Structure Of The Data Blocks; Data Structure; Fault Handling; Reply In The Event Of Fault - Siemens RS-485 User Manual

Structure of the data blocks

Data structure

Fault handling

Error codes
Reply in the event
of fault
Example
Special cases
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All data blocks use the same structure:
Slave address
1 byte
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:
-
-
-
Checksum
Identification of transmission errors
3 different error codes are used:
1 Invalid function
2 Invalid parameter address
8 Write access to parameter rejected
Slave address
1 byte
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
Reply:
01 82
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
-
Function code
1 byte
Word address
Number of words
Word value
Function
XX OR 80 h
1 byte
00 70
01 CRC16
Data field Checksum CRC16
x byte
Error code Checksum CRC16
1 byte
00 04
2 bytes
2 bytes
CRC16
5/14
CC1A7865.1en
31.10.2002