemetering
meritve (telemetrija)
35.100.10 Fizični sloj Physical layer
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 13757-2:2018+A1
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2023
EUROPÄISCHE NORM
ICS 33.200; 35.100.10; 35.240.99; 91.140.50 Supersedes EN 13757-2:2018
English Version
Communication systems for meters - Part 2: Wired M-Bus
communication
Systèmes de communication pour compteurs - Partie 2 Kommunikationssysteme für Zähler - Teil 2:
: Communication M-Bus filaire Drahtgebundene M-Bus-Kommunikation
This European Standard was approved by CEN on 8 February 2018 and includes Amendment approved by CEN on 22 October
2023.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 13757-2:2018+A1:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 5
Introduction . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions !and abbreviations" . 8
3.1 !Terms and definitions" . 8
3.2 !Abbreviations" . 9
4 Physical layer specifications . 9
4.1 General . 9
Figure 1 — Representation of bits on the M-Bus . 10
4.2 Electrical requirements slave . 10
4.2.1 Master to slave bus voltages . 10
4.2.2 Slave bus current and multiple unit loads . 11
4.2.3 Dynamic requirements . 12
4.3 Electrical requirements master . 12
4.3.1 Parameters . 12
4.3.2 Function types . 13
4.3.3 Requirements . 13
4.4 Electrical requirements mini-master . 15
4.4.1 Definition of a mini-master . 15
4.4.2 Requirements . 15
4.5 Repeaters . 15
4.5.1 General requirements . 15
4.5.2 Additional requirements . 16
4.6 Burst and surge requirements . 16
4.6.1 General . 16
4.6.2 Requirements for devices intended for domestic use . 16
4.6.3 Requirements for devices intended for industrial use . 16
5 Link Layer (master and slave) . 16
5.1 General . 16
5.2 Baud rate . 16
5.2.1 Required baud rate . 16
5.2.2 Recommended additional baud rates . 16
5.2.3 Special baud rates . 17
5.2.4 Baud rate after reset . 17
5.2.5 Baud rate set . 17
5.2.6 Auto speed mode . 17
5.2.7 Transmit baud rate accuracy . 17
5.3 Bit position . 17
5.3.1 Synchronous transmit bit distortion. 17
5.3.2 Gross transmit bit distortion and minimum signal element . 17
5.3.3 Character interval requirement . 18
5.3.4 Practical receive margin and character interval requirement . 18
5.3.5 Minimum signal element . 18
5.4 Byte format . 18
5.5 Block format . 18
5.5.1 Transmission interbyte gaps . 18
5.5.2 Reception interbyte gaps . 18
5.5.3 Idle time between datagrams . 18
5.6 Datagram abort on collision . 18
5.7 Datagram description . 19
5.7.1 General . 19
5.7.2 Data integrity . 19
5.7.3 !Communication types" . 19
5.7.4 Datagram coding . 20
5.7.5 Addressing . 20
5.7.6 Link layer time schedule . 20
5.7.7 Datagram sequencing. 20
6 Tables and figures . 22
Table 1 — Signal quality characteristics for slaves and masters . 22
Figure 2 — Start stop distorsion (example for bit 4), minimum signal element (example for
bit 7) (Transmit) . 23
Figure 3 — Character interval requirement (Transmit) . 23
Figure 4 — Practical receive margin (example for two falling slopes) . 24
Figure 5 — Character interval requirement (Receive) . 24
Figure 6 — Minimum duration start element (Receive) . 25
Figure 7 — Reception of datagram packets . 25
Figure 8 — Quiescent time after reception. 26
Annex A (informative)  Schematic implementation of slave . 27
Figure A.1 — Slave transceiver . 27
Annex B (informative) !Examples of protection techniques for M-Bus meters against
surge/lightning" . 28
Annex C (informative)  Slave powering options . 33
Annex D (informative)  Slave collision detect . 34
Annex E (informative)  Wire installation . 35
E.1 General . 35
E.2 Type A: small in house installation . 35
E.2.1 Description . 35
E.2.2 Usage . 35
E.3 Type B: large in house installation . 35
E.3.1 Description . 35
E.3.2 Usage . 35
E.4 Type C: small wide area net . 35
E.4.1 Description . 35
E.4.2 Usage . 36
E.5 Type D: large wide area net . 36
E.5.1 Description . 36
E.5.2 Usage . 36
E.6 Type E: mini installation (meter cluster) . 36
E.6.1 Description . 36
E.6.2 Usage . 36
Annex F (informative)  Protocol examples . 37
F.1 Startup . 37
F.2 Slave (meter) readout . 37
Bibliography . 38
European foreword
This document (EN 13757-2:2018+A1:2023) has been prepared by Technical Committee CEN/TC 294
“Communication systems for meters”, the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by June 2024, and conflicting national standards shall be
withdrawn at the latest by June 2024.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes !EN 13757-2:2018".
