Modbus Protocol Data Unit Formats - Mitsubishi Electric MELSEC iQ-F FX5 User Manual

MODBUS protocol data unit formats

This section explains MODBUS protocol data unit formats of the MODBUS standard functions.
The MODBUS protocol data unit contains request messages sent from the master to a slave and response messages sent
from the slave to the master.
How to see the request/response message formats
■Request/Response message format diagram
The following shows how to see the request/response message format diagrams provided in Page 66 Read coils to
Page 74 Read/Write multiple registers.
Area name
Frame contents
[For request message format]
Setting range
[For response message format]
Value stored to the response
message
■Response message format
The response message formats issued from the slave to the master differs depending on whether the slave has normally
completed or failed to handle the requested processing (read/write, diagnosis etc.).
The formats for normal and error completions are shown in Page 66 Read coils to Page 74 Read/Write multiple
registers.
Precautions
■When the slave receives a broadcast request message
Although the processing requested by the request message is performed, no response message is sent to the master.
■When the processing is completed with error at the slave
When the processing (read/write, diagnosis etc.) requested by the request message is completed with error, an exception
code is sent to the master.
Refer to "Response message formats (when completed with an error)" in Page 66 Read coils to Page 74 Read/Write
multiple registers.
■Storage location of exception code and error code
Refer to the following sections for the storage location, confirmation methods, and other detailed contents.
MODBUS serial communication: Page 35 Related Devices
MODBUS/TCP communication : Page 58 Related Devices
Function code
Function
Head coil number
code (0000H to FFFFH)
(01H)
(H)
1 byte 1 byte
(8 bits) (8 bits)
Data
Read points
(0001H to 07D0H)
(L) (H) (L)
When a single data is expressed by 2 bytes, set the upper
byte (8 bits) as (H) and the lower byte (8 bits) as (L).
Note: no switching of the high and low byte takes place (big
endian).
Appendix 3 The frame specifications
A
APPX
65