A field device of automation technology, is fed via a two-wire line or via an energy source of limited energy reserve associated with the field device. The field device includes: a hardwired communication interface having at least one communication channel; and, associated with the communication interface, a functional unit, which is embodied in such a manner, that it performs sending and/or receiving of digital communication signals via the communication interface. The functional unit, for the purpose of energy saving, is turned on only in active, operating phases, while it is switched off in inactive, resting phases; and a detection circuit is provided, which detects a communication signal applied to the communication interface and automatically activates the functional unit.