This document includes Amendment 1 approved by CEN on 22 October 2023.
The start and finish of text introduced or altered by amendment is indicated in the text by tags !".
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association.
The following significant technical changes have been incorporated in the new edition of this document:
a) more precise definition of collision state under 4.3.3.8;
b) modification of application under 5.7.3.4 from “required” to “optional”;
c) additional explanations for usage of REQ-SKE under 5.7.3.4;
d) addition of new datagram SND-UD2 under 5.7.3.4;
e) alignment of Annex D with revised definition of collision state under 4.3.3.8 and
f) editorial alignments with other parts of this standard, e.g. replacement of $E5 with ACK.
EN 13757 is currently composed with the following parts:
— Communication systems for meters — Part 1: Data exchange;
— Communication systems for meters — Part 2: Wired M-Bus communication;
— Communication systems for meters — Part 3: Application protocols;
— Communication systems for meters and remote reading of meters — Part 4: Wireless meter readout
(Radio meter reading for operation in SRD bands);
— Communication systems for meters — Part 5: Wireless M-Bus relaying;
— Communication systems for meters — Part 7: Transport and security services;
— CEN/TR 17167, Communication systems for meters — Accompanying TR to EN 13757-2,-3 and -7,
Examples and supplementary information.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Introduction
This European Standard belongs to the EN 13757 series, which covers communication systems for
meters. EN 13757-1 contains generic descriptions and a communication protocol. EN 13757-3 contains
detailed description of the application protocols especially the M-Bus Protocol. EN 13757-4 describes
wireless communication (often called wireless M-Bus or wM-Bus). EN 13757-5 describes the wireless
network used for repeating, relaying and routing for the different modes of EN 13757-4. EN 13757-6
describes a twisted pair local bus for short distance (Lo-Bus). EN 13757-7 describes transport
mechanism and security methods for data. The Technical Report CEN/TR 17167 contains informative
annexes from EN 13757-2, EN 13757-3 and EN 13757-7.
An overview of communication systems for meters is given in EN 13757-1, which also contains further
definitions.
The Physical and Link Layer parameters for baseband communication over twisted pairs have first been
specified in EN 1434-3:1997 (“M-Bus”) for heat meters. This standard is a compatible and interworking
update of a part of EN 1434-3:2015 and includes also other measured media (e.g. water, gas, thermal
energy, heat cost allocators), the master side of the communication and newer technical developments.
It should be noted that EN 1434-3: 2015 covers also other communication techniques.
It can be used with various application layers especially the application layer of EN 13757-3.
1 Scope
This document is applicable to the physical and link layer parameters of baseband communication over
twisted pair (M-Bus) for meter communication systems. It is especially applicable to thermal energy
meters, heat cost allocators, water meters and gas meters.
NOTE It is usable also for other meters (like electricity meters) and for sensors and actuators. For generic
descriptions concerning communication systems for meters and remote reading of meters see EN 13757–1.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
EN 13757-1:2014, Communication systems for meters - Part 1: Data exchange
EN 60870-5, (all parts), Telecontrol equipment and systems (IEC 60870-5 series)
EN 60870-5-1, Telecontrol equipment and systems - Part 5: Transmission protocols - Section 1:
Transmission frame formats
EN 60870-5-2:1993, Telecontrol equipment and systems - Part 5: Transmission protocols - Section 2: Link
transmission procedures
EN 61000-4-4, Electromagnetic compatibility (EMC) - Part 4-4: Testing and measurement techniques -
Electrical fast transient/burst immunity test
EN 61000-4-5, Electromagnetic compatibility (EMC) - Part 4-5: Testing and measurement techniques -
Surge immunity test
3 Terms, definitions !and abbreviations"
3.1 !Terms and definitions"
For the purposes of this document, the terms and definitions given in EN 13757-1:2014 and the
following apply.
