An interchangeable electronic lock mechanism provides selective access to a motor controlled latching system including a motorized pin to lock and unlock a knob assembly. The lock mechanism may be used to replace key operated locking cores, on the exterior of a storage unit, with a plug and optional adapter inserted into a remaining shell housing, and a driver to control access to a storage unit. Manual rotation of the knob activates the drive assembly to control access to the storage unit. An optional break away security feature in the knob inhibits unauthorized unlatching of the lock. When the lock is unlatched, the knob rotates the drive assembly including the plug and adapter within the shell housing, and in turn, activates the driver to operate the lock assembly in the storage unit. An optional modular chassis assembly includes a removable array of components for testing, maintenance and repair.