Mitsubishi Electric LE7-40GU, Instruction Manual, Page: 120 / 154

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7 MODBUS/RTU, ASCII (SLAVE)
7.3 Communication Setting
• Holding register (for master station monitoring)
The status of the slave station monitored by the master station is stored in this area.
Slave station monitoring is performed by the master station by reading the area of the holding register by the master station.
• Holding register (for slave station RAM write)
An area to store data to be written to the RAM of the slave station.
• Holding register (for slave station RAM + ROM write)
An area to store the data to be written to the RAM and ROM of the slave station.
Frame specifications
The frame specification of MODBUS communication is shown below.
The frame specification detail is as below.
Frame mode
There are two kinds of frame modes for the message which reads/writes the coil and holding registers.
■ RTU mode
In RTU mode, frames are transmitted in binary code.
When the message is interrupted for a time corresponding to the communication time (depending on the transmission speed)
of 3.5 characters, it is regarded as the end of the message.
■ ASCII mode
This mode is used to transmit and receive frames in ASCII code characters (2 bytes).
Type Address Data reading/writing by the
master station
Coil (bit data) For Master station

Slave station H0000 to H002F Writable
For Slave station

Master station H1000 to H102F Readable
Holding register (word data) For monitoring by the master station H0000 to H015F Readable
For writing to RAM in the slave
station Readable/Writable
For writing to RAM + ROM in the
slave station H1000 to H115F Writable
No. Area name Direction Contents
1 Address field Master station

Slave station 0: Broadcast
1 to 247: Send the request message to the specified slave station.
Slave station

Master station Returns the own station No. of the slave station.
2 Function code Master station

Slave station Specify the function code that the master station instructs for the slave station.
Slave station

Master station • Normal completion: Returns the function code at the time of request.
• Abnormal completion: Returns with the most significant bit ON.
3 Data Master station

Slave station Stores information for executing the function specified in the function code.
Slave station

Master station • Normal completion: Returns the execution result of the function specified by
the function code.
• Abnormal completion: Returns the error response code.
4 Error checking Master station

Slave station The sender computes the error check bit and gives it.
Slave station

Master station
Start Address field Function code Data Error check
(CRC) End (Start) Address field
Interval of 3.5
character time or
more 1 byte 1 byte 0 to 252 bytes 2 bytes Interval of 3.5
character time or
more 1 byte
Start Address field Function code Data Error check (LRC) End
1 byte (3AH) 2 bytes 2 bytes N

2 bytes (N=0 to
252)
2 bytes 2 bytes ((0DH)+(0AH))