how to interface 8051 and matlab

im trying to turn led on and off by sending an 'o' for on and 'f' for off serially from matlab to (at89c51) the problem is nothing is happening and both my cods are error free. i have never done serial communication before so i have little idea of what im doing wrong. please help! here are my codes:

for 8051

org 100h

mov tmod,#20h

mov th1,#-3

mov scon,#50h setb tr1

again: jnb ri,here

mov a,sbuf

cjne a,#'o',ledoff

setb p2.0

ledoff: clr p2.0

clr ri

sjmp again

end

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for matlab:

>> s = serial('COM1','baudrate',9600);

>> fopen(s)

>> a=inputdlg('enter o to turn led on and f to turn it off','s');

>> fprintf(s, 'a');

>> fclose(s)

>> clear s