The Siemens 6ES7421-1BL01-0AA0 is a SIMATIC S7-400 SM 421 digital input module providing 32 isolated inputs rated for 24 V DC, compliant with IEC 61131 type 1 input characteristic. It accepts 2-wire sensors and defines signal "0" from -30 V DC to +5 V DC and signal "1" from 13 V DC to 30 V DC, with a typical signal "1" current of 7 mA. The module draws 20 mA from the 5 V DC backplane bus and dissipates 6 W typical power loss.
Deployment and integration
This module occupies one slot in the S7-400 rack (25 mm wide, 290 mm high, 210 mm deep) and connects via the backplane bus. All 32 inputs are simultaneously controllable; no channel-to-channel isolation is provided, but the input group (all 32) is isolated from the backplane bus. The isolation is tested at 500 V AC / 707 V DC, type test. Status indicators (green LEDs) are present per digital input.
Input timing and sensor compatibility
Input filtering is configurable within a range: transition from "0" to "1" takes 1.2 ms minimum to 4.8 ms maximum; "1" to "0" similarly 1.2 ms to 4.8 ms. This covers both fast and debounced sensing. For 2-wire sensors, the permissible quiescent current at signal "0" is max. 1.3 mA, and the overall permissible quiescent current is max. 1.5 mA — exceeding these will cause false "1" detection. Unshielded cable runs up to 600 m; shielded up to 1 000 m.
This part carries a lifecycle status of NRND (Not Recommended for New Designs) per the manufacturer, with a maturity stage of "mature".