6ES7421-5EH01-0AA0 - Digital Input Module - SIEMENS

SM 421 DI 16x24...125VUC HF 6ES7421-5EH01-0AA0, the digital I/O reference for S7-400

Designed for demanding industrial environments, the SIEMENS SM 421 DI 16x24...125VUC HF digital input module, part number 6ES7421-5EH01-0AA0, secures the acquisition of your discrete signals at the heart of SIMATIC S7-400 architectures. It meets the needs of maintenance managers, automation engineers and integrators facing severe electromagnetic environments, heterogeneous sensor fleets, and the availability constraints of continuously operating equipment. The wide permissible voltage range, from 24 V UC to 125 V UC, reduces the variety of stock references and simplifies engineering. Galvanic isolation per channel improves EMC immunity and the functional safety of the upstream system. Each channel features configurable diagnostics and a process alarm to detect field anomalies earlier and speed up troubleshooting. The adjustable input delay, suitable for DC signals, filters contact bounce and optimises detection reliability. Configuration is standardised via STEP 7 (TIA Portal) with support via GSD file for controlled and traceable integration into your project repository.

Measurable productivity, risk control and integration flexibility

• Single 24 to 125 V UC range – One module covers multiple field topologies, reducing specification errors, lowering the number of stock items and simplifying maintenance.
• Galvanic isolation per channel – Better immunity to interference and reduced ground loops, contributing to measurement stability and protection of the PLC input stages.
• Channel diagnostics and process alarms – Rapid localisation of faults and out-of-tolerance signals, shortening analysis times and facilitating preventive maintenance.
• Configurable input delay – Debounce filtering and adaptation to machine dynamics, reducing spurious triggers and improving repeatability.
• Compatible with 2-, 3- or 4-wire sensors – Smooth integration of common detectors and switches without complex rewiring, for fast, controlled modernisations.

Technical architecture and design quality

The SM 421 DI 16x24...125VUC HF provides 16 isolated digital inputs, with galvanic isolation per group of 1, effectively per channel. This architecture strengthens signal processing robustness in noisy environments. The module natively integrates with SIEMENS SIMATIC S7-400 controllers. Configuration and parameterisation are performed via STEP 7 within the TIA Portal, with support via GSD file, ensuring a standardised hardware description and consistent diagnostics management. Each channel can be configured for diagnostic functions and process alarms, allowing granular supervision of field states. The adjustable input delay for DC signals allows tailored filtering, useful for neutralising bounce and micro-disturbances. The HF designation indicates enhanced suitability for high-performance applications in demanding EMC contexts. The product continues SIEMENS standards for industrial reliability and longevity.

Target uses and application cases

The module is aimed at industrial automation installations using standard digital signals, notably in discrete manufacturing, intralogistics, materials handling and robotic cells. It is suitable for reading inputs from dry contacts, relays, inductive sensors or limit switches, wired as 2-, 3- or 4-wire. The wide voltage range enables reuse of heterogeneous installations during S7-400 retrofits while maintaining a single digital I/O reference. Channel diagnostics facilitate field intervention management and commissioning during test phases.

Technical differentiation and reassurance

SIEMENS builds on a proven ecosystem where hardware, engineering and diagnostics work coherently. The SM 421 DI 16x24...125VUC HF stands out through the combination of an extended voltage range, galvanic isolation per channel and configurable diagnostics—three tangible levers to secure operation and speed up troubleshooting. Standard Exchange Industry supports your B2B needs with exchange-standard supply options, new or refurbished units, as well as repair. Our teams commit to short lead times, typically 24 to 72 h depending on availability, and warranties of 6 to 24 months depending on the selected service. Each unit is handled under documented procedures to ensure traceability and functional compliance.

If you need to put an S7-400 line back into service without delay or prepare a controlled retrofit operation, contact our specialists for a quick quote on the SM 421 DI 16x24...125VUC HF 6ES7421-5EH01-0AA0. Express delivery depending on stock, exchange standard and repair available, with dedicated technical and logistical support.