An interface card is capable of communicating with an external device and includes a power supplier; a non-volatile memory which stores executable instructions to operate in an active-mode and a sleep-mode; a small-capacity volatile memory which is supplied with power in the sleep mode; a transmitter-receiver which transmits and receives packet data to/from the external device; and a controller which retrieves sleep-mode instructions stored in the non-volatile memory and loads the sleep mode instructions in the small-capacity volatile memory to transition the interface card into the sleep mode if the transmitter-receiver does not receive the packet data for predetermined time period in an active mode. The interface card processes certain packet data in the sleep mode and transitions back into the active mode when sleep mode operations determine that the packet data cannot be processed in the sleep mode. The non-volatile memory, and other components of an external circuit, is powered down when the interface card is in the sleep mode.