The Modbus block functions according to the client/server principle. The client is the active station and the server is the passive station in the communication process. Data is exchanged between the communication partners by means of various different function codes. The S7 can be operated as a client as well as a server during transmission.

In the initialization phase, it is determined on which S7 data blocks the Modbus registers and coils should be represented.

A distinction is made between client and server functionality during cyclic operation:

• If the S7 is operating as a client, when a task is activated, a Modbus message frame is generated from the specified current parameters and sent to the coupling partner over the TCP/IP connection. After the response has been received and the data has been checked and found to be consistent, the required actions, such as reading or writing data, will be performed. Any errors that occur during evaluation or processing are indicated on the Modbus block.
• If the S7 is operating as a server, the block waits for a request from the client. If a frame is received from a client, it is checked and evaluated. After the frame has been checked, the response is generated and the required actions, such as reading or writing data, will be performed. A processed request or any errors that have occurred during evaluation are indicated on the Modbus block.