SMG-5420 User Manual Ver. 1.1
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1
Modbus Overview
1.1
Modbus
Modbus is a serial communications protocol originally published by Modicon (now
Schneider Electric) in 1979 for use with its programmable logic controller (PLCs).
As a matter of fact, Modbus has become a standard communication protocol.
Modbus serial line protocol is a master-slave protocol that a master communicates
with one or more slaves. A master sends Modbus queries to slaves and slaves send
responses to the master.
There are two transmission modes in Modbus serial line protocol and those are RTU
and ASCII. Comparisons of those two modes are as follows:
Division
Modbus/RTU
Modbus/ASCII
Basic
Commonly used
More efficient than ASCII mode
Less efficient than RTU mode
Used when the timer-related
requirements of the RTU mode
cannot be met
Coding System
8-bit binary
Hexadecimal, ASCII characters
Bits per Byte
1 start bit
8 data bits, LSB sent first
1 parity bit (or no parity bit)
1 stop bit (or 2 bits with no
parity)
1 start bit
7 data bits, LSB sent first
1 parity bit (or no parity bit)
1 stop bit (or 2 bits with no
parity)
Message
Framing
Use time intervals
char interval
≤
1.5 char
frame interval
≥
3.5 frame
Use ASCII characters
Start
–
0x3A (‘:’)
End
–
0x0D0A (CR+LF)
char interval
≤
1 second
Frame
Checking
Filed
CRC
LRC
Frame
description
Slave Address (1)
Function Code (1)
Data (0 ~ 252)
CRC (2)
Start (1)
Slave Address (2)
Function Code (2)
Data (0 ~ 504)
LRC (2)
End (2)
Table 1-1 Comparisons of Modbus/RTU and Modbus/ASCII
With the evolution of networks, the demand for network communications of
industrial equipment has also increased. As a result, a network version became a
necessity for the Modbus protocol, the TCP/IP version called Modbus/TCP is released.
