MBusConnector

Introduction

IoTSyS is an integration middleware for the Internet of Things. It provides a communication stack for embedded devices based on IPv6, Web services and oBIX to provide interoperable interfaces for smart objects. The IoTSyS middleware aims providing a gateway concept for existing sensor and actuator systems found in nowadays home and building automation systems (KNX, BACnet, ZigBee, Wireless M-Bus), a stack which can be deployed directly on embedded 6LoWPAN devices and further addresses security, discovery and scalability issues.

This connector extends IoTSyS by the M-Bus (Meter Bus) standard. Therefore, the API from the Wireless M-Bus has been altered to make the communication with the meters as easy as possible. To use this API in IoTSyS a M-Bus connector was created. As Raspberry Pi is be the a target platform for IoTSyS a piggy back board M-Bus-Master for a Raspberry Pi was created.

M-Bus Protocol Overview

The M-Bus standard is defined in EN 13757-2 (physical and link layer) and EN 13757-3 (application layer). A detailed description is available at the official M-Bus homepage {1}.

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, which can, for example, be a hand-held computer, connected at periodic intervals to read all utility meters of a building. {1}

MBusConnector.jpg

Physical Layer

The M-Bus is a hierarchical system, with communication controlled by a master. The M-Bus consists of the master, a number of slaves (end-equipment meters) and a two-wire connecting cable. The slaves are connected in parallel to the transmission medium.

In order to realize an extensive bus network with low cost for the transmission medium, a two-wire cable was used together with serial data transfer. In order to allow remote powering of the slaves, the bits on the bus are represented as follows:

The transfer of bits from master to slave is accomplished by means of voltage level shifts. A logical “1” (Mark) corresponds to a nominal voltage of +36V at the output of the bus driver (repeater), which is a part of the master; when a logical “0” (Space) is sent, the repeater reduces the bus voltage by 12V to a nominal +24V at its output. Bits sent in the direction from slave to master are coded by modulating the current consumption of the slave. A logical “1” is represented by a constant (versus voltage, temperature and time) current of up to 1.5mA, and a logical “0” (Space) by an increased current drain requirement by the slave of additional 11-20mA. The mark state current can be used to power the interface and possibly the meter or sensor itself.

Data Link Layer

The physical layer makes certain demands on the data link layer. Besides half-duplex asynchronous serial transmission with data rates between 300 and 9600 Baud, these include the requirement that at least every eleventh bit should be a logical 1, and also that there should be a Master-Slave structure, since the slaves can not communicate with each other. The protocol of the data link layer is based on the international standard IEC 870-5, which defines the transmission protocols for telecontrol equipment and systems. The M-Bus protocol described below derives from the above standard, but doesn´t use all the IEC functions.

Transmission Parameters

This protocol uses asynchronous serial bit transmission, in which the synchronization is implemented with start and stop bits for each character. There must be no pauses within a telegram, not even after a stop bit. Since quiescence on the line corresponds to a 1 (Mark), the start bit must be a Space, and the stop bit a Mark. In between the eight data bits and the even parity bit are transmitted, ensuring that at least every eleventh bit is a Mark. The bits of data are transmitted in ascending order, i.e. the bit with the lowest value (LSB = least significant bit) is the first one to be found on the line. The transmission takes place in half duplex and with a data rate of at least 300 Baud.

Telegram Format

In the M-Bus specification there are three different telegram formats, which can be recognized by means of special start characters. In the table below the telegram formats used for the M-Bus will now be explained.

Single Character Short Frame Control Frame Long Frame
E5h Start 10h Start 68h Start 68h
C Field L Field L Field
A Field L Field L Field
Check Sum Start 68h Start 68h
Stop 16 h C Field C Field
A Field A Field
CI Field CI Field
Check Sum User Data (0-252 Byte)
Stop16h Check Sum
Stop16h

Single Character: This format consists of a single character, which the E5h (decimal 229), and serves to acknowledge receipt of transmissions.

Short Frame: This frame with a fixed length begins with the start character 10h, and besides the C and A fields includes the check sum (this is made up from the two last mentioned characters), and the stop character 16h.

Long Frame: With the long frame, after the start character 68h, the length field (L field) is first transmitted twice, followed by the start character once again. After this, there follow the function field (C field), the address field (A field) and the control information field (CI field). The L field gives the quantity of the user data inputs plus 3 (for C,A,CI). After the user data inputs, the check sum is transmitted, which is built up over the same area as the length field, and in conclusion the stop character 16h is transmitted.

Control Frame: The control sentence conforms to the long sentence without user data, with an L field from the contents of 3. The check sum is calculated at this point from the fields C, A and CI.

