Safety relay base unit for distributed fail-safe I/O
The Siemens 3SK2122-1AA10 is a SIRIUS safety relay base unit configured as 20 fail-safe digital inputs (F-DI), 4 fail-safe digital outputs (F-DQ), and 2 standard digital outputs (DQ). It is designed for use in safety circuits up to SIL 3 / PL e when paired with the appropriate PROFIBUS DP or PROFINET IO interface module — the base unit supports both protocols via a plug-in module, carrying 64-bit cyclic data per the datasheet.
The 20 F-DI channels accept 24 V DC signals (15 to 30 V DC range per channel), with an input delay configurable from 0 to 150 ms. That delay window lets you tune out nuisance trips from contact bounce or short-duration sensor glitches without adding external timers. Monitored start-up and valve monitoring are built in — the base unit checks that the start button and downstream valves are in the correct state before enabling the safety outputs. The light test period is 3 ms, and the readback time for output status is a maximum of 400 ms. Total current draw from the backplane is rated at a maximum of 7 A. The unit operates across -25 to +60 °C ambient, with storage and transport range of -40 to +80 °C. Unshielded cable runs up to 600 m are permitted; shielded runs up to 1,000 m.