Siemens LMV3 series - Section 7 Modbus; Modbus General Principles

LMV Series Technical Instructions
Document No. LV3-1000
SCC Inc. Page 1 Section 7
Section 7: Modbus
General
The physical connection to the Modbus system is made via an external OCI412.10 module.
Master-slave principle
Communication between Modbus users takes place according to the master-slave principle. The LMV3
always works as a slave. Every device on the bus line must be assigned a different address.
Data transmission
Modbus interface settings
In addition to the interface parameters that can be set on the LMV3 (parameters 141-149), the following
parameters for the communication interface are already set:
Number of data bits
8
Number of start bits
1
Number of stop bits
1
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 A
ddress
Function C
ode
Data F
ield
Checksum CRC16
1 byte
1 byte
x byte
2 bytes
Slave A
ddress
Device address of a certain slave
Function
C
ode
Function selection (reading /
writing words)
Data F
ield
Contains the following information:
- Word address
- Number of words
- Word value
Checksum
Identification of transmission errors