Meaning of the Fields

C Field: The following table shows the function codes used in the calling and the replying directions:

Name Hex Value Telegram Description
SND_NKE 40 Short Frame Initialization of Slave
SND_UD 53/73 Long/Control Frame Send User Data to Slave
REQ_UD2 5B/7B Short Frame Request for Class 2 Data
REQ_UD1 5A/7A Short Frame Request for Class1 Data
RSP_UD 08/18/28/38 Long/Control Frame Data Transfer from Slave to Master after Request

A Field: The address (A) field is used to address a slave in calling direction and to identify the sender of information in receiving direction. The addresses 1 to 250 can be allocated to the individual slaves, up to a maximum of 250. The address 0 is reserved for unconfigured slaves and the addresses 254 and 255 are used for broadcasts.

CI Field: The control information (CI) field is already a part of the Application Layer, and is described in more detail in section Application Layer. It is used to distinguish between the formats of the long and the control frames. The control information allows the implementation of a variety of actions in the master or the slaves.

Check Sum: The Check Sum is used to recognize transmission and synchronization faults. The Check Sum is calculated from the arithmetical sum of the data mentioned above without taking carry digits into account.

Communication Process

The Data Link Layer uses the two kinds of transmission services Send/Confirm (SND/CON) and Request/Respond (REQ/RSP).

Send/Confirm Procedures:

SND_NKE → Single control character This procedure is used to start up after the interruption or beginning of communication. The slave responds to a correctly received SND_NKE with an acknowledgment using of a single character (E5h).

SND_UD → Single control character With this procedure the master sends user data to the slave. The slave can either confirm the correct receipt of data with a single character acknowledge (E5h), or by omitting a confirmation signal that it did not receive the telegram correctly.

Request/Respond Procedures:

REQ_UD2 → RSP_UD The master requests data from the slave according to Class 2. The slave can either transfer its data with RSP_UD, or give no response indicating that the REQ_UD2 telegram has not been received correctly or that the address contained in the REQ_UD2 telegram does not match.

Application Layer

The standardized application protocol is defined in the standard EN1434-3 for data exchange with heat meters. This standard is also suitable for other consumer utility meters, e.g. for gas and water. However, EN1434-3 only covers the data structure in the reply direction, the data structure generally used in the direction master to slave will be presented here.

The CI field encodes the mode of operation of the data transfer between the master and a slave. To send the requested data from a slave to the master there are two possible data structures, namely fixed data structure and variable data structure.

The configuration of slaves is also defined in this layer, but won’t be described here, because M-Bus meters are only read with IoTSyS.

MBusConnector Schematic

Fixed Data Structure

In the reply direction with a long frame two different data structures are used. The fixed data structure, besides a fixed length, is limited to the transmission of only two counter states of a predetermined length, which have binary or BCD coding. In contrast the variable data structure allows the transmission of more counter states in various codes and further useful information about the data. The number of bytes of the transmitted counter states is also variable with this data structure. Contrary to the fixed structure, the variable structure can also be used in calling direction. For this reasons the fixed data structure is not recommended for future developments.

The frame of the fixed data structure is shown in the following table:

Identification No. Access No. Status Medium/Unit Counter 1 Counter 2
4 Byte 1 Byte 1 Byte 2 Byte 4 Byte 4 Byte

Variable Data Structure

The frame of the variable data structure is shown in the table below:

Fixed Data Header Variable Data Blocks (Records) MDH Mfg.specific data
12 Byte variable number 1 Byte variable number

Fixed Data Header:

The first twelve bytes of the user data consist of a block with a fixed length and structure :

Ident. Nr. Manufr. Version Medium Access No. Status Signature
4 Byte 2 Byte 1 Byte 1 Byte 1 Byte 1 Byte 2 Byte

Variable Data Blocks:

Each data record contains one value with its description as shown in the table below, a data record, which consists of a data record header (DRH) and the actual data. The DRH in turn consists of the DIB (data information block) to describe the length, type and coding of the data, and the VIB (value information block) to give the value of the unit and the multiplier.

| DIF | DIFE | VIF | VIFE | Data | |:—-|:—–|:—-|:—–|:—–| | 1 Byte | 0-10 (1 Byte each) | 1 Byte 0-10 | (1 Byte each) | 0-N Byte |

The DIB contains at least one byte (DIF, data information field), and can be extended by a maximum of ten DIFE’s (data information field extensions). After a DIF or DIFE without a set extension bit there follows the VIB (value information block). This consists at least of the VIF (value information field) and can be expanded with a maximum of 10 extensions (VIFE). The VIF and also the VIFE’s show with a set MSB that a VIFE will follow. In the value information field VIF the other seven bits give the unit and the multiplier of the transmitted value. The meaning of the multiplier can be look up in the M-Bus standard {1}.

Manufacturer Specific Data Block:

The manufacturer specific data block consists of the manufacturer data header (MDH) and manufacturer specific data. This can’t be encoded because it depends on the manufacturer how this part of frame structure looks like.

