What this module is — and why you're here
The Siemens 6ES7228-1RC51-0AA0 is an SM 1228 failsafe digital input module for the S7-1200 family. It's NRND (not recommended for new designs), so the procurement angle is clear: you're filling a BOM line on an existing safety loop, not spec'ing a new one. The module runs on 24 V DC, carries 2 interfaces (backplane bus plus PROFIsafe), and is rated IP20.
Supply tolerance is tight: 20.4 to 28.8 V DC — keep the bus clean. The 2 interfaces mean one for the backplane bus (power and internal comms) and one for PROFIsafe; no extra fieldbus expansion slot to juggle.