sub SerRead {
$ser_dev = "/dev/ttyS3";
$AckOk = "\x06";
$AckErr = "\x15";

        sysopen ( SerialRead, "$ser_dev", O_RD | O_NONBLOCK ) || die "cannot sysopen";  

        $len="";
        while (length($len) == 0){
               $len = sysread(SerialRead, $buf, 1 );
        }
        if ($buf == $AckOk) {
                print "ACK_OK\n";
        }
        if ($buf == $AckErr) {
                print "ACK_ERR\n";
        }
        
        close SerialRead;

}