A system for controlling access to one or more enclosed areas comprises at least one access card reader and controller powered via a Power-over-Ethernet (PoE) interface, each access card reader and controller being capable of controlling access through a particular entrance to a particular enclosed area and an access control server in communication with the at least one access card reader and controller, the access control server being capable of controlling the operation of the at least one access card reader and controller, and a signal converter disposed between the access card reader and the access control server. In a network mode of operation, the access control server is configured to perform authentication of a card identifier (ID) received from the at least one access card reader and controller and to signal the at least one access card reader and controller to unlock a door at the particular entrance to the particular enclosed area when the access control server has successfully authenticated the received card ID. In a standalone mode of operation, the at least one access card reader and controller is configured to perform local authentication of a received card ID independently of the access control server and to unlock a door at the particular entrance to the particular enclosed area when the at least one access card reader and controller has successfully authenticated the received card ID.