Mitsubishi Electric 800 Series Instruction Manual page 66

• Details of protocol
The following table explains the four message fields.
Start Address
T1 8 bits
Message field
0 to 247 can be set in single byte lengths (8 bits). Set "0" when sending broadcast messages (instructions to all
addresses), and "1 to 247" to send messages to individual slaves.
Address field
The address set by the master is also returned when the response from the slave is.
The value set to Pr.331 RS-485 communication station number is the slave address.
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
Function field
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 + the function code is returned.
The format changes according to the function code. (Refer to
Data field
count, number of bytes and accessing content of holding registers.
Errors in the received message frame are detected. Errors are detected in the CRC check, and the message is
appended with data 2 bytes long. When the message is appended with the CRC, the lower bytes are appended
first, followed by the upper bytes.
CRC Check field
The CRC value is calculated by the sender that appends the message with the CRC. The receiver recalculates the
CRC while the message is being received, and compares the calculation result against the actual value that was
received in the error check field. If the two values do not match, the result is treated as an error.
Function
n  8 bits
8 bits
Data CRC Check
L H
8 bits 8 bits
Description
page 66.) The data, for example, includes the byte
End
T1
65
PARAMETER
4