Mitsubishi Electric 800 Series Instruction Manual page 504

 Message frame (protocol)
• Communication method
Basically, the master sends a Query message (question), and slaves return the Response message (response). At normal
communication, the Device Address and Function Code are copied as they are, and at erroneous communication (illegal
function code or data code), bit7 (= 80 h) of the Function Code is turned ON, and the error code is set at Data Bytes.
Query message from Master
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Message frames comprise of the four message fields shown in the figures above.
A slave recognizes message data as a message by the message data being prefixed and appended with a no data time of 3.5
characters (T1: start/end).
• Details of protocol
The following table explains the four message fields.
Start Address
T1 8 bits
Message field
Address 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.
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.
Function field 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 + the function code is returned.
Data field The format changes according to the function code. (Refer to
includes the byte count, number of bytes and accessing content of holding registers.
CRC Check field 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.
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.
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Response message from slave
Function Data
n  8 bits
8 bits
CRC Check
L H T1
8 bits 8 bits
Description
page 504.) The data, for example,
5.12 (N) Operation via communication and its settings
End
5. PARAMETERS
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