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7 Communication protocol
7.1 Modbus-RTU communication protocol
7.1.1 Introduction
In six pieces of products, the communication between the AIM-M300(or AIM-M300/SG) insulation
monitor and the upper computer by the Modbus-RTU communication protocol. The Modbus protocol
particularly defines the check code, the data sequences and so on, which are the necessary contents
of the specific data exchange. The Modbus protocol uses a master-slave responsive connection
(half-duplex) on a communication line, which means the signal on a single communication line is
transmitted in two opposite directions. Firstly, the signal from the main computer is addressed
to a unique terminal device (slave computer), and then the answering signal emitted from the terminal
device is transmitted to the host in the opposite direction.
The Modbus protocol only permits communication between hosts (PC, PLC, etc.) and terminal
devices, without allowing the data interchange between independent terminal devices. So that
terminal devices do not occupy communication lines when they are initialized and are limited to
in response to the query signals arriving at the computer.
7.1.2 Introduction to the function code
7.1.2.1 Function code 03H or 04H: Read the registers
This function allows the user to acquire the data collected and recorded by equipment and the system
parameters. The number of data requested by hosts has no limit, but cannot exceed the defined address
range.
The following example shows how to read a measured insulation resistance value from No.01
slave
computer, with the address of the insulation resistance value of 0008H.
The host computer
sends
Send
message
The slave computer
returns
Return
message
Address code
01H
Address code
01H
Function code
03H
Function code
03H
Start
address
High
byte
00H
Bytes
02H
Low byte
08H
Register
data
High
byte
00H
Number of
register
s
High
byte
00H
Low
byte
50H
Low byte
01H
CRC
check
code
High
byte
21H
