frank197662:
My project is that my clinet has 8 buttons, if I put 1 of the button then send message to server, Server receive the button message then send message to arduino by using RS232 and let the pin to low for control machine's status. Totat 48 clients.
What sort of machines are you talking about? That can affect the type of control system that is appropriate.
How far apart are the 48 machines? Why not just use a single Mega for the complete job?
Do the 48 Arduinos have to send data back to the server?
Also, note that an Arduino does NOT use RS232 voltage levels - they can damage an Arduino. If your PC produces RS232 voltage levels then you need a MAX232 chip to convert to the Arduino's TTL voltage levels.
...R