rosserial_python fails but rosserial_server doesn't

edit

Hi guys,

I have a sketch in an Arduino that is publishing into ROS (with a custom message) some data that he gets form a bluetooth connection (HC-05) from another arduino.

Here is the code:

#include <SoftwareSerial.h>
#include <ros.h>
#include <ros/time.h>
//#include <sensor_msgs/Range.h>
#include <pkg_2/msg_2.h>

//variables globales
char Buffer[11];
int SoF=0x01; 
int Eof=0x04;
int i=0;
int trama_ok=0;
//declaracion objetos
SoftwareSerial BT(10,11);      //Rx,Tx: pins 7,8 de la placa OBJETO BLUETOOTH

ros::NodeHandle  nh;

pkg_2::msg_2 range_msg;
ros::Publisher pub_range( "/ultrasound", &range_msg);

void setup()
{
  nh.initNode();
  nh.advertise(pub_range);
  delay(1);

  //Serial.begin(9600);
  BT.begin(9600);
  delay(1);
}

void loop()
{
     if (BT.available())
  {
    int a=BT.read();
    Buffer[i]=a;
    i++;
    if (i==11){
               range_msg.obstacle = Buffer[1];
         range_msg.state_m1 = 9;
         range_msg.state_m2 = Buffer[3];
         range_msg.state_m3 = Buffer[4];
         range_msg.state_m4 = Buffer[5];
         range_msg.d_left = Buffer[6];
         range_msg.d_right = Buffer[7];
         range_msg.d_center = Buffer[8];
    pub_range.publish(&range_msg);
          nh.spinOnce();
   i=0;
   delay(1000);
  }
}
}

Pay no attention to the names of the topics and stuff, it's just a recycled code from an Ultrasound sketch.

The thing is, when I try to connect to the arduino with rosserial_python:

$ rosrun rosserial_python serial_node.py /dev/ttyACM0

It gives me the next error:

dani@ROS:~/catkin_ws$ rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0
[INFO] [WallTime: 1462188244.111483] ROS Serial Python Node
    [INFO] [WallTime: 1462188244.122188] Connecting to /dev/ttyACM0 at 57600 baud
    [ERROR] [WallTime: 1462188261.230066] Unable to sync with device; possible link problem or link software version mismatch such as hydro rosserial_python with groovy Arduino

Fortunately, when I use rosserial_server:

$ roslaunch rosserial_server serial.launch

Everything is published efficiently and smoothly.

The question: Why is this happening? How is that rosserial_server can handle the connection and rosserial_python can't? I must say that the previous ROS sketches that i worked with (which didn't involve any SoftwareSerial external connection) worked perfectly with rosserial_python.