Understanding how modbus communication works

Hello everyone,

Currently i have an ESP32 board (server) sending data to PLC Logo! (client) through Modbus TCP. Everything is going correcctly but i would like to comprehend exactly how it works. 

I mean, according to the Logo Soft Comfort Manual "Each Modbus connection can be configured as a client or a server, in which the local IP and remote IP are configured respectively. The server is the data source side in communication, it sends information that responds to the requests from the other side - the client." 

i don't undersant how the Logo! make those requests, automatically? 

How often the Logo as a client sends each request? It depends of the synchronize interval which can be seen in the photo wich i have append? 

In my ESP32 the order to write data on X register is not carried out until the Logo! has request it previously? 

My problem is that, theorecally the client sends requests and later, the server response.But in my case, i only see the ESP32 writing in the register every x time that i adjust in the code of Arduino IDE ( i program the ESP32 with Arduino IDE) . I don't notice any response of my ESP32 to any request received. 

The only part of my code which perhaps is handling  that is:

I copy the server code from internet, so now i need to understand how the protocol works internally.

Sorry for this long words.

Berguinsson

Closed cause question posted twice.

The second thread with the subject Communication through Modbus is published within the conference LOGO! - Communication.