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.
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.
CRC check field
Tab. 5-242: Protocol details
Query message from master
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.
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.
The format changes according to the function code. (Refer to page 5-492.) The data, for
example, includes the byte 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.
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.
(N) Operation via communication and its settings
Response message from slave
n × 8 bits
5 - 491