The game pad controllers were more-or-less copied directly from the Game & Watch machines, although the Famicom design team originally wanted to use arcade-style joysticks, even taking apart ones from American game consoles to see how they worked. However, it was eventually decided that children might step on joysticks left on the floor and their durability was also questioned. Katsuyah Nakawaka attached a Game & Watch D-pad to the Famicom prototype and found that it was easy to use and had no discomfort. Ultimately though, they did install a 15-pin expansion port on the front of the console so that an arcade-style joystick could be used optionally. The controllers were hard-wired to the console with no connectors for cost reasons.
What was installed on the front of the console so the joystick couldn't be used?