Nominal Mode

Most applications are comprised of a PC used for supervision applications. The supervisor is a Modbus master that communicates with different slaves. This operating mode is called the nominal mode.

The figure below illustrates the nominal mode (supervisor to slave):

The Exception Mode

On request, a PLC should be able to toggle from Modbus slave mode to Modbus master mode in order to send requests to other PLCs or devices. This toggling is called the exception mode.

When the exception mode is terminated, the system should be able to return to the initial state (to nominal mode).

The figure below illustrates the exception mode (emulated master to slave):