To try this tutorial on you Arduino board you have to connect a Led. The pin used here is the 13 but of course you can change it in the code if you want to use another one.

As in the Arduino Publisher Tutorial you first have to include the libraries. One of them depends on the message you are transferring with ROS. In the following example we will choose an empty message. This means that the message doesn’t contain anything. So when the subscriber receives the message, it doesn’t react depending on its content but it only reacts because a message has been sent. As the type of the std_msg is «Empty», you have to include the following lines at the beginning of your Arduino code :

#include <ros.h>
#include <std_msgs/Empty.h>

Here again, like the Arduino Publisher Tutorial code, you have to start a ROS node with this line :

ros::NodeHandle nh;

Then you initiate the callback function where you have to specify the name of the callback (messageCb), the type of the message (std_msgs::Empty) and the name of the message (toggle_msg which contains the transferred message). The callback is the function called each time you receive a message. If toggle_msg had a content you could use it in the callback but it is not the case here as the message is of type «Empty».

void messageCb( const std_msgs::Empty& toggle_msg){
  digitalWrite(13, HIGH-digitalRead(13));   // blink the led
}

The following line of code is used to instantiate the subscriber. To do so you have to specify two arguments : the topic name (we chose in this example : «toggle_led») and the callback function we defined previously (messageCb).

ros::Subscriber<std_msgs::Empty> sub("toggle_led", &messageCb );

As this is a blink led example, in the setup part of your Arduino code you first have to use the number 13 pin as an output.

pinMode(13, OUTPUT);

Then for the ROS part of the setup you have to first initialize the node :

nh.initNode();

And subscribe to the node you want to :

nh.subscribe(sub);

So finally this is what the void setup() in your Arduino code should look like :

void setup()
{
  pinMode(13, OUTPUT);
  nh.initNode();
  nh.subscribe(sub);
}

Finally you just have to add in the void loop of your Arduino code the following lines where the spinOnce passes arguments to the callback :

void loop()
{
  nh.spinOnce();
  delay(1);
}

Now that you know how each part of the code works you can test the blink led example available on the wiki.ros.org website http://wiki.ros.org/rosserial_arduino/Tutorials/Blink :

/*
 * rosserial Subscriber Example
 * Blinks an LED on callback
 */
#include <ros.h>
#include <std_msgs/Empty.h>

ros::NodeHandle nh;

void messageCb( const std_msgs::Empty& toggle_msg){
  digitalWrite(13, HIGH-digitalRead(13));   // blink the led
}

ros::Subscriber<std_msgs::Empty> sub("toggle_led", &messageCb );

void setup()
{
  pinMode(13, OUTPUT);
  nh.initNode();
  nh.subscribe(sub);
}

void loop()
{
  nh.spinOnce();
  delay(1);
}

You have to upload this code on the Arduino board before connecting it to the Raspberry and before starting the test on ROS.

To test it you first have to connect the Arduino board to the Raspberry. Then you launch the terminal and execute the following commands :

on a first window you type :

roscore

on another window :

/dev/ttyXXXX is the usb port the Arduino is connected to and you’ll have to find the exact name in order to replace the XXXX by the name of the port (examples : ttyUSB0, ttyACM0, ttyACM1, ...). You can find the name of the port ttyXXXX in the Arduino IDE or you can find it in the terminal by typing :

ls /dev/tty*

you should first execute this previous command and only then plug the Arduino in the Raspberry usb port and execute this command again to see which port has been added to the list displayed on the terminal and thus know which one is the Arduino board.

The first time you use the usb port the Arduino is connected to, you have to give permissions to use that port. To do so, you have to type :

sudo chmod 666 /dev/ttyXXXX

Now that you know the port you are using and you have permissions, you can type this :

rosrun rosserial_python serial_node.py /dev/ttyXXXX

Finally, in order to send a single message to the Arduino, you can publish on a specific topic by typing on another terminal window :

rostopic pub toggle_led std_msgs/Empty --once

where toggle_led is the topic you are publishing on, std_msgs/Empty is the type of the message and we use «—once» to publish this message only once.