serial communication problem with open port

Hello,

I work with a PIC microchip connected to the serial COM port of my computer to send and receive data to and from the microchip PIC.

This device works well with the hyperterminal under Windows XP.

When I work with the command windows of Matlab these commands are working well:

s= serial('COM3','BaudRate',9600,'DataBits',8,'terminator',17);
fopen(s);
fprintf(s,'F')
out = fscanf(s);
disp(out)
fclose(s);
delete(s);
clear s;

and it is possible to launch again this program without any error. But when I write these lines inside of a m file and run it the error pops up as following:

??? Error using ==> serial.fopen
Port: COM3 is not available. No ports are available.
Use INSTRFIND to determine if other instrument objects are connected to the requested device.

Error in ==> PIC_I2C_Translator at 2
fopen(s);

-----------------------

Running INSTRFING it comes:

>> INSTRFIND

     Instrument Object Array

     Index:    Type:     Status:   Name:  
     1         serial    open      Serial-COM3
     2         serial    closed    Serial-COM3
     3         serial    closed    Serial-COM3
     4         serial    closed    Serial-COM3
     5         serial    closed    Serial-COM3

----------------------

What is the wrong thing that makes the program not able to close the COM port when working with the m file instead of working directly through the command windows with exactly the same code ?

Is there anybody out there who met such a problem ... and solve it ?