Modbus Data Model
Discretes Input | Single bit | Read-Only | Data can be provided by the I/O system. |
Coils | Single bit | Read-Write | Data can be alterable by the application program. |
Input Registers | 16-bit word | Read-Only | Data can be provided by the I/O system |
Holding Registers | 16-bit word | Read-Write | Data can be alterable by the application program. |
Reference: MODBUS Application Protocol Specification V1.1b3, April 26 2012
For more information refer to the Modbus organisation website.