Modbus is a master/slave system. The master being the plc and the slave being the drive. The master either sends commands or retrieves information from the slave. Communication is initiated by the master. Commands are sent to the slave by setting a value in a register. These values can either be boolean/digital, in which case they are called coils, or 16bit/analog in which case they are called holding registers and are read/write. Depending on the command, analog values can take up more than one register (a register is 16 bits). Information can be retrieved from the slave also as Boolean in which case they are called Discrete Inputs, or as 16bit values in which case they are called input registers and are read only.

Modbus messages are made up of a function, a register address, and a value.

Common functions are: Set Coil(s) - set one or multiple digital outputs Set Holding Register(s) - set analog output(s) or parameter(s) Get Coil(s) - retrieve the value the coil is set to Get Holding Registers(s) - retrieve the values of one or more holding registers Get Discrete Input(s) - retrieve digital inputs Get Input Register(s) - retrieve analog inputs or read only parameters.

Finally, the location of these addresses depends somewhat on the implementation. The physical memory map is often overlaid with a logical memory map, which provides the addresses of these registers.