Tutorial 3: Modbus RTU - Function 01 (01hex) Read Coils

Function 01 (01hex) Read Coils
Reads the ON/OFF status of discrete coils in the slave.
Request
The request message specifies the starting coil and quantity of coils to be read.
Example of a request to read 10...22 (Coil 11 to 23) from slave device address 4:

Response
The coil status response message is packed as one coil 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 coil addressed in the request. The other coils follow toward the high-order end of this byte and from low order to high order in subsequent bytes. If the returned coil 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.