Siemens 6ES7421-7BH01-0AB0 SIMATIC S7-400 SM 421 16DI 24V DC PLC I/O Module

The Siemens 6ES7421-7BH01-0AB0 SIMATIC S7-400 SM 421 module provides 16 isolated digital inputs rated 24 V DC, meeting IEC 61131 Type 2 input characteristics. Two isolated encoder supply outputs with electronic short-circuit protection are built in, and the module reports diagnostic alarms plus hardware interrupts to the CPU. Input delay is parameterizable from 3 microseconds to 3 ms per channel.

Technical Specifications

Selection Notes

The input delay is parameterizable from 3 µs up to 3 ms, so the same module handles fast counting signals – up to 2 kHz at the 0.1 ms setting – and slower contact inputs that need debounce filtering. Two built-in, short-circuit-protected encoder supply outputs remove the need for a separate 24 V sensor supply, and the optional redundant feed keeps inputs live if one supply channel fails. Channels are isolated in groups of 8, so mixing different 24 V DC signal sources across the 16 inputs is only possible in two-group blocks, not per individual channel.