What this module does and where it fits
The Siemens 6ES7421-7DH00-0AB0 is a SIMATIC S7-400 SM 421 digital input module providing 16 isolated inputs accepting UC 24-60 V AC/DC — meaning it can read both 24 V DC proximity sensors and 48 V or 60 V AC limit switches from the same card without external level conversion. That voltage flexibility is the headline: it spans 24 V UC to 60 V UC (47-63 Hz), so a single module type covers the mixed-voltage field wiring common in older plants where 48 V DC panels coexist with 24 V DC control loops. The input threshold for a reliable '1' is 15-72 V DC or 15-60 V AC; '0' is -6 to +6 V DC or 0-5 V AC, giving a clean noise margin even on long cable runs. Each input is isolated channel-to-channel in groups of one, and the entire bank is isolated from the backplane bus at 1500 V AC — essential for maintaining galvanic separation when the module sits in a rack alongside high-voltage I/O or drives. The module draws up to 150 mA from the 5 V DC backplane bus and dissipates 8 W typical at 60 V DC supply, so budget that into the rack power calculation.
Key ratings and what they mean for your wiring
Input filtering is parameterizable: you can set the debounce time to 0.5 ms, 3 ms, 10 ms, or 20 ms. Maximum cable length depends on that filter setting. Wire-break detection is built in: the module monitors each input for a current below 0.7 mA, which triggers a diagnostic alarm if the sensor or wiring opens. The quiescent current threshold for 2-wire sensors is adjustable between 0.5 mA and 2 mA, and the fixed current-limiting resistors for the wire-break monitoring circuit are selected automatically based on the rated voltage — 18 kΩ at 24 V, 39 kΩ at 48 V, 56 kΩ at 60 V. That means you don't need external pull-up resistors for proximity sensors; the module handles it. Diagnostic alarms and hardware interrupts are both parameterizable, so the module can signal a fault or a change-of-state directly to the CPU without polling. The status indicator per channel (green LED) gives a quick visual on each input's state; there is no red channel fault LED, but the diagnostic information is readable via the backplane bus.
Lifecycle and sourcing reality
This module carries a lifecycle status of NRND — Not Recommended for New Designs. That means Siemens is not actively promoting it for new projects, but it remains available for existing S7-400 installations that need a spare or a last-buy. There is no official successor order code published for this specific module; the S7-400 platform itself is being phased down in favor of the S7-1500 series, so any replacement would involve a migration to a different form factor and backplane. This module is sourced to order against an RFQ through independent channels. NRND status means you should secure last-buy quantities for long-term support.