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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 describe the different byte of encoding and show how the data is encoded as it is
within the Modbus/TCP packet.
5.2.5.1
Binary
A binary item is represented as a single bit within a data word. All binary is packed
into 16-bits data words, which are accessed using function code 01 and 02. Therefore, a
single register contains 16 bits of binary data, each having a specific meaning.
(1010101001010101)
Table 5-3: A single register contains 16 bits of binary data
ALM-06-WF User's Manual
Value
0xAA55
ALM-06-WF
User's Manual (Rev1.0, Feb./2019) ------------- 34
1st
0xAA
(10101010)
2nd
0x55
(01010101)
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