!3.1.1
communication type
frame type as defined in EN 60870-5-2:1993 and identified by the function code
Note 1 to entry: Other parts of EN 13757 also use the term message type as an equivalent."
3.1.2
unit load
one unit load (1 U ) is the maximum mark state current of 1,5 mA
L
!3.1.3
ACK
acknowledge frame coded with E5h according to EN 60870-5-2:1993, 3.2 “Format FT 1.2”
3.1.4
NACK
negative acknowledge frame coded with A2h according to EN 60870-5-2:1993, 3.2 “Format FT 1.2”"
3.2 !Abbreviations"
!Abbreviation Term
FCB frame count bit
FCV frame count valid
bit "
4 Physical layer specifications
4.1 General
Figure 1 shows the principal electrical concept of the physical layer: Information from the master to the
slaves is transmitted via voltage level changes. A mark state voltage U (idle state, typically 36 V)
Mark
and an space state voltage which is typically 12 V below U (but at least 12 V) is used for the data
Mark
transmission. The high voltage step improves the noise immunity in the master to slave direction. The
required minimum voltage supports a stable remote powering of all slaves of a segment. Signalling via a
voltage change rather than by absolute voltage levels supports even large voltage drops due to wiring
resistance of the cable installation. All slaves are constant current sinks. Their mark state current of
typically 1,0 mA to 1,5 mA can be used for powering the transceiver IC in the slave and optionally also
the slave (meter). The active (space state) current transmit of a slave is signalled by an increase of this
constant current by (11 to20) mA. Signalling via constant current improves the immunity against
induced voltages and is independent on wiring resistance. On the input of each slave transceiver a
rectifier bridge makes each slave independent of the wiring polarity and reduces installation errors.
Protective resistors in front of each slave transceiver simplify the implementation of overvoltage
protection and safeguards, the bus against a semiconductor short circuit in a slave by limiting the
current of such a defective slave to 100 mA. Annex A shows the principal function of a slave transceiver.
Integrated slave transceivers which include a regulated buffered voltage output for slave (meter)
powering, support of battery supply with supply switchover and power down signalling are
commercially available.
!
"
Key
1 Bus Voltage at Repeater, Master
transmits to Slave
2 Current composition of a Slave, Slave
transmits to Master
t time
Figure 1 — Representation of bits on the M-Bus
All specification requirements shall be held over the full range of temperature and operating voltage for
the responsible system component.
4.2 Electrical requirements slave
4.2.1 Master to slave bus voltages
Maximum permanent voltage: - 50 V to 0 V to + 50 V (no damage).
Voltage range for meeting all specifications: ± (12 V to 42 V).
The Bus voltage at the slave terminals in mark-(quiescent) state of master slave communication
(= U ) shall be ± (21 V to 42 V).
Mark
The mark voltage shall be stored by a voltage maximum detector with an asymmetric time constant.
The discharge time constant shall be greater than 30 × (charge constant) but less than 1 s.
The stored voltage maximum U may drop in 50 ms by not more than 0,2 V for all voltages between
Mark
12 V and U .
Mark
a) Bus voltage Mark/Space state for master slave communication:
1) Space: U < U - 8,2 V;
Bus Mark
2) Mark: U ≥ U - 5,7 V;
Bus Mark
b) maximum space state time: 50 ms;
c) maximum space state duty cycle: 0,92.
4.2.2 Slave bus current and multiple unit loads
4.2.2.1 General
A slave device may require a maximum mark state current of an integer multiple N (in the range 1 to 4)
unit loads. Each terminal device shall be marked with the unit load number N (If > 1) and the device
description shall contain a note on the multiple unit loads for this device.
4.2.2.2 Mark state current of a slave device
The mark state current I shall be ≤ N unit loads.
Mark
4.2.2.3 Variation of the mark state current over bus voltage
For bus voltages in the range of ± (12 V to 42 V) a voltage variation of 1 V to 15 V shall not change the
bus current by more than N × 3 µA/V.
4.2.2.4 Short-term variation of the mark state current
At constant bus voltage the bus current shall not change by more than ± 1 % within 10 s.
4.2.2.5 Total variation over allowed temperature and voltage range of slave device
The total variation of the mark state current of a slave device shall not vary by more than ± 10 % over
the full voltage and temperature range of the slave device.
4.2.2.6 Maximum bus current for any single semiconductor or capacitor defect
1 min after any single semiconductor or capacitor defect the maximum current of any slave device shall
be less than 100 mA for any bus voltage ≤ 42 V.
