Mitsubishi Electric 800 Series Instruction Manual page 60

 Message frame (protocol)
• Communication method
Basically, the master sends a query message (inquiry), and slaves return a response message (response). At normal
communication, the transaction identifier, protocol identifier, unit identifier, and function code are copied as they are, and
at erroneous communication (illegal function code or data code), bit 7 (= H80) of the function code is turned ON, and the
error code is set at data bytes.
Query message from Master
Transaction Identifier
Protocol Identifier
Length Field
Unit Identifier
Function Code
Eight-Bit
Data Bytes
Message frames have the six message fields shown in the figures above.
• Details of protocol
The following table explains the six message fields.
Transaction identifier
2 × 8 bits
Message field
The master adds the data for the purpose of transaction control.
Transaction identifier
The same data is returned in the response from the slave.
Fixed to 0. (When the slave receives data other than 0, it does not send the response message.)
Protocol identifier
"0" is returned in the response from the slave.
Length field The data length from the unit identifier to the data is stored in byte.
Unit identifier Fixed to 255
"1 to 255" can be set as the function code in the single-byte (8-bit) length filed. The master sets the function to
be sent to the slave as the request, and the slave performs the requested operation. Refer to the function code
list for details of the supported function codes. An error response is generated when a function code other than
Function code
those in the function code list is set.
The normal response from the slave contains the function code set by the master. The error response contains
H80 and the function code.
The format changes according the function code. (Refer to
Data
count, number of bytes, and accessing content of holding registers.
 Function code list
Function name Read/write
Read holding registers Read
Write Single Register Write
Diagnostics Read
Transaction Identifier
Protocol Identifier
Function Code
Response message from slave
Protocol identifier Length field
2 × 8 bits 2 × 8 bits
Code
The data of the holding registers is read.
The various data of the inverter can be read from MODBUS
registers.
System environmental variable (Refer to
H03 Real time monitor (Refer to the FR-E800 Instruction Manual
(Function).)
Fault history (Refer to
Model information monitor (Refer to
Converter unit parameters (Refer to
Data is written to a holding register.
Data can be written to MODBUS registers to output instructions
H06 to the inverter or set parameters.
System environmental variable (Refer to
Inverter parameters (Refer to
Functions are diagnosed. (communication check only)
A communication check can be made since the query message
H08 is sent and the query message is returned as it is as the return
message (subfunction code H00 function).
Subfunction code H00 (Return query data).
Length Field
Unit Identifier
Eight-Bit
Data Bytes
Unit identifier
8 bits 8 bits
Description
page 60.) The data, for example, includes the byte
Outline
page 67.)
page
page
page 66.)
Function Data
n × 8 bits
Message
format
reference page
page 64.)
page 60
67.)
66.)
page 61
page 64.)
page 61
2. Ethernet Communication
2.7 MODBUS/TCP
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