the number of registers being requested. As registers are used in pairs, then this must be an
even number.
Error Check (Lo): The bottom (least significant) eight bits of a 16-bit number representing the
error check value.
Error Check (Hi): The top (most significant) eight bits of a 16-bit number representing the error
check value.
Response
The example illustrates the normal response to a request for a single floating point parameter
i.e. two 16-bit Modbus Protocol Registers.
First Byte
Last Byte
Slave
Address
Function
Code
Byte
Count
First
Register
(Hi)
First
Register
(Lo)
Second
Register
(Hi)
Second
Register
(Lo)
Error
Check
(Lo)
Error
Check
(Hi)
Slave Address: 8-bit value representing the address of slave that is responding.
Function Code: 8-bit value which, when a copy of the function code in the query, indicates that
the slave recognised the query and has responded. (See also Exception Response).
Byte Count: 8-bit value indicating the number of data bytes contained within this response
First Register (Hi)*: The top (most significant) eight bits of a 16-bit number representing the
first register requested in the query.
First Register (Lo)*: The bottom (least significant) eight bits of a 16-bit number representing
the first register requested in the query.
Second Register (Hi)*: The top (most significant) eight bits of a 16-bit number representing the
second register requested in the query.
Second Register (Lo)*: The bottom (least significant) eight bits of a 16-bit number representing
the second register requested in the query.
Error Check (Lo): The bottom (least significant) eight bits of a 16-bit number representing the
error check value.
Error Check (Hi): The top (most significant) eight bits of a 16-bit number representing the error
check value.
*These four bytes together give the value of the floating point parameter requested.
Exception Response
If an error is detected in the content of the query (excluding parity errors and Error Check
mismatch), then an error response (called an exception response), will be sent to the master.
The exception response is identified by the function code being a copy of the query function
code but with the most-significant bit set. The data contained in an exception response is a
single byte error code.
First Byte
Last Byte
Slave
Address
Function
Code
Error
Code
Error
Check
(Lo)
Error
Check
(Hi)
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