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Response
If no error occurs related to the Modbus function requested in a properly received
Modbus PDU (Protocol Data Unit) the data field of a Modbus response from a server
to a client contains the data requested. If an error related to the Modbus function
requested occurs, the field contains an exception code that the server application
can use to determine the next action to be taken.
For example a client can read the ON/OFF states of a group of digital input or output
or it can read/write the data contents of a group of registers.
When the server responds to the client, it uses the function code field to indicate
either a normal response or that some kind of error occurred (called an exception
response). For a normal response, the server simply echoes to the request the
original function code.
For an exception response, the server returns a code that is equivalent to the
original function code from the request PDU with its most significant bit set to logic 1.
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