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5
Chapter
Message Composition
The command message and response message are composed of four parts: the station number, function code, data part, and
error check code. These four parts are sent in that order.
The following describes each part of the message.
Station No.
This is the number specifying the slave. Commands can only be processed by slaves that have the same value set in the "STno"
parameter.
Function Code
This code specifies the function for the slave to perform.
Data Part
This data is required to run the function code. The composition of the data part is different depending on the function code.
The data in the micro controller is assigned a coil number or resistor number. This coil number or resistor number is specified
when the data is read or written through communication.
The coil number or resistor number used by the message employs a relative address.
The relative address is calculated using the following formula.
Relative address = (last four digits of the coil number or resistor number) – 1
(Ex.) When a function code specifies resistor number "40003"
Relative address= (the last four digits of 40003) – 1
= 0002
is used in the message.
Error Check Code
This code detects whether there are errors (changes in the bits) during the signal transmission processes. MODBUS protocol
(RTU mode) uses CRC-16 (Cyclic Redundancy Check).
Field name No. of bytes
Station No. 1 byte
Function Code 1 byte
Data Part 2 to 125 bytes
Error Check Code (CRC-16) 2 bytes
Refer to
For more about setting the "STno" parameter, see "Chapter 4,
Setting Communication Parameters" (p. 15).
Refer to
For more about function codes, see "Function Code" (p. 23).
Refer to
See "Chapter 6, Message Details" (p. 27).
Refer to
For more about calculating CRC, see Section 5,
"Calculating Error Check Code (CRC-16)" (p. 24).
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