PaulS:
I have to save 'recibido' in a String because and array has the character '\0' at the end, so if I use 'recibido' in the statement of 'if' function, this doesn't works.
No, you don't. The NULL at the end of a char array makes it a string, and people dealt with strings for 40 years before there were Strings. The strcmp() function will tell you if two strings are equal, with wasting ANY resources.
longitud=Serial.readBytes(recibido,11);
I tried using 'recibido' in the stament of 'if' functions and this didn't work and in the way that I am doing now the program works.
I don't know much about programming, so many times I try in a way or in another way until I get that the program works.
I don't know much English too, so I am sure that my explanations are no clear and my grammar is not good enough.
I will use the function that you say to compate 'recibido' and what I need in the 'if' statement in order to understand what is the problem.
Thank you for your help.
I will write my progress.