To use AVR UART engine you need external inverter in front of RX and TX pin.
Otherwise you can software serial routine to communicate the keyboard.
-This converter uses software method, you doesn't need any inverter part.
+This converter uses software method by default, so you don't need any inverter part. But
+it can also be built with 'make HARDWARE_SERIAL=y' to enable hardware serial if there
+is an inverter present. Good results have been obtained using a 74LS04 and hardware serial.
Commands From System To Keyboard