Mitsubishi Electric FR-A820-00046 Instruction Manual page 989

Parameters
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, and function
code are copied as they are, and at erroneous communication (illegal function code or data code),
bit7 (= H80) of the function code is turned ON, and the error code is set at data bytes.
Fig. 5-389:
Message frames comprise 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
Transaction identifier
Protocol identifier
Length field
Unit identifier
Function code
Data
Tab. 5-368: Protocol details
FR-A800
Query message from master
Transaction identifier
Protocol identifier
Length field
Unit identifier
Function code
Eight-Bit
Data Bytes
Data transmission
Protocol
Length field
identifier
2 × 8 bits 2 × 8 bits
Description
The master adds the data for the purpose of transaction control.
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.)
0 is returned in the response from the slave.
The data length from the unit identifier to the data is stored in byte.
Fixed to 255
1 to 255 can be set in single byte lengths (8 bits) for the function code. The master sets the
function 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.
The format changes according to the function code. (Refer to page 5-789.) The data, for
example, includes the byte count, number of bytes and accessing content of holding registers.
Ethernet communication (FR-A800-E)
Transaction identifier
Protocol identifier
Length field
Unit identifier
Function code
Eight-Bit
Data Bytes
Response message from slave
Unit identifier Function
8 bits 8 bits
I001228E
Data
n × 8 bits
5 - 787