Description of the Application

At a Glance

The application described in this document is a Modbus communication application via modems.

Example Illustration

The figure below illustrates the example:

The devices communicate with each other using modems. The supervisor is Modbus master whereas the X and Y PLCs are slaves.

The goal of the example is to write the data area values of PLC X to PLC Y.

To do this, the PLC X must become Modbus Master.

Each day, the supervisor communicates with the PLCs to recover information.

If an alarm is raised on the PLC X, it switches in Modbus Master mode and sends data to PLC Y.

To simplify programming, the modems have been initialized with the correct parameters via a programming terminal. These parameters are stored in non-volatile memory by the AT&W commands.

Operating Mode

The operating of the application is as follow:

Step	Action
1	The PLC X port is switched to Character mode.
2	The PLC X sends a dial message to the modem.
3	The PLC X port is switched to Master Modbus mode.
4	The Master PLC (X) sends data to the Slave PLC (Y).
5	The port is switched to character mode.
6	The PLC X sends a disconnection message to the modem.
7	The PLC X port is switched to Slave Modbus mode.