Smart Acknowledge (Smart Ack) is the EnOcean protocol for bidirectional communication with battery-less devices: the sensor registers with a Postmaster (a relay) that buffers commands destined for the sensor and delivers them during the brief listening window that follows each of the sensor's own transmissions.

Smart Ack solves the fundamental asymmetry of an energy-harvested sensor: it can transmit cheaply but cannot afford to keep a receiver running continuously. A mains-powered Postmaster acts as a persistent relay; the sensor registers its short listening windows (typically a few milliseconds after each outgoing telegram), and the Postmaster queues any inbound commands (configuration, ACK, OTA updates) in a mailbox. As soon as the sensor next transmits, the Postmaster fires the queued command inside the listening window. This is what makes over-the-air configuration of battery-less devices possible at all.

When commissioning Smart Ack devices, always verify that the Postmaster has mains power backup; a mailbox loss after a power cut will silently drop pending commands and confuse the field technician.