How to have 2 Arduinos Communicate a variable over Ethernet?

Question

So here is how it goes, I have 2 arduino mega's, both with a w5100(wiznet) shield on them. one has a light sensor, I need the other one to be able to get that light sensor's value from another location. I have searched but was unable to find anything like this. I have it client.Println(); the value, but I'm unsure of how to fetch, and than store it.

Please help, Joey

Answer 1

The code given at http://arduino.cc/en/Reference/EthernetServer tells you pretty much exactly what you need. Highlights copied / adapted here.

First the "receiving end":

#include <SPI.h>
#include <Ethernet.h>

// network configuration.  
// gateway and subnet are optional.

 // the media access control (ethernet hardware) address for the shield:
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };  

//the IP address for the shield:
byte ip[] = { 10, 0, 0, 177 };    

// the router's gateway address:
byte gateway[] = { 10, 0, 0, 1 };

// the subnet:
byte subnet[] = { 255, 255, 0, 0 };

// telnet defaults to port 23
EthernetServer server = EthernetServer(23);

void setup()
{
  // initialize the ethernet device
  Ethernet.begin(mac, ip, gateway, subnet);

  // start listening for clients
  server.begin();
}

void loop()
{
  // if an incoming client connects, there will be bytes available to read:
  char incoming[100];
  EthernetClient client = server.available();
  if (client == true) {
    // read bytes from the incoming client and write them back
    // to any clients connected to the server:
    int ii = 0;
    while ((c = client.read()) != '\n') 
    {
      incoming[ii++] = c;
    }
    // the variable incoming[] now contains the most recent value sent
    // so you can do something with it  
  }
}

Now the "sending part" (sketch running on the Arduino that is the data source):

#include <SPI.h>
#include <Ethernet.h>
// inspired by/copied from http://arduino.cc/en/Tutorial/TelnetClient

// Enter a MAC address and IP address for your controller below:
// The IP address will be dependent on your local network:
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192,168,1,176);  // this is the data source card IP address

// the IP address of the server you're connecting to:
IPAddress server(192,168,1,177);

// Initialize the Ethernet client library
// with the IP address and port of the server 
// that you want to connect to (port 23 is default for telnet;
EthernetClient client;
int port = 23;  // telnet default port

char myVar[100]; // contains string with variable to transmit

void setup() {
  // start the Ethernet connection:
  Ethernet.begin(mac, ip);
 // Open serial communications and wait for port to open:
  Serial.begin(9600);
   while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }

  // give the Ethernet shield a second to initialize:
  delay(1000);
  Serial.println("connecting...");

  // if you get a connection, report back via serial:
  if (client.connect(server, port)) {
    Serial.println("connected");
  } 
  else {
    // if you didn't get a connection to the server:
    Serial.println("connection failed");
  }
}

void loop()
{
  // if there are incoming bytes available 
  // from the server, read them and print them:
  // the server code above doesn't send anything…
  // but if it did, this is where you would echo it
  int ii;
  if (client.available()) {
    char c = client.read();
    Serial.print("***Server says:***\n");
    Serial.print(c);
  }

  // assume your variable myVar will have a valid string in it...
  strcpy(myVar, "123.456\n");
  // tell the serial port what you are sending:
  Serial.print("sending variable: ");
  Serial.print(myVar);
  for(ii = 0; ii < strlen(myVar); ii++) {
    if (client.connected()) {
      client.print(myVar[ii]); 
    }
  }

  // if the server's disconnected, stop the client:
  if (!client.connected()) {
    Serial.println();
    Serial.println("disconnecting.");
    client.stop();
    // do nothing:
    while(true);
  }
  // add appropriate delay here before sending next data element
}

I do not have two Arduino's with Ethernet shields, so I have had to piece this together from what I know / could look up. Let me know how you get on with this!

Answer 2

A couple of tips for people trying this:

Firstly, at the time of me writing this, the above code does not work on any version ABOVE 1.0.5 (1.0.5r2 works fine).

Also the sender has the following:

// the IP address of the server you're connecting to:
IPAddress server(192.168,1,177);

Should be (comma, not space):

// the IP address of the server you're connecting to:
IPAddress server(192,168,1,177);

Just my findings so far! :)