Serial resistors in a matrix keypad are used to prevent ghosting. When you press a key on a matrix keypad, it shorts together two or more of the rows and columns that make up the keypad. This creates a circuit that completes the path to ground (or some other voltage reference), telling the device what key was pressed. Sometimes, when you press one key, another key near it will also get pressed because of the way that the keys are connected together. This is called ghosting, and it can cause problems when you're trying to enter data into a device. ...