Mitsubishi Electric fr-e700 series, Instruction Manual

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MODBUS®/TCP (FR-E700 SC ENE) Parameter
6 - 334
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.
Message frames comprise the six message fields shown in the figures above.
●
Details of protocol
The following table explains the six message fields.
I001228E
Fig. 6-173: Data interchange
Transaction
identifier
Protocol
identifier
Length field Unit identifier Function Data
2 x 8 Bit 2 x 8 Bit 2 x 8 Bit 8 Bit 8 Bit n x 8 Bit
Nachrichtenfeld Beschreibung
Transaction identifier
The master adds the data for the purpose of transaction control.
The same data is returned in the response from the slave.
Protocol identifie
Fixed to 0. (When the slave receives data other than 0, it does not send the response message.)
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
Function code
1 to 255 can be set in single byte length (8 bits) for the function code. The master sets the func-
tion to be sent to the slave as the request, and the slave performs the requested operation.
"Function code list" summarizes the supported function codes. An error response is generated
when a function code other than "Function code list" is set.
At a response from the slave, the function code set by the master is returned in the case of a
normal response. At an error response, H80 and the function code are returned.
Data
The format changes according the function code. (Refer to page 6-336.) The data, for example,
includes the byte count, number of bytes, and accessing content of holding registers.
Tab. 6-122: Structure of the protocol
Query messages from master
Transaction identifier Transaction identifier
Protocol identifier Protocol identifier
Length field Length field
Unit identifier Unit identifier
Function code Function code
Eight-bit
Data bytes
Eight-bit
Data bytes
Response message from slave