Doubt regarding serial communication

edit

Hi all,

I am new to ROS and Linux. I am using ROS Indigo on Ubuntu 14.04 LTS. I am trying to establish serial communication with an microcontroller using ROS serial package ( https://github.com/wjwwood/serial ). Came across an example program by garyservin ( https://github.com/garyservin/serial-... ). The program works well.

The instruction packet I want to send to MCU has start bytes "@@". However, when I try to publish to this to the topic "write" I get this error:

rostopic: error: Argument error: while scanning for the next token found character '@' that cannot start any token   in "<string>", line 1, column 1:
    @@1233
    ^

Similarly if I try to send a number I get this error:

ERROR: Unable to publish message. One of the fields has an incorrect type:
  field data must be of type str

The datatype used in the example program is std_msgs/String.

What I want to send is actually data in hex code. Something like 0x40 0x40 0x09 0x01 0x00 0x00 0x02 0x0d 0x0a

Could someone guide me on how to solve this problem?

UPDATE

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

serial::Serial ser;

void write_callback(const std_msgs::String::ConstPtr& msg){
    ROS_INFO_STREAM("Writing to serial port" << msg->data);
    ser.write(msg->data);
}

int main (int argc, char** argv){
    ros::init(argc, argv, "serial_example_node");
    ros::NodeHandle nh;

    ros::Subscriber write_sub = nh.subscribe("write", 1000, write_callback);
    ros::Publisher read_pub = nh.advertise<std_msgs::String>("read", 1000);

    try
    {
        ser.setPort("/dev/ttyACM0");
        ser.setBaudrate(9600);
        serial::Timeout to = serial::Timeout::simpleTimeout(1000);
        ser.setTimeout(to);
        ser.open();
    }
    catch (serial::IOException& e)
    {
        ROS_ERROR_STREAM("Unable to open port ");
        return -1;
    }

    if(ser.isOpen()){
        ROS_INFO_STREAM("Serial Port initialized");
    }else{
        return -1;
    }

    ros::Rate loop_rate(5);
    while(ros::ok()){

        ros::spinOnce();

        if(ser.available()){
            ROS_INFO_STREAM("Reading from serial port");
            std_msgs::String result;
            result.data = ser.read(ser.available());
            ROS_INFO_STREAM("Read: " << result.data);
            read_pub.publish(result);
        }
        loop_rate.sleep();

    }
}

Thanks!