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=== Variable Data Structure === The variable data structure is the part of an M-Bus frame that contains the measurement data. Its format is defined as part of the M-Bus application layer in EN 13757-3. A variable data structure consists of the following components: Data Header (optional) – Depending on the value of the CI field the variable data structure contains a long data header (CI=0x72), short data header (CI=0x7a), or no data header (CI=0x78). The header contains mainly information about the metering device. A short header contains the following fields: Access number (1 byte) – is a counter of how many times a meter has been read (wired M-Bus) or a meter has sent data (wireless M-Bus). For detailed information on when the access number is incremented see EN 13757-3. Status (1 byte) – Codes the status of the metering device. For example a device can signal that it is running out of power or is in an error state. Configuration (2 bytes) – the configuration field contains information about the encryption mode and the number of encrypted bytes. Wired M-Bus usually uses no encryption while wireless M-Bus meters often do. Only data after the data header may be encrypted. A long data header contains the secondary address of the meter in addition to all the fields of the short header. Data records – Data records (sometimes called variable data blocks) contain the measured data. Each data record is made up of a data information block (DIB), a value information block (VIB) and a value. Similar to OBIS codes DIBs and VIBs code information such as the meaning of a value. The DIB codes: Storage number – a meter can have several storages e.g. to store historical time series data. The storage number 0 signals an actual value. Function – Data can have the following four function types: instantaneous value, max value, min value, value during error state Data value type – The length and coding of the data value field following the DIB and VIB. Possible value types are 8/16/24/32/48/64 bit integer, 32 bit real, 2/4/6/8/12 digit binary coded decimals (BCD), date and string. In addition the value type “none” exists to label data records that have no data value field. Tariff – Indicates the tariff number of this data field. The data of tariff 0 is usually the sum of all other tariffs. Subunit – Can be used by a slave to distinguish several subunits of the metering device. The VIB codes: Description – The meaning of the data value (e.g. “Energy”, “Volume” etc.) Unit – The unit of the data value. Multiplier – A factor by which the data value coded in the data field has to be multiplied with Manufacturer specific data (optional) – Manufacturer specific data may optionally be appended at the end of the frame. This is essentially an array of bytes that is not standardized by the M-Bus standard. <br><br>
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