The Siemens 3SK2122-2AA10 is a SIRIUS safety relay base unit — the control backbone for a fail-safe I/O block on a machine safety circuit. This is the piece that sits between your safety-rated sensors (light curtains, e-stops, interlock switches) and the contactors or motor starters that drop power. The 20 F-DI count means it can handle a decent-sized guarding zone without needing a second node.
At 45 mm wide and 124.5 mm deep, it fits a standard DIN-rail footprint — plan for about 4.5 cm of rail per unit. The IP20 enclosure means it belongs inside a panel, not out on the machine frame where washdown or dust would get to it. Input delay is adjustable from 0 to 150 ms, which lets you tune out nuisance trips from vibration or brief sensor shadows without slowing the safety reaction below what the risk assessment demands. Monitored start-up and valve monitoring are built in — the unit checks that the start button circuit and any downstream valve feedback are healthy before it lets the outputs energize. That's a standard feature on SIRIUS safety relays, but worth confirming if your BOM was carrying a separate monitoring module. It supports PROFIBUS DP and PROFINET IO when paired with the respective interface module, passing 64 bits of cyclic data. That lets the safety I/O talk to the standard PLC without a separate safety controller, as long as the CPU handles PROFIsafe.