Pc card compatibility?

The original standard was defined for both 5 volt and 3.3 volt cards. The 3.3V cards have a key on the side to protect them from being damaged by being put into a 5V-only slot. Some cards and some slots operate at both voltages as needed. If your card will not go into the slot, this means you have a 3.3V card and a 5V slot. CardBus cards are 3.3V only.