use warnings;
use Fcntl;

my $tty = "COM3";
my $text = shift || "ATH0";

sysopen(COM,$tty,O_RDWR) || die "Can't open $tty $^E";

transmit("$text");
receive();

sub transmit {
	my $data = shift;
	$data .= "\r";
	@ascii = unpack("C*",$data);
	print "transmit: [@ascii]\n";
	syswrite(COM,$data);
}

sub receive {
	sysread(COM,$buf,4);
	@ascii = unpack("C*",$buf);
	print "receive: [@ascii]\n";
	return $buf;
}