4.2.2.7 Slow start
For any bus voltage in the range of (0 to ± 42) V the bus current shall be limited to ≤ N × U .
L
4.2.2.8 Fast change
After any bus voltage change the bus current shall be ≤ N × U within 1 ms.
L
4.2.2.9 Space state current
The bus current for a slave space state send shall be higher by (11 to 20) mA than in the mark state for
all allowed bus voltages:
I = I + (11 to 20) mA
Space Mark
4.2.2.10 Input capacitance at the slave terminals: ≤ 0,5 nF
This capacitance shall be measured with a DC bias of (15 to 30) V.
4.2.2.11 Startup delay
In case of a bus voltage drop below 12 V for longer than 0,1 s the recovery time after applying an
allowed mark state voltage until reaching full communication capabilities shall be less than 3 s.
4.2.2.12 Galvanic Isolation
The isolation resistance between any bus terminal and all metal parts accessible without violating seals
shall be > 1 MΩ. Excluded are terminals for the connection of other floating or isolated external
components. The test voltage is 500 V. For mains operated terminal devices the appropriate safety rules
apply.
4.2.2.13 Optional reversible mains protection
The slave interface can be equipped with an optional reversible mains protection. This guarantees that
even for a prolonged period (test duration: 1 min) the slave interface can withstand mains voltages of
230 V + 10 % and 50 Hz or 60 Hz and that afterwards all specifications are met again. This mains
protection function is recommended for all mains operated terminal devices. For possible
implementations see Annex B.
4.2.3 Dynamic requirements
Any link layer or application layer protocol of up to 38 400 Baud is acceptable if it guarantees that a
mark state is reached for at least one bit time at least once in every 11 bit times and not later than after
50 ms. Note that this is applicable for any asynchronous protocol with 5 data bits to 8 data bits (with or
without a parity bit) for any baud rate of at least 300 Baud, including a break signal (see 4.3.3.8). It is
also applicable for many synchronous protocols with or without bit coding.
4.3 Electrical requirements master
4.3.1 Parameters
4.3.1.1 Max current (I )
Max
A master for this physical layer is characterized by its maximum current I . For all bus currents
Maximum
between zero and I it shall meet all functional and parametric requirements. For example a
Max
maximally loaded segment with up to 250 slaves with 1 U each (375 mA) plus an allowance for one
L
slave with a short circuit (+ 100 mA) plus the maximum space send current (+ 20 mA) an I ≥ 0,5 A is
max
required.
4.3.1.2 Max allowable voltage drop (U )
r
The maximum voltage drop U (>0 V) is defined as the minimum space state voltage minus 12 V. U
r r
divided by the maximum segment resistance between the master and any terminal device (meter) gives
the maximum usable bus current for a given combination of segment resistance and master.
4.3.1.3 Max baud rate (B )
Max
Another characterization of a master is the maximum baud rate B up to which all specifications are
Max
met. The minimum baud rate is always 300 Baud.
4.3.1.4 Application description
Each master device shall include a description about the required cable and device installation for
proper functioning.
4.3.2 Function types
4.3.2.1 Simple level converter
The master function can be realized as a logically transparent level converter between the M-bus
physical layer and some other (standardized) physical layer (e.g. V24). It is then bit transparent for
allowable baud rates of 300 to B . No bit time recovery is possible. Hence a simple level converter
Max
cannot be used as a repeater.
4.3.2.2 Intelligent level converter
An intelligent level converter can perform space bit time recovery for any asynchronous byte protocol
at its maximum baud rate B . Other baud rates B /L (L = 2 to L ) are allowed, but bit time
Max Max Max
recovery cannot be guaranteed for these other baud rates. Such a level converter can be used as a
physical layer repeater for its maximum baud rate.
4.3.2.3 Bridge
The master function can be integrated with a link layer unit thus forming a (link layer) bridge. If this
bridge can support the required physical and link layer management functions it can support also
multiple baud rates.
4.3.2.4 Gateway
The master function can be integrated into the application layer of a gateway or it can be fully
integrated into an application.
4.3.3 Requirements
4.3.3.1 Mark state voltage
For currents between:
0.I :U 24VU+ .42V
( )
max Mark r
4.3.3.2 Space state voltage
, and
UU< −12V U ≥12VU+
Space Mark Space r'
=
4.3.3.3 Bus short circuit
Reversible automatic recovery shall guarantee full function not later than 3 s after the end of any
current higher than I .
