See cassette port for details.
The same composite video port found on earlier Apple II models remained present; however, gone were the cassette ports and internal DIP-16 game port.
These changes make it impossible to use a standard unmodified C64 Centronics parallel printer interface, since it took a +5V voltage from the cassette port.
It plugged into a proprietary cassette port on the Commodore 64's motherboard.
An improved model was in development which provided a cassette port but the project was canceled when Texas Instruments canceled the 99/4A and left the home computer field.
Square wave tones could be produced by outputting data to the cassette port and plugging headphones or an amplifier into the Data Out line.
This could be overcome by using special cabling, and by doing a "dummy" write to the cassette port while triggering the printer.
New were a cassette port and parallel printer port.
Ariolasoft also developed the cassette ports of those titles, and also developed original games.
The clipped signal therefore fits into the TTL electrical level window of the schmitt trigger step that in turn feeds the digital cassette port.