72
Modbus RTU control (RS-485 communication)
11.2.2 Response
Slave-returned responses are classied into three types: normal response, no response, and exception response.
The response message structure is the same as the query message structure.
Slave address Function code Data Error check
8 bits 8 bits N x 8 bits 16 bits
Normal response
Upon receiving a query from the master, the slave executes the requested process and returns a response corresponding
to the function code.
No response
The slave may not return a response to a query sent by the master. This condition is referred to as "No response."
The causes of no response are explained below.
z
Transmission error
The slave discards the query if any of the transmission errors in the table is detected. No response is returned.
Cause of transmission
error
Description
Framing error Stop bit 0 was detected.
Parity error A mismatch with the specied parity was detected.
Mismatched CRC The calculated value of CRC-16 was found not matching the error check value.
Invalid message length The message length exceeded 256 bytes.
z
Other than transmission error
A response may not be returned without any transmission error being detected.
Cause Description
Broadcast If the query was broadcast, the slave executes the requested process but does not return a response.
Mismatched slave address When the slave address in the query is not matched the slave address of the driver.
Exception response
An exception response is returned if the slave cannot execute the process requested by the query. Appended to this
response is an exception code indicating why the process cannot be executed. The message structure of exception
response is as follows.
Slave address Function code Exception code Error check
8 bits 8 bits 8 bits 16 bits
z
Function code
The function code in the exception response is a sum of the function code in the query and 80h.
Function code of query Exception response
03h 83h
06h 86h
08h 88h
10h 90h
17h 97h
To Next Page
Loading...
