Hi Maxmilian,
your understanding of the functionality of the Packet Input block is almost correct. The only difference is that the block does not wait (you cannot wait in a real-time model), but signals the availability of new data by its Data Ready port. If Data Ready is false, the block outputs the same data as in the previous time step. When Data Ready becomes true, it means that a new message has been received in this time step. You are also correct that the block only returns new message when it is complete. That is, if you set message length to 100, the block does not set Data Ready until 100 bytes are received.
The above suggests how to handle the variable-size messages. You need several Packet Input ports for this purpose. First, you read a two-byte message containing the size. Then, according to the size read, you read the rest of the message by other Packet Input blocks set to read the appropriate number of bytes. And because you only want to read a message if it has newly arrived, you use the Data Ready port to drive an Enable subsystem and only enable message reading if there is a new one ready.
Please see the attachment for an example.
Good Luck, Jan
