The Siemens 3RK3122-1AC00 is a SIRIUS modular safety system central unit — the brains of a distributed safety node. It packs 2/4 fail-safe digital inputs, 4 standard digital inputs, 1 fail-safe relay output, and 1 fail-safe digital output into a 45 mm wide housing that snaps onto DIN rail or screws in with a push-in lug. Rated SIL CL 3 per IEC 61508 and PL e per IEC 62061, it's sized for guarding zones up to that integrity level — think emergency stops, light curtains, and two-hand controls on a production line.
SIL CL 3 per IEC 61508 means this unit can be used in safety functions up to Safety Integrity Level 3 — the highest common level for machine guarding. The 2/4 F-DI count means you can wire two dual-channel or four single-channel safety sensors directly, no extra expansion module. Input delay is parameterizable from 0 to 150 ms, letting you filter out sensor chatter without adding external timers. The fail-safe relay output (F-RO) switches up to 45 mA at 30 V DC — enough to drive a safety contactor or a small solenoid valve directly. The PROFIBUS DP interface (with the optional DP module) carries 64 bits of cyclical safety data, so the central unit can talk to a safety PLC over the fieldbus without hardwiring every signal.
Snap-mounted to DIN rail or screw-mounted with an additional push-in lug — standard panel integration. Mounting position is vertical. The enclosure is IP20, so it lives inside a cabinet, not on the machine frame. Shock resistance is 15g for 11 ms, typical for a control panel on a vibrating machine base.