Could I write a program for control 8 arduino devices (MEGA 2560 R3 ) on pc via rs 232 , that mean is control 8 com port at the same time?
A Mega has 4 hardware serial ports.
The most number of Megas I have run (at the same time) on a PC is two, but then again I only have two Megas.
Each Arduino you plug into a USB port will have a different COM number.
Hi,
Welcome to the Forum.
What is the application that needs 8 Megas?
Thanks.. Tom.. 
frank197662:
Could I write a program for control 8 arduino devices (MEGA 2560 R3 ) on pc via rs 232 , that mean is control 8 com port at the same time?
Have you considered connecting one Mega to your PC and connecting the other Megas to the first one rather than to the PC?
If you tell us all about the project you are trying to create it will be much easier to help.
...R
frank197662:
Could I write a program for control 8 arduino devices (MEGA 2560 R3 ) on pc via rs 232 , that mean is control 8 com port at the same time?
You have our permission to.
That you had to ask, though, makes me suspect that the answer is that no, you can't.
larryd:
A Mega has 4 hardware serial ports.The most number of Megas I have run (at the same time) on a PC is two, but then again I only have two Megas.
Each Arduino you plug into a USB port will have a different COM number.
Thank you
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.
Welcome to the Arduino forum, Frank.
A block diagram of your project will help clarify what you are doing.
Paul
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
Signalling like RS485 is not restricted to serial comms.
You could use it to carry the switch contacts to the input pins of the Arduino’s.