Abstract: A system for unlocking any of several vehicles by a passive electronic key coupled to base units in the vehicles by a radio link uses short range LF transmission for signals to the key and VHF transmission for signals to the base units. The key has a ferrite antenna connected to a receiver comprising a comparator. When an operator pulls the door handle, codes stored in the base are transmitted to identify any authorized key within the range. The key has a memory partitioned into separate blocks for different base units and another block for a scratchpad memory. A state machine stores the block addresses and controls access to the blocks according to the transmitted codes. When a block is accessed, a secure ID in the block is sent to the base unit to verify that the key is authorized to unlock the vehicle. The ID can be updated upon command from the base unit using the same codes to assure that an authorized unit is receiving the new ID.