Connect multiple Arduinos to one computer

1/* GREEN or RED LED-Arduino. Copy and paste the
2code into each program  connected to a different
3Arduino, giving one the  name GREEN_LED and the 
4other  RED_LED.
5The basic code and the  circuit setup is public 
6domain  but  taken  from  Massimo Banzi's book, 
7"Getting Started with Arduino". In his version,
8the  LED is plugged into 13 and GND, next to it, 
9on  the  Arduino  board - and  you  can use that 
10method if you prefer. I extended  this onto the 
11breadboard,  the colour-coding is easily visible.
12*/
13
14const int LED = 13; /* "const  int" because uses 
15less memory and does not change */
16const int BUTTON =  7; // Pin 7 for LED test
17                      // = ON or OFF
18int val = 0;  // val stores the button state
19int old_val = 0; // stores the old val-ue 
20int  state = 0; // 0 sets LED OFF 1 sets LED ON
21
22void setup() {
23  pinMode(LED,  OUTPUT); // Defines Pin 13 as output
24  pinMode(BUTTON, INPUT); // BUTTON is an  input to
25                          //record when pressed.
26}
27
28void loop()  {
29val = digitalRead(BUTTON); // read and store the LED 
30                           //state  is it HIGH? Maybe
31                           //change state.
32if ((val == HIGH)  && (old_val == LOW)){
33    state = 1 - state;
34    delay(10);
35}
36old_val  = val; // 
37if (state == 1) {
38  digitalWrite(LED, HIGH); // turn LED on
39}  else {
40  digitalWrite(LED, LOW);
41}
42}

1/* GREEN or RED LED-Arduino. Copy and paste the
2code into each program
3  connected to a different
4Arduino, giving one the  name GREEN_LED and the 
5other
6  RED_LED.
7The basic code and the  circuit setup is public 
8domain  but  taken
9  from  Massimo Banzi's book, 
10"Getting Started with Arduino". In his version,
11the
12  LED is plugged into 13 and GND, next to it, 
13on  the  Arduino  board - and  you
14  can use that 
15method if you prefer. I extended  this onto the 
16breadboard,
17  the colour-coding is easily visible.
18*/
19
20const int LED = 13; /* "const
21  int" because uses 
22less memory and does not change */
23const int BUTTON =
24  7; // Pin 7 for LED test
25                      // = ON or OFF
26int val = 0;
27  // val stores the button state
28int old_val = 0; // stores the old val-ue 
29int
30  state = 0; // 0 sets LED OFF 1 sets LED ON
31
32void setup() {
33  pinMode(LED,
34  OUTPUT); // Defines Pin 13 as output
35  pinMode(BUTTON, INPUT); // BUTTON is an
36  input to
37                          //record when pressed.
38}
39
40void loop()
41  {
42val = digitalRead(BUTTON); // read and store the LED 
43                           //state
44  is it HIGH? Maybe
45                           //change state.
46if ((val == HIGH)
47  && (old_val == LOW)){
48    state = 1 - state;
49    delay(10);
50}
51old_val
52  = val; // 
53if (state == 1) {
54  digitalWrite(LED, HIGH); // turn LED on
55}
56  else {
57  digitalWrite(LED, LOW);
58}
59}