LSIS XBL-C41A

Chapter 9 Modbus Communication
9-1
Chapter 9 Modbus Communication
9.1 General
Modbus protocol is specified open protocol used between client-server, which executes reading/writing data
according to function code. Communication between devices that use Modbus protocol uses Client-server
function in which only one client processes the data.
9.2 Modbus Protocol
9.2.1 Kind of modbus protocol
There are two communication modes of Modbus, ASCII and RTU.
9.2.2 Structure of modbus protocol
Modbus protocol’s structure is as follows.
.
In case of normal communication, process step is as follows.
Characteristic
ASCII mode
RTU mode
Coding method
ASCII code
8 bit binary code
No. of data per
one character
Start bit
1
1
Data bit
7
8
Parity bit
Even,Odd,None
Even,Odd,None
Stop bit
1 or 2
1 or 2
Error check
LRC(Longitudinal Redundancy Check)
CRC (Cyclical Redundancy Check)
Start of frame
Colon (:)
3.5 Character no response time