The Siemens 3RG7122-3GA00 is a SIMATIC PXO130 M12 thru-beam sensor receiver — the receiving half of a photoelectric pair. It operates on DC 10-36V, with a sensing range of 4m, NPN output configured dark-on, rated at 200mA, and terminates in a 4-pole M12 connector. The housing is M12x50, rated IP67.
The 4m range is the maximum specified detection distance between emitter and receiver under ideal conditions (clean optics, aligned). In practice, derate for dust, condensation, or misalignment. The NPN dark-on output means the load (PLC input, relay) is energized when the light beam is interrupted — standard for presence detection in packaging or material handling. The 200mA rating is the maximum switched current; typical PLC digital inputs draw 10-20mA, so headroom is ample. Supply voltage 10-36V DC covers 24V nominal systems with tolerance for brownout or long cable runs. The M12 connector is 4-pole: typically pin 1 = brown (V+), pin 2 = white (not used or alarm), pin 3 = blue (V-), pin 4 = black (output). Verify pinout against your existing wiring before swap.