Siemens 6ES7421-7DH00-0AB0 SIMATIC S7-400 SM 421 16DI 24-60V UC PLC I/O Module

The Siemens 6ES7421-7DH00-0AB0 SIMATIC S7-400 SM 421 module provides 16 individually isolated digital inputs rated 24-60 V UC, accepting both AC and DC field signals. Input delay is fixed to four selectable steps from 0.5 to 20 ms, and the module reports diagnostic alarms plus hardware interrupts to the CPU when parameterized. Wire-break detection triggers above 0.7 mA.

Technical Specifications

Selection Notes

This variant accepts 24-60 V on AC or DC (UC) field wiring, wider than a fixed 24 V DC-only input, so it fits mixed AC-driven and DC-driven field devices without a separate module. Each channel is isolated individually (groups of 1) rather than in larger blocks, which allows different signal sources per channel without cross-coupling. Input delay is fixed to four steps – 0.5, 3, 10 or 20 ms – suited to slower AC-driven field contacts rather than high-speed DC counting. Wire-break detection triggers above 0.7 mA, useful where open-wire detection on the field circuit matters.