The EP-1901 is the safety feed-input module in Emerson's RSTi-EP slice I/O platform — a 1-channel device that accepts a 2-channel safety input (Type 3 per IEC 61131-2) from an emergency stop button, safety gate interlock, or light curtain, processes it through dual-channel diagnostics with 96.64% diagnostic coverage, and provides a single OSSD safety output rated at 8 A. The module is certified to SIL 3 (IEC 61508), SIL CL3 (IEC 62061), and PLe Cat.4 (ISO 13849-1) with a PFH of 6.27×10⁻⁹ per hour and an MTTFd exceeding 100 years. At 11.5 mm wide — roughly the thickness of two smartphone stacked together — the EP-1901 adds functional safety to a RSTi-EP slice without consuming the physical space that a standalone safety relay or a dedicated safety PLC would occupy.

Safety integrated into the I/O slice, not bolted on as a separate system

In a traditional machine safety architecture, the emergency stop circuit runs through a dedicated safety relay (a 22.5 mm DIN rail module with force-guided contacts), which is wired in series with the main contactor coil and monitored independently of the standard PLC I/O. The EP-1901 replaces this external safety relay by bringing the safety input and the safety-rated output into the same RSTi-EP backplane as the standard I/O — the safety signal travels over PROFIsafe (via the EPXPNS001 head module) or FSoE (via the EPXETC001 head module) to the safety PLC, while the OSSD output directly controls the safety contactor or safe torque off (STO) input of a VFD. The safety and standard I/O share the same DIN rail, the same 24 VDC power bus, and the same Ethernet cable to the controller — but the safety data is transmitted in a PROFIsafe or FSoE black channel, certified end-to-end for SIL 3 integrity. The economic and spatial advantage is immediate: a machine that needs 8 safety inputs (e-stops, gates, light curtains) and 64 standard I/O points can be built on a single RSTi-EP slice with 8 EP-1901 modules occupying a total of 92 mm of DIN rail, versus a traditional architecture with 8 × 22.5 mm safety relays (180 mm) plus a separate standard I/O block.