The Siemens 3SF7844-8MD17-0SS1 is a Type 4 safety light curtain receiver from the SIMATIC FS420i family, with 30 mm resolution across a 1200 mm protective height and a 0–18 m operating range. It communicates over Profisafe and supports muting, so it fits into a safety-rated machine guard circuit that needs to pause protection during a material pass-through without stopping the line.
Type 4 per IEC 61496-1/2 means this receiver is rated for the highest level of fault tolerance and can be used in safety applications up to SIL 3 / PL e. The 30 mm resolution catches fingers approaching the hazard zone — that's finger-protection class. Mount the receiver opposite its matching emitter on a guarded machine opening; the Profisafe bus connection ties directly into a SIMATIC safety CPU without a separate safety relay.