Message Frame (Protocol); Communication Method; Details Of Protocol - Mitsubishi Electric A800-E Instruction Manual

(N) Operation via communication and its settings
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), bit 7 (H80) of the function code is turned ON, and the error code is set
at data bytes.
Query messages from master
Transaction identifier
Protocol identifier
Length field
Unit identifier
Function code
Eight-bit
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.
Transaction identifier
2  8 bits
Message field
Transaction identifier
Protocol identifier
Length field
Unit identifier
Function code
Data
40
PARAMETER
Protocol identifier Length field
2  8 bits 2  8 bits
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 length (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 the function code. (Refer to
count, number of bytes, and accessing content of holding registers.
Transaction identifier
Protocol identifier
Length field
Unit identifier
Function code
Eight-bit
Data bytes
Response message from slave
Unit identifier
8 bits
Description
page
Function Data
n  8 bits
8 bits
42.) The data, for example, includes the byte