LSIS XBE-DC08A, User Manual

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Chapter 2 Built-in Cnet communication
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2.7 Modbus Protocol
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.
2.7.1 Modbus Protocol
There are two communication modes of Modbus, ASCII and RTU.
1) Structure of Modbus protocol
Modbus protocol’s structure is as follows.
In case of normal communication, process step is as follows.
In case of abnormal 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