The 6AG1221-1BH30-2XB0: Sixteen digital inputs reading 24 V DC signals, grouped in fours with a common return. Parameterizable so you can set input delay and diagnostics without reaching for a screwdriver — the diagnostics alarm reports channel faults back to the CPU. It's the kind of module you drop into a S7-1200 rack and forget about, except the wide -40 to 70 °C range means it will keep running when the panel heater fails.
Where it fits
DIN-rail mount, 45 mm wide, 100 mm tall, 75 mm deep. Requires a front connector (not included). Draws 130 mA from the 5 V backplane bus and dissipates 2.5 W typical. The load voltage must stay within 20.4 to 28.8 V DC; each input sinks 4 mA typ at 24 V. Unshielded cable runs up to 300 m, shielded to 500 m.