CHAPTER 13 RS-485 COMMUNICATIONS

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

・Query/response message: Data are set according to the FC.

Variable length ・An exceptional code (1 byte) is returned in the exceptional

response from the amplifier.

CRC check

・Query/response message: CRC-16 is added to the bottom

(L)

of the frame.

・The sender calculates CRC-16 for the data sent, add it to the

16 bits

bottom of the frame, and send the frame.

(2 bytes)

・The receiver calculates CRC-16 of the received data. If the

(H)

calculation results are not equal to the received CRC-16,

an error occurs.

If an error is detected, no response message is returned.

13

Modbus RTU Communications 13-5