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Any other function code request will be returned with an error response indicating
the function code is not supported, as well as a request for too much data or data at a
register address that not present.
5.2.3
Data
The data field of Modbus request sent from a client to server devices contains
additional information that the server uses to take the action defined by the function code.
This can include items like discrete and register addresses, the quantity of items to be
handled, and the count of actual data bytes in the field.
The data field may be nonexistent (of zero length) in certain kinds of requests, in this
case the server does not require any additional information. The function code alone
specifies the action.
5.2.4
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.
5.2.5
Data Encoding
Modbus uses a "big-endian" representation for address and data items. This means
that when a numerical quantity larger than single byte is transmitted, the most significant
byte (MSB, also called the high-order byte) is send first. The following sub-topics
tWF-DA1PD2R1
tWF-DA1PD2R1 User's Manual
User's Manual (Rev1.0, APR./2019) ------------- 28
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