Hi Rafael,
In short, the Serial (UART) protocol transmits packets of 8 bits each that will contain your data. The structure of that data is defined by the device you'd like to communicate with. Some devices expect ASCII characters, some a specific stream of bits, possibly preceded by headers used to identify the packets.
The value passed to the port called "Number of Bytes to Write" will define how many of the bytes passed to the "Data" port will effectively be transmitted at the current time step of your model.
You will find a detailed description of these parameters, as well as example models showing how the Serial (UART) functionality of your IO306 module can be configured in the Speedgoat documentation here. If you require more help, please feel free to contact the Speedgoat Support team.
I hope this is helpful!
Dimitri