Max
1 ms after the beginning of a short circuit situation the bus current shall be limited to < 3 A.
4.3.3.4 Minimum voltage slope
The transition time between space state and mark state voltages from 10 % to 90 % of the steady-state
voltages shall be ≤ 1/2 of a nominal bit time. The asymmetry of these transition times shall be ≤ 1/8 of a
nominal bit time.
Test conditions (C selected from the E12 value series):
Load
— baud rate 300 Baud: C = 1,5 μF;
Load
— baud rate 2 400 Baud: C = 1,2 μF;
Load
— baud rate 9 600 Baud: C = 0,82 μF;
Load
— baud rate 38 400 Baud: C = 0,39 μF.
Load
4.3.3.5 Effective source impedance
The voltage drop of the bus voltage for a short (<50 ms) increase of the bus current by 20 mA shall
be ≤ 1,2 V.
4.3.3.6 Hum, ripple and short-term (<10 s) stability of the bus voltages
Hum, ripple and short-term (<10 s) stability of the bus voltages: < 200 mV peak to peak.
4.3.3.7 Data detection current (Reception of slave current pulses)
Bus current ≤ Bus idle current + 6 mA: Mark state receive.
Bus current ≥ Bus idle current + 9 mA: Space state receive.
Measurement with current pulses of < 50 ms, duty cycle < 0,92.
4.3.3.8 Reaction at large data current (collision state, break signal)
A current increase beyond a certain level shall be considered as a collision state. Current
increases ≤ 25 mA shall never be detected as collision state. Current increases between 25 mA and
50 mA may be considered as collision state. Current increases of ≥ 50 mA shall be considered as
collision state.
For collision detection the collision state shall persist for at least 2 bit times at all supported baud rates.
If a collision state persists for ≤ 50 µs the master shall not emit a break signal. If a collision state persists
for > 50 µs to < 6,6 ms the master may emit a break signal. If a collision state persists for ≥ 6,6 ms the
master shall emit a break signal.
A break signal is characterized by a bus voltage = U and a duration of 40 ms up to 50 ms. This state
Space
shall also be signalled to the user side.
If the bus current is > I , the master may switch off the bus voltage completely. Note that for voltage
Max
switch off the requirements for minimum recovery time (switch off time > 100 ms, please refer to
4.2.2.11) and for reversible automatic recovery and current limitation (refer to 4.3.3.3) shall be taken
into account.
4.3.3.9 Galvanic isolation
The isolation resistance between any bus terminal and all metal parts accessible without violating seals
shall be > 1 MΩ. The test voltage is 500 V. For mains powered masters or masters with connection to
ground based systems (e.g. connection to the V24 port of a mains powered PC) this includes isolation
from these power respective signal lines. For mains powered masters the appropriate safety rules
apply.
4.3.3.10 Ground symmetry
For mains powered masters or masters with connection to ground based systems (e.g. connection to the
V24 port of a mains powered PC the static and dynamic bus voltages shall be symmetric (40 % to 60 %)
with respect to ground. This requirement is only valid for ground based systems.
4.4 Electrical requirements mini-master
4.4.1 Definition of a mini-master
A Mini-Master can be used in systems which can accept the following restrictions:
— maximum wiring length of its segment: ≤ 50 m;
— B : 2 400 baud;
Max
— no function required if any device fails with overcurrent;
— no automatic search for secondary addresses (collision mode) required.
A Mini-Master can be implemented as a simple level converter to some other standardized physical
layer interface (e.g. V24) or it can be integrated into a data processing device. It usually cannot be used
as a repeater. It can be implemented as a stationary or as a portable device. It can be powered from
mains or it can be battery powered.
4.4.2 Requirements
A Mini-Master has the following reduced requirements as compared to a full standard master:
— Minimum transition slopes:
For a load capacitance of 75 nF: Transition time between mark and space state voltages in both
directions between 10 % and 90 % of the voltage step of the two static signal voltages: Maximum
transition time t ≤ 50 μs.
max
— Behaviour at higher data currents (collision): No requirements.
4.5 Repeaters
4.5.1 General requirements
A physical layer repeater shall meet at its slave side all requirements for a slave and at its master side
all requirements of a master. Such a repeater is required in a net where one or several limits of the
installation concerning maximum number of meters, maximum total cable length, maximum number of
meters per segment or maximum distance are exceeded for the desired baud rate.
