Siemens LMV5 Series Technical Instructions page 271

LMV Series
8-1: Modbus
Master-slave principle
Communication between Modbus users takes place according to the master-slave principle. The AZL5
always works as a slave.
Data Transmission
Transmission mode (RTU)
– The transmission mode used is RTU (Remote Terminal Unit)
– Data is transmitted in a binary format (hexadecimal) with 8 bits
– The LSB (least significant bit) is transmitted first
– ASCII operating mode is not supported
Structure of data blocks
All data blocks use the same structure with the following four fields:
Slave Address
1 byte
Slave Address
Function Code
Data Field
Checksum
Checksum (CRC16)
The checksum (CRC16) is used to detect transmission errors. If, during evaluation, an error is detected,
the relevant device will not respond.
CRC = 0xFFFF
Calculation
scheme
while (not all ByteOfMessage handled)
SCC Inc.
Function Code
1 byte
Device address of a certain slave
Function selection (reading / writing words)
Contains the following information:
- Word address
- Number of words
- Word value
Identification of transmission errors
CRC = CRC XOR ByteOfMessage
For (1 through 8)
CRC = SHR (CRC)
if (flag shifted at right = 1)
then
CRC = CRC XOR
0xA001
The low-byte of the checksum is transmitted first.
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Technical Instructions
Document No. LV5-1000
Data Field Checksum CRC16
x byte 2 bytes
else
Section 8