An input/output (I/O) level shifter for a subscriber identification module (SIM) interface includes a controller configured to apply a first enable signal to turn ON a first transmitter when the direction of packet flow is from an interface device to a SIM card, and is configured to apply a second enable signal to turn ON a second transmitter when the direction of packet flow is from the SIM card to the interface device. The controller is configured to not apply the first and the second enable signals concurrently. The controller selectively controls the ON/OFF period of the first and the second transmitter to maintain half-duplex communication on the interface I/O line and the SIM I/O line to prevent undesired positive data feedback.