Frame Specifications - Mitsubishi Electric LE7-40GU Instruction Manual

• 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.
Type
Coil (bit data)
Holding register (word data)

Frame specifications

The frame specification of MODBUS communication is shown below.
The frame specification detail is as below.
No. Area name
1 Address field
2 Function code
3 Data
4 Error checking
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.
Start Address field
Interval of 3.5 1 byte
character time or
more
■ASCII mode
This mode is used to transmit and receive frames in ASCII code characters (2 bytes).
Start Address field
1 byte (3AH) 2 bytes
7 MODBUS/RTU, ASCII (SLAVE)
118
7.3 Communication Setting
For Master stationSlave station
For Slave stationMaster station
For monitoring by the master station
For writing to RAM in the slave
station
For writing to RAM + ROM in the
slave station
Direction
Master stationSlave station
Slave stationMaster station
Master stationSlave station
Slave stationMaster station
Master stationSlave station
Slave stationMaster station
Master stationSlave station
Slave stationMaster station
Function code Data
1 byte 0 to 252 bytes
Function code
2 bytes
Address
H0000 to H002F
H1000 to H102F
H0000 to H015F
H1000 to H115F
Contents
0: Broadcast
1 to 247: Send the request message to the specified slave station.
Returns the own station No. of the slave station.
Specify the function code that the master station instructs for the slave station.
• Normal completion: Returns the function code at the time of request.
• Abnormal completion: Returns with the most significant bit ON.
Stores information for executing the function specified in the function code.
• Normal completion: Returns the execution result of the function specified by
the function code.
• Abnormal completion: Returns the error response code.
The sender computes the error check bit and gives it.
Error check
(CRC)
2 bytes
Data Error check (LRC)
N2 bytes (N=0 to 2 bytes
252)
Data reading/writing by the
master station
Writable
Readable
Readable
Readable/Writable
Writable
End (Start) Address field
Interval of 3.5 1 byte
character time or
more
End
2 bytes ((0DH)+(0AH))