User Manual
iM871A Wireless M-Bus Module Firmware
iM871A_WMBus_UserManual.docx, Wireless Solutions, v1.2 Page 12
3.1.3 WM-Bus Telegrams
In this section the message format on the air interface is described.
There are two different telegram formats specified in EN13757-4. Telegram Format A and
Telegram Format B. The operating modes S, T and R2 use Telegram Format A. The C-mode
supports both telegram formats.
Wireless M-Bus Telegram Format A
Preamble-
sequence
specified by mode 18 bytes
Block 1 Postamble
18 or less bytes
Block 2 Block n -1 Block n
Sync-
word
12 bytes 18 bytes max. 1 byte
Figure 3-2: Wireless M-Bus Telegram Format A
Every Wireless M-Bus telegram starts with a preamble sequence followed by a synchronization
word. The length of these fields is mode-dependent. The implementation is done according to
EN13757-4 [1]. The postamble contains normally 8 chips. Only for even packet sizes in T
mode (Meter) the postamble consists of four chips.
Block 1:
L-Field
Address
Type
Version
1 byte
4 bytes 1 byte 1 byte
6 bytes1 byte
C-Field M-Field A-Field
2 bytes
CRC-Field
2 bytes
Figure 3-3: Wireless M-Bus Telegram Format A, block1 (header)
The first byte of block 1 is the length byte. It describes the number of the following user bytes
including C-Field-, M -Field- and A-Field-Data, but without any CRC byte.
C-Field, M-Field and A-fields can be pre-configured and stored in the non-volatile memory.
L-Field and CRC-fields are filled by the firmware at transmission.
Block 2:
CI-Field
1 byte 15 bytes or, if last block, (((L-9) MOD 16) -1) bytes
Data-Field CRC-Field
2 bytes
Figure 3-4: Wireless M-Bus Telegram Format A, block2
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