These days if you find a slot on a piece of Nintendo hardware it's likely that it's a place for a standardized piece of technology such as an SD card or a USB port, but the company used to have a knack for adding proprietary ports and slots to its consoles that were used for increasingly esoteric add-ons and upgrades. Check out this fun article from the Nintendo World Report archives chronicling the history of Nintendo's various expansion ports from Nintendo Entertainment System to Nintendo 64 to Nintendo GameCube and beyond. How many of these add-ons did you own? Hint: likely zero.
First up is the Famicom and NES. Unlike the NES, the Famicom came with hard-wired controllers. Any extra controllers and peripherals could be plugged into Nintendo's first expansion port, which was located at the front of the machine. This port was used to host light guns, 3D shutter glasses, keyboards, extra controllers, and other items. Many system expansions plugged directly into the cartridge slot, such as the Famicom Disk System and the Famicom Modem. The Sharp Twin Famicom, a system that combined the Famicom and Disk System into one machine, added an additional three expansion ports, but these remained unused.
The NES shipped with an expansion port on the bottom of the console. On multiple occasions, modems were planned to be connected there. However, the NES expansion port never received a commercial application. Originally, the port was covered by a snap-in cover, but later model systems actually had a plastic tab covering the port completely. The port was still there, but the plastic actually had to broken off to access the port. The lack of expansion port utilization outside of Japan was an ongoing trend that started with Nintendo's first system.
Nintendo had lofty goals that usually went underwhelming fulfilled with most of the expansion port accessories debuting in Japan to provide niche gameplay experiences with experimental ideas and then appearing nowhere else. Third parties filled the gap with increasingly obscure hardware that used the ports without achieving much success. The NES, Super NES, and N64 all featured commonly unused expansion ports overseas and it wasn't until the GameCube era that the ports saw a mainstream use with the Game Boy Player (a pair of networking add-ons which also made use of the ports were offered for sale online in limited quantities and worked with a handful of games). Hobbyists have long since cracked the mysteries of these ports, too. While Nintendo didn't get around to doing much with these ports internationally, I'm glad they were there. Had history unfolded a little differently, we could have been able to experience some of the unique ideas made possible by the expansion hardware and those ports were the gateway to making that happen.