Siemens RS-485 - Manual (Page 5)

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Siemens Building Technologies CC1A7865.1en
HVAC Products 31.10.2002
Structure of the data blocks
All data blocks use the same structure:
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
Function code Function selection (reading or writing words)
Data field Contains the following information:
- Word address
- Number of words
- Word value
Checksum Identification of transmission errors
Fault handling
3 different error codes are used:
1 Invalid function
2 Invalid parameter address
8 Write access to parameter rejected
Slave address Function
XX OR 80 h
Error code Checksum CRC16
1 byte 1 byte 1 byte 2 bytes
The function code is OR linked with 0 x 80, that is, the MSB (most significant bit) will be
set to 1.
Data query:
01 02 00 70 00 04 CRC16
Reply:
01 82 01 CRC16
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
Data structure
Error codes
Reply in the event
of fault
Example
Special cases