Protocol Data Unit Formats Grouped By Function - Mitsubishi Electric MELSEC iQ-R Series Reference Manual

1.3

Protocol Data Unit Formats Grouped by Function

This section describes the protocol data unit formats of the MODBUS standard functions.
When the device number is specified within the message
When specifying the device number within the message, specify "device number - 1".
However, this instruction is not applicable to the file number and the device number specified for reading or writing extended
file register.
Ex.
When the status of input 32 (100032) is read by the read discrete inputs (function code: 02H)
Function code
Start input number specification
02H 001FH
(H)
(1) When reading the status of input 32 (100032), specify 31 (001FH) for the start input number.
The device number stored in the response message is "the device number of the device that actually performed reading/
writing - 1".
Descriptions of request message and response message formats
This section describes descriptions of the request message and response message formats of the MODBUS standard
functions.
■ Message format for MODBUS RTU mode and MODBUS/TCP
(1) Function code
(2)
Start coil number specification
(3)
01H (0000H to FFFFH)
(H)
1 byte 1 byte
(8 bits) (8 bits)
(1) Area name
(2) Frame description
(3) For request message format: Setting range
For response message format: Value stored in the response message
(4) When one piece of data consists of two bytes, the upper byte (eight bits) is (H) and the lower byte (eight bits) is (L).
■ Message format for MODBUS ASCII mode
The message format shown in Chapter 3 is for RTU mode or MODBUS/TCP.
In the ASCII mode, read the values shown in Chapter 3 in ASCII code.
Data
Number of read points
(1)
0001H
(L) (H) (L)
Data
Number of read points
(0001H to 07D0H)
(L) (H)
(4)
(L)
1 FRAME SPECIFICATIONS
1.3 Protocol Data Unit Formats Grouped by Function
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