Data Encoding - ICP DAS USA WF-2000 Series User Manual

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.
Table 5-3: A single register contains 16 bits of binary data
0xAA55
(1010101001010101)
WF-2000 Series DIO
Value
(10101010)
User's Manual (RevB1.0, Oct./2016) ------------- 42
1st
0xAA
(01010101)
2nd
0x55