Tutorial 2: Modbus TCP/IP - Function 02(02hex) Read Discrete Inputs

Function 02(02hex) Read Discrete Inputs
Reads the ON/OFF status of discrete inputs in the slave.
Request
The request message specifies the starting input and quantity of inputs to be read.
Example of a request to read 10...22 (input 10011 to 10023) from slave device address 4:

Response
The input status response message is packed as one input per bit of the data field. Status is indicated as: 1 is the value ON, and 0 is the value OFF. The LSB of the first data byte contains the input addressed in the request. The other inputs follow toward the high-order end of this byte and from low order to high order in subsequent bytes. If the returned input quantity is not a multiple of eight, the remaining bits in the final data byte will be padded with zeroes (toward the high-order end of the byte). The byte count field specifies the quantity of complete bytes of data.