Siemens SM421 digital input module, 16 x DC 24 V inputs, is designed for SIMATIC S7-400 backplanes and is commonly listed under part number 6ES7421-7BH01-0AB0 (also seen as 6ES74217BH010AB0). It accepts standard 24 V DC signals from sensors and switches and features an approximate 0.05 ms input delay for fast, synchronized state changes across all channels. The module provides diagnostic and alarm feedback to the PLC, supporting reliable monitoring of field devices such as pushbuttons, limit switches, and proximity sensors in automated production lines.
