Abstract: A system in which two circuits which are spatially mobile with respect to one another communicate with one another over a preselected range to identify themselves. So long as the communication remains, one of the circuits, called a detector, maintains the input/output of a third circuit enabled. Upon breaking communication, for example by one of the circuits moving outside the preselected range, input/output of the third circuit is disabled. In one embodiment, the third circuit is a computer workstation to which the detector is affixed, and the other circuit, called the agent, is worn or carried by an authorized user of the workstation. This permits the workstation to be available to the user while he is present, and become unavailable in general when he is not.