What it is and what it does
The 6ES7421-5EH00-0AA0 is a SIMATIC S7-400 digital input module — specifically the SM 421 — packing 16 optically isolated inputs that read 120 V AC signals. It's the part you reach for when you need to bring 120 V AC field devices (limit switches, pushbuttons, proximity sensors) back into the S7-400 rack without mixing voltage domains. Each channel group is isolated from the backplane bus, and the module carries a 1500 V AC isolation test between channels and bus. That means you can land a 120 V AC sensor on one channel and a dry-contact limit on the next without worrying about sneak paths through the module.
Signal thresholds and timing — the stuff that matters for fit
The input thresholds are set for 120 V AC nominal: a logic "1" needs 74 V to 132 V AC, and a logic "0" is 0 V to 20 V AC. That's a clean 54 V deadband between off and on, which filters out noise from motor starters and contactors common in a 120 V control panel. The module draws 100 mA max from the 5 V DC backplane bus and dissipates 3 W typical. That's a light load — it won't stress your power supply budget, even in a full rack.
Where it lives in the panel
The 16 inputs share one common point — one M terminal for the whole group. That's typical for AC input modules and keeps wiring simple, but it means all 16 inputs must share the same voltage reference. If you need isolated groups, you'd need multiple modules.