Example: Slave Read Out

Master sends a request (REQ_UD2) for reading data of slave with address 1: 10 7B 01 7C 16

Encoded request send from master to slave with address 1: 10 Start: Short frame 7B C-Feld: REQ_UD2: 01FV 1011 = 4B/5B/6B/7B: Request for Class 2 Data 01 A-Feld : primary address = 1 7C Checksum 16 Stop

Received data from slave with address 1: 68 6A 6A 68 08 01 72 43 53 93 07 65 32 10 04 CA 00 00 00 0C 05 14 00 00 00 0C 13 13 20 00 00 0B 22 01 24 03 04 6D 12 0B D3 12 32 6C 00 00 0C 78 43 53 93 07 06 FD 0C F2 03 01 00 F6 01 0D FD 0B 05 31 32 4D 46 57 01 FD 0E 00 4C 05 14 00 00 00 4C 13 13 20 00 00 42 6C BF 1C 0F 37 FD 17 00 00 00 00 00 00 00 00 02 7A 25 00 02 78 25 00 3A 16

Encoded data from slave with address 1: 68 Start : STart of telegram, Long or control frame 6A L-Feld : Length of user data plus 3
6A L-Feld : Length of user data plus 3 68 Start : 2. Start 08 C-Feld : RSP_UD 01 A-Feld : primary address = 1 Begin user data: 72 CI-Feld : variable data structure, Mode 1 Fixed data block: 43 53 93 07 Identification#= 07935343 65 32 Manufacturer = 3265h equals LSE 10 04 Version = 10 Medium = Heat CA 00 Access number = CA Status = 00h 00 00 Signatur = 00 00h 1.Datenrecord: 0C DIF: 8 digit BCD 05 VIF: E0000nnn = Energy 10^(nnn-3) Wh = 0.001Wh to 10000Wh: 10^(5-3) Wh = 100 Wh 14 00 00 00 date = 14 * 100 Wh = 1.4 kWh 2.Datenrecord: 0C DIF: 8 digit BCD 13 VIF: E0010nnn = Volume 10(nnn-6) m3 = 0.001l to 10000l: 10^(3-6) m3 = 10^-3 m3 = 1l 13 20 00 00 date = 2013 * 1l = 2013 l 3.Datenrecord: 0B DIF: 6 digit BCD 22 VIF: E010 00nn On Time: nn = 00 seconds, 01 minutes, 10 hours, 11 days 01 24 03 date = 032401 hours 4.Datenrecord: 04 DIF: 32 Bit Integer 6D VIF: E110110n: Time Point n = 0 date = data type G, n = 1 time & date = data type F 12 0B D3 12 date: Type F = Compound CP32: Date and Time: 5.Datenrecord: 32 DIF: 16 Bit Integer, fehlerbehaftet 6C VIF: E110110n Time Point n = 0 = data type G 00 00 date: Type F = Compound CP32: Date and Time: 6.Datenrecord: 0C DIF: 8 digit BCD 78 VIF: E110110n Time Point n = 0 = data type G 43 53 93 07 date: E111 1000 Fabrication No 07935343 7.Datenrecord: 06 DIF: 48 Bit Integer FD VIF: 1111 1101 Extension of VIF-codes true VIF is given in the first VIFE and is coded using table 8.4.4 a) 0C VIFE: E000 1100 Model / Version F2 03 01 00 F6 01 date: Model / Version 01F6000103F2h = 2156073649138 8.Datenrecord: 0D DIF: variable Länge FD VIF: 1111 1101 Extension of VIF-codes true VIF is given in the first VIFE and is coded using table 8.4.4 a) 0B VIFE: E000 1011 Parameter set identification 05 LVAR: ASCII string with LVAR characters, Länge 5 31 32 4D 46 57 date: 57 46 4D 32 31 = WFM21 9.Datenrecord: 01 DIF: 8 bit Integer FD VIF: 1111 1101 Extension of VIF-codes true VIF is given in the first VIFE and is coded using table 8.4.4 a) 0E VIFE: E000 1110 Firmware version # 00 date: 0 10.Datenrecord: 4C DIF: 8 digit BCD, LSB of storage number 1 05 VIF: E0000nnn = Energy 10^(nnn-3) Wh = 0.001Wh to 10000Wh: 10^(5-3) Wh = 100 Wh 14 00 00 00 date = 14 * 100 Wh = 1.4 kWh 11.Datenrecord: 4C DIF: 8 digit BCD, LSB of storage number 1 13 VIF: E0010nnn = Volume 10(nnn-6) m3 = 0.001l to 10000l: 10^(3-6) m3 = 10^-3 m3 = 1l 13 20 00 00 date = 2013 * 1l = 2013 l 12.Datenrecord: 42 DIF: 16 Bit Integer, LSB of storage number 1 6C VIF: E110110n Time Point n = 0 = data type G BF 1C date: Type F = Compound CP32: Date and Time: 13.Datenrecord: 0F DIF: special function 37 FD 17 00 00 00 00 00 00 00 00 02 7A 25 00 02 78 25 00 manufacturer specific data 3A Checksum 16 Stop

