Structure of the data blocks
| All data blocks use the same structure: |
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Data structure |
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Slave address | Function code | Data field | Checksum CRC16 | |
| 1 byte | 1 byte | x byte | 2 bytes |
Every data block contains 4 fields:
| Slave address | Device address of a certain slave |
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| Function code | Function selection (reading or writing words) |
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| Data field | Contains the following information: |
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| - | Word address |
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| - | Number of words |
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| - | Word value |
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| Checksum | Identification of transmission errors |
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Fault handling |
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Error codes | 3 different error codes are used: |
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| 1 | Invalid function |
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| 2 | Invalid parameter address |
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| 8 | Write access to parameter rejected |
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Reply in the event |
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| Slave address |
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| Error code | Checksum CRC16 |
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of fault |
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| XX OR 80 h |
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| 1 byte |
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| 1 byte |
| 2 bytes |
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| The function code is OR linked with 0 x 80, that is, the MSB (most significant bit) will be |
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Example | Data query: |
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| 01 |
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| 70 |
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| 04 |
| CRC16 |
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| Reply: |
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01
82
01
CRC16
Special cases | In the following cases, the slave does not reply: | |
| - The checksum (CRC16) is wrong | |
| - | The instruction given by the master is incomplete or overdefined |
| - | The number of words or bits to be read is zero |
| 5/14 |
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Siemens Building Technologies | CC1A7865.1en |
HVAC Products | 31.10.2002 |