(Here are some general rules to controllers).

A controller is a device which makes the decisions at each door in an access control system.
It is essentially the brains of your system.
A Controller may be integrated with the reader on the outside of the door (unsecured version)
or mounted on the inside of the building (secured version).
audit trails (each use of  the door) are retained in the controller.
Controllers can work independently or be linked together.
Some controllers can work with various reader technologies.