Siemens RS-485 User Manual

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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

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:

Invalid function

Invalid parameter address

Write access to parameter rejected

Slave address

Function

XX OR 80 h

Error code

Checksum CRC16

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:

CRC16

Reply:

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