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Wireless M-Bus uses the 3-layer IEC model, which is a subset of the 7-layer OSI model (see Figure 1). Figure 1. Stack Layers (modulation, center frequency, and frequency deviation). application TX buffer to the MbusPhy TX buffer, adding the required headers and CRCs. amount of code to accommodate any kind of data to the meter would add unnecessary code and cost to the meter. GUI. For these reasons, the application layer is implemented using example applications for a meter and reader.
2 Rev. 0.1 1.2. Required Standards 1.2.1. EN 13757-4 EN 13757-4 Communication system for meters and remote reading of meters Part 4: Wireless meter readout Radio meter reading for operation in the 868 MHz to 870 MHz SRD band 1.2.2. EN 13757-3 Communication system for meters and remote reading of meters Part 3: Dedicated application layer 1.2.3. IEC 60870-2-1:1992 Telecontrol equipment and systems Part 5: Transmission protocols Section 1:Link transmission procedure 1.2.4. IEC 60870-1-1:1990 Telecontrol equipment and systems Part 5: Transmission protocols Section 1: Transmission frame formats 1.3. Definitions M-Bus—M-Bus is a wired standard for meter reading in Europe. Wireless M-Bus—Wireless M-Bus for meter reading applications in Europe. PHY—Physical Layer defines how data bits and bytes are encoded and transmitted. API—Application Programmer interface. LINK—Data Link Layer defines how bloc ks and frames are transmitted. CRC—Cyclic Redundancy Check. FSK—Frequency Shift Keying. Chip—Smallest unit of transmitted data. One data bit is encoded as multiple chips. Module—A C code source .c file.
- M-Bus PHY Functional Description
one is defined as the higher frequency, and a zero is defined as the lower frequency. The Preamble options for the Si443x is an integer number of nibbles consisting of alternating ones and zeros. would be misaligned by one bit. Synchronization Word registers. synchronization word is complemented before writing to the Si443x registers. Table 1. Synchronization Word for Mode S and Mode R
than specified. Subtracting six chips for the preamble gives the minimum number of chips for the Si443x preamble. minimum setting for the Preamble Detection Control register. Table 2. Synchronization Word for Mode T Meter to Other Table 3. Transmit Preamble Length
false preamble detection slightly higher for this mode. the receiver will interoperate with any M-bus-compliant transmitter. receive any benefit from the longer preamble. the short preamble Mode S receiver configuration. Table 4. Preamble Detection
link uses 3 out of 6 encoding. data rate for the Si443x is effectively doubled when not using Manchester encoding. Si443x. As a consequence, the Manchester encoding and decoding must be performed by the MCU. of data is encoded and written to the FIFO at a time. call the encode nibble function twice, first for the most significant nibble and then for the least significant nibble. shifting two-bits for each symbol. However, it is faster to just use a simple 16 entry look-up table for each nibble. before writing to the FIFO to account for the inverted preamble requirements. 10 µs for 100 encoded chips. The benefit is a RAM savings 512 bytes. Table 5. Manchester Encoding
shortest transmission time and the longest battery life for a wireless meter. chip symbols are aggregated as three bytes. software. The symbol is written to the least significant byte of the shift register. The register is shifted left twice. still be one nibble left in the shift register. This is handled with the postamble as explained in the next section. look-up table for the corresponding symbol takes considerably longer. Table 6. Three Out of Six Encoding
having at least two alternating chips. the radio will insert the length field after the header bytes. actual packet length. It is the number of link layer payload bytes not including the CRC bytes or encoding. then written to the Packet Length register (0x3E). The postamble is discarded. 1.28 milliseconds. This is no problem for an 8051 running at 20 MIPs. postamble is ignored; so, if the postamble is not transmitted, four chips of noise will be received and ignored. Table 7. Postamble Length
