Modbus registers

A Modbus device holds transient (real-time) data and often persistent (configuration) data in addressable registers. Here, the term “registers” implies all addressable data, but this is a loose interpretation. Using Modbus nomenclature, all accessible data in a Modbus slave is contained in the following four available groups of data flags and registers (including the Modbus-master access that is possible):

Coil status

Or simply “coils”: single-bit flags that represent the status of digital (NiagaraAX: boolean) outputs of the slave, that is, On/Off output status. A Modbus master can both read from and write to coils.
Input status

Or simply “inputs”: single-bit flags that represent the status of digital (NiagaraAX: boolean) inputs of the slave, that is, On/Off output status. A Modbus master can read (only) inputs.
Input registers

Are 16-bit registers that store data collected from the field by the Modbus slave. The Modbus master can read (only) input registers.
Holding registers

Are 16-bit registers that store general-purpose data in the Modbus slave. The Modbus master can both read from and write to input registers.