What it is and the rating that decides fit
The Siemens 6AG1221-1BF30-4XB0 is a SIPLUS 8-channel digital input module for the S7-1200 system. The defining spec is the ambient temperature range: -40 to 70°C, which means it runs in freezer lines, unheated warehouses, or outdoor cabinets without derating. That range is the operating limit, not just storage — the module stays powered and functional across the full span. Inputs are 24 VDC, grouped in pairs for electrical isolation (groups of 2). Signal '1' threshold is 15 V DC at 2.5 mA min, 4 mA typical — standard for 3-wire proximity sensors without adjustment.
What else matters for commissioning
Built-in diagnostics and a diagnostic alarm output, parameterizable via the hardware config. The module also supplies voltage for 2-wire transmitters — one less external loop supply to wire. Power consumption is 1.5 W typical, drawing 105 mA from the backplane 5 V and 4 mA per input channel from load voltage L+. That leaves headroom for additional modules in the rack. Cable length: 500 m shielded, 300 m unshielded. IP20 housing, 45 mm wide — one slot on the rail.
Compliance: CE marked and FM approved for use in hazardous locations. That covers EU and North American installations for Class I, Division 2 environments.