Each on-board I/O pin has a minimum amount of protection consisting of a 100-Ohm Current limit resistor and a 5.6VDC Zener diode. Each I/O pin also has a 5Kohm pull-up resistor. When assigned as outputs, they act as a push-pull amplifier that drives hard to either the positive or negative 5VDC rail. This means they are not open-collector I/O pins. Each I/O Pin can sink up to 12mA and source up to 4mA. Exceeding this could result in damage to the I/O port.
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