ET-7X00/PET-7X00 User Manual, version 1.1.2 Page: 92
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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.