Output signal switching device

An output signal switching device (or OSSD) is the interface of a sensor (such as a light curtain) designed for reliably signalling a safety-related event, usually the light curtain beams being "broken". OSSD signals are outputs from the protective device (light curtain or scanner) to a safety relay. OSSD outputs are typically semiconductor or transistor outputs, as opposed to relay or contact type outputs. There are usually two independent channels, so-called OSSD1 and OSSD2.
The non-tripped state is typically 24 VDC, and the tripped state (when the safety barrier has been violated) 0 VDC. If a wire were to break between the light curtain and the safety relay, the safety relay would trip to the safe state.
The OSSD outputs are self-checked. In the non-tripped state, the outputs periodically pulse low. The protective device checks the output, to make sure it does indeed go low when commanded. If not, the output may have failed or has shorted to 24V somewhere else. Between OSSD1 and OSSD2 the pulse intervals are staggered to check for criss-crossed wiring between the two.
The technology relies on two independent channels carrying the same information output by the device:
Some related terms: