CHAPTER 13 RS-485 COMMUNICATIONS
Modbus RTU Communications 13-5
13
13.1.3 Transmission Protocol
1. Message types
Communications are configured as the single master and multiple slaves method. The amplifier
operates as a slave.
The messages sent/received between the master and amplifier are classified into the two types below:
Query: Messages transferred from the master to the amplifier
Response message: Messages transferred from the amplifier to the master
Communications are started by a query from the master. Communications are not performed between
the amplifiers.
2. Message fields
The message frame is as follows for both the query from the master and the response message from
the amplifier.
Station No. 1 byte
・0: Broadcast query to all amplifiers.
(No response message is issued.)
・1-31: Query for each station number.
Self station numbers 1-31 are responded in the response
messages from the amplifiers.
FC (function code) 1 byte
・Master: Specify an FC according to the processing that you
wish to execute.
・Amplifier: Returns the specified FC.
(If the amplifier has not finished processing successfully,
the message is returned with the MSB of the FC set to 1.)
...Exceptional response
Information Variable length
・Query/response message: Data are set according to the FC.
・
n exceptional code (1 byte) is returned in the exceptional
response from the amplifier.
(L)
CRC check
16 bits
(2 bytes)
(H)
・Query/response message: CRC-16 is added to the bottom
of the frame.
・The sender calculates CRC-16 for the data sent, add it to the
bottom of the frame, and send the frame.
・The receiver calculates CRC-16 of the received data. If the
calculation results are not equal to the received CRC-16,
an error occurs.
If an error is detected, no response message is returned.
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