sub build_string {
	my( $string ) = @_;
	my $chksum = 0;
	$chksum += $_ for map{ $_ & 0x7F } unpack 'C*', $string;
	$chksum = (~$chksum & 0xFFFF);

	return chr(1) . $string . sprintf( '%4x', $chksum ) . chr(3);
}