You can use the ADB port to connect input devices such as a keyboard, mouse, trackball, etc. The Apple Desktop Bus protocol can accomodate up to 16 devices. However, the practical limit is three or four devices.
ADB devices are typically daisy-chained together. Some devices may contain enough internal resistance to limit the number of devices you can connect. For instance, a maximum of four Apple Extended Keyboards plus one Apple Desktop Bus mouse may be daisy-chained together before increases in DC resistance result in drops below TTY levels. This will prevent other devices on the ADB chain from communicating with the computer.
If input isn't reaching the computer (you type but nothing happens, or you click the mouse button but nothing happens), turn the computer off and disconnect any extra devices you are not using. Connect only those that are necessary. Some Macintosh computers have two ADB ports. In this case, you could break a single chain into two chains.
Note: you should only connect and disconnect ADB devices when the computer is turned off. Damage to the device and the computer may result if you connect an ADB device with the computer turned on.
Note: Apple computers introduced after August, 1998 do not use the Apple Desktop Bus. They use the Universal Serial Bus (USB) to connect input devices.