M-Bus Java API

The M-Bus Java Api is shown as class diagram in the picture below:

Class diagramm: https://jschober88-mbus.googlecode.com/hg/iotsys-mbus/doc/MBus.png’ />

The class MBusConnector is used to create a new instance of the class ComPortReader, which handles the serial communication with the M-Bus master. To initialize the ComPortReader correctly there are a some useful methods in the class MBusConnector: * connect(): Creates a new instance of ComPortReader and open a serial connection. * disconnect(): Closes the serial connection. * setInterval(): Sets the periodic reading interval of the meters. * setAdress(): Sets the address of the meter. * refresh(): Forces a start of the read out procedure of a meter.

If the ComPortReader receives a new telegram, it is added to a TelgramManager. The class Telegram represents the data structure of a M-Bus long frame. Therefore, Telegram is split into TelegramHeader (representing the fields start, L, C, A, CI, check sum and stop) and the and TelegramBody (representing the user data).

The TelegramBody can be split furthermore as seen in section Data Link Layer into the TelegramBody header representing the fixed data header and the TelegramBodyPayload representing the variable data blocks, which can be again differentiated in the following fields: * DIFTelegramField * DIFETelegramField * VIFTelegramField * VIFETelegramField

The information of all data fields is encoded using the tables of the appendix of the M-Bus documentation {1}. For debugging the class Telegram and all the related classes have a method debugOutput() to print the encoded data to the console.

Connection and Device Configuration

For the connection you need to specify the serial port of the Raspberry Pi (see UART configuration). Further, the serial number and the address of the meter and polling interval need to be specified.

“` MBus Smart Meter Linux true /dev/ttyS80 at.ac.tuwien.auto.iotsys.gateway.obix.objects.iot.sensors.impl.mbus.SmartMeterImplMBus 01 60 07935343 smartmeter true 1000

MBus Smart Meter Windows false COM17 at.ac.tuwien.auto.iotsys.gateway.obix.objects.iot.sensors.impl.mbus.SmartMeterImplMBus 60 01 07935343 smartmeter true 1000 “`

M-Bus-Master for Raspberry Pi

To create a master for the M-Bus a layout for simple M-Bus-Master for up to 3 slaves {2} has been used. This layout had to be extended, because the Raspberry Pi doesn’t have a RS232 port, which is necessary for this M-Bus-Master. This master is on the right side of the picture below. The M-Bus-Master operates with a supply voltage of +/- 15V. The Raspberry Pi only provides 3.3V and 5V as output voltages. Hence, a DC/DC converter IH0515S {3} produced by the company XP POWER was used to convert the 5V output of the Raspberry Pi to +15V and -15V with a maximum output current of 66mA for each output. The output currents are sufficient, because a M-Bus slave consumes between 12mA and 20mA and the Mini M-Bus-Master is designed to operate with up to 3 slaves.

The Raspberry Pi GPIO (TXD GPIO14 and RXD GPIO15) voltage levels are 3.3V and therefore, a MAX3232 has to be used to operate correctly with the M-Bus-Master input and output, which uses the RS232 voltage levels of +/-12V.

https://jschober88-mbus.googlecode.com/hg/iotsys-mbus/doc/MBus-Master_Circuit_BW.png’ />

https://jschober88-mbus.googlecode.com/hg/iotsys-mbus/doc/MBus-Master_Board.png’ />

Raspberry Pi

UART

In order to use the dedicated UART pins on the Raspberry Pi, first they have to be removed from their default application which is debugging. To do this edit “/boot/cmdline.txt” and “/etc/inittab” {4}. This files should be backuped to make it possible to return to the default configuration: cp /boot/cmdline.txt /boot/cmdline.bak cp /etc/inittab /etc/inittab.bak

Remove “console=ttyAMA0,115200” and “kgdboc=ttyAMA0,115200” configuration parameters from the “/boot/cmdline.txt” configuration file using nano editor. nano /boot/cmdline.txt

Comment the last line on the “/etc/inittab” file. Put a ‘#’ before “T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100. nano /etc/inittab

Now the RXD (GPIO15) and TXD (GPIO14) pins are available for general UART use.

To use the Raspberry Pi UART interface with the Java RXTX library one more step is required. A symbolic link the serial port (UART) has to be created: sudo ln -s /dev/ttyAMA0 /dev/ttyS80

For more detail: MBusConnector

Current Project / Post can also be found using:

  • M-Bus master hat for Raspberry Pi
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