4.5.2 Additional requirements
4.5.2.1 Isolation
The bus terminals at the master side shall be isolated from the bus terminals at the slave side. The
isolation resistance shall be ≥ 1 MΩ for the test voltage of 500 V. Any pertinent safety regulations for
mains powered devices shall be considered.
4.5.2.2 Bit recovery
Incoming data bytes with acceptable bit time distortions for a reception according to the requirements
of the link layer used shall be transmitted at the other side in such a way that all the transmit timing
requirements of the link layer are met.
A repeater may therefore be restricted to certain baud rate(s) or may be restricted to certain byte
formats or link layers.
4.6 Burst and surge requirements
4.6.1 General
A device according to this standard shall fulfil at least the following burst and surge requirements
according to EN 61000-4-4 and EN 61000-4-5 for the M-bus connection. Note that device standards
might impose further requirements or might impose higher requirements regarding burst and surge.
Note also that the values have been updated from EN 1434-3 due to field experience.
4.6.2 Requirements for devices intended for domestic use
Burst test voltage: 1 kV (Severity class 2).
4.6.3 Requirements for devices intended for industrial use
Burst test voltage: 1 kV (Severity class 2).
Surge test voltage: 1 kV (Severity class 2).
5 Link Layer (master and slave)
5.1 General
The alphabetic percent designations (e.g. “W %”) in the following clauses refer to the value specified in
Table 1.
5.2 Baud rate
5.2.1 Required baud rate
300 Baud shall be supported.
5.2.2 Recommended additional baud rates
2 400 Baud, 9 600 Baud or 19 200 Baud are recommended.
5.2.3 Special baud rates
By special arrangement between a net operator and a meter manufacturer also one or several of the
following baud rates could be used: 600 Baud, 1 200 Baud, 4 800 Baud or 38 400 Baud.
The total segment size and the number of connected slaves limits the technically safe maximum baud
rate. (See cable installation section in Annex E).
5.2.4 Baud rate after reset
The baud rate shall be kept after a reset of the device.
5.2.5 Baud rate set
The default baud rate of any device after fabrication is 300 Baud. A desired baud rate may be set by link
layer management commands. (See the appropriate application layer commands). Broadcast baud rate
set is not recommended. Immediately (<2 min) after such a baud rate set command for a slave to a baud
rate other than 300 Baud (transmitted at the old baud rate) a valid communication at the new baud rate
shall be attempted. If (even after the appropriate number of retries) no acknowledge is received, the
master shall set the slave baud rate back to the original baud rate via a baud rate set command at the
attempted baud rate and then continue communication at the original baud rate. If the communication
is acknowledged, the master knows that the slave and its segment can both operate at the new baud
rate. A slave without an auto speed detect shall monitor after the reception of a baud rate set command
to a baud rate other than 300 Baud for a valid communication at the new baud rate within 2 min to
10 min after the baud rate set command. If such a communication is not properly received, the slave
shall switch back automatically to the previous baud rate to save it from being permanently lost in a
baud rate which is not supported by its segment.
5.2.6 Auto speed mode
Devices may support communication with all supported baud rates without a prior baud rate set
command (auto speed mode). In this case no baud rate switch command monitoring and auto fall back
is required. All baud rate set commands shall still be acknowledged but can be ignored otherwise except
for their FCB-administration (if required).
5.2.7 Transmit baud rate accuracy
The transmission baud rate averaged over any RSP-UD datagram may vary under all acceptable
parameters (i.e. supply voltages, temperature, current operating state and function) by not more
than ± M % of the nominal baud rate (see Table 1).
5.3 Bit position
5.3.1 Synchronous transmit bit distortion
For data transmission the individual bit transitions may have a non-accumulating maximum deviation
from their nominal time position (calculated from the actual baud rate) of up to N % of a bit time
(Synchronous start-stop-distortion, see also Figure 2).
5.3.2 Gross transmit bit distortion and minimum signal element
For data transmission the individual bit transitions may have a non-accumulating maximum deviation
from their nominal time position (calculated from the nominal baud rate) of up to P % of a bit time
(gross start-stop-distortion, see also Figure 2), assuming that each bit time is at least Q % of a nominal
bit time (minimum signal element, see also Figure
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