small to get the best possible battery life. determines the required size for the receive buffer (USER_RX_BUFFER_SIZE). not be used in that case. This is feasible, but it is more convenient to use the packet handler, if possible. encoded packet may not fit within the 64-byte buffer. enabled. Most short packets will be sent in one FIFO transfer. almost Full threshold is then set to 48 bytes. The RX FIFO almost full and Valid Packet interrupts are enabled. have been read and stops reading after the last byte. Table 8. Packet Size Limits
10 Rev. 0.1 3. Data Link Layer The data link layer module implements a 13757-4:2005 compliant link layer. The data link layer (LINK) provides an interface between the physical layer (PHY) and the application layer (AL). The Data Link Layer performs the following functions: Provides functions that transfer data between PHY and AL Generates CRCs for outgoing messages Detects CRC errors in incoming messages Provides physical addressing Acknowledges transfers for bidirectional communication modes Frames data bits Detects framing errors in incoming messages 3.1. Link Layer Frame Format The Wireless M-Bus frame format used in EN 13757-4:2005 is derived from the FT3 (Frame Type 3) frame format from IEC60870-5-2. The frame consists of one or more blocks of data. Each block includes a 16-bit CRC field. The first bock is a fixed length block of 12 bytes that includes the L-field, C-field, M-field, and A-Field. 3.1.1. L-Field The L-field is the length of the Link layer data payload. Th is does not include the L-fiel d itself or any of the CRC bytes. It does include the L-field, C-field, M-field, and A-Field. These are part of the PHY payload. Because the number of encoded bytes is limited to 255 by tes, the maximum supported value for the M-field is 110 bytes for Manchester encoded data and 148 bytes for Mode T Three-Out-of-Six encoded data. The Link layer is responsible for calculating the L-fi eld on transmission. The lin k layer will use the L-field on reception. Note the L-field does not indicate the PHY payload length or the number of encoded bytes. Upon transmission, the PHY will calculate the PHY payload length and the number of encoded bytes. Upon reception, the PHY will decode the L-field and calculate the number of bytes to decode. 3.1.2. C-Field The C-field is the frame control field. This field identifies the frame type and is used for the link data exchange service primitives. The C-field indicates the frame type - SEND, CONFIRM, REQUEST, or RESPOND. In the case of SEND and REQUEST frames, the C-field indicates whether a CONFIRM or RESPOND is expected. When using the basic Link TX function, any value of C can be used. When using the Link Service Primitives, the C field is populated automatically according to EN 13757-4:2005. 3.1.3. M-Field The M-field is the manufacturers code. Manufacturers ca n request a three letter code from the following web address: http://www.dlms.com/flag/INDEX.HTM Each character of the three letter code is encoded as five bits. The 5-bit code may be obtained by taking the ASCII code and subtracting 0x40 (“A”). The three 5-bit codes are concatenated to make 15-bits. The most significant bit is zero. 3.1.4. A-Field The address field is a unique 6-byte address for each device. The unique address should be assigned by the manufacturer. It is the responsibility of each manufacturer to ensure that each device has a unique 6-byte address. The address for Send and Request frames is the self-a ddress of the meter or other device. The confirm and response data frames are sent using the address of the originating device.
Rev. 0.1 11 3.1.5. CI-Field The CI-field is the application header and specifics t he type of data in the application data payload. While EN13757-4:2005 specifies a limited number of values, the Link Service Primitives will permit any value to be used. 3.1.6. CRC The CRC is specified in EN13757-4:2005. The CRC Polynomial is: X16 + x13 + x12 + x11 + x10 + x8 +x6 + x5 +x2 + 1 Note that the M-Bus CRC is calculated over each 16-byte bl ock. The result is that each 16 bytes of data require 18 bytes to be transmitted, 3.2. Additional Information For additional information about the Link Layer Implem entation, see “AN452: Wireless M-Bus Stack Programmers Guide”.
Figure 3. Meter Transceiver Power Management C8051F930 RTC and put the MCU into sleep mode. be used instead of sleep so that the USB and UART may interrupt the MCU.
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