What it is and where it fits
The Siemens SIMATIC 3RK1908-0AP00-0KP0 is a BaseUnit — the DIN-rail-mounted backplane that carries one fail-safe digital input (F-DI), with no AC/DC infeed onboard. It feeds the input from a 24 V DC supply rail (20.4 to 28.8 V permissible) and handles up to 7 A total ampacity. The single input is Type 1 per EN 61131-2, meaning it's designed for standard industrial proximity switches and pushbuttons, not the higher-threshold Type 2/3 variants used on some 2-wire sensors. 75 mm deep, 30 mm wide, 215 mm tall — it slots into a standard DIN-rail panel alongside other SIMATIC safety modules.
Termination and wiring
Push-in spring terminals on both the main current circuit and the auxiliary/control circuit. Accepts solid or stranded wire from 0.5 to 2.5 mm². Standard slot screwdriver, 0.6 mm x 3.5 mm tip, for release. Strip length is not given here, but 8 mm is typical for this terminal family — verify against the terminal marking on the unit. Spring-cage holds firm under vibration; the unit is rated for 6g shock at 11 ms and 2g vibration up to 500 Hz.
Environmental and mounting
Mounts vertically or horizontally on DIN rail. Requires 50 mm clearance above and below for airflow and wiring access. Operating temperature range is -25 to +60 °C (derate above that per the manual — not covered here). Storage and transport range is -40 to +70 °C. Pollution degree 2 (non-conductive dust only). Relative humidity 10 to 95%, non-condensing. Air pressure range 900 to 1060 hPa — fine for altitudes up to about 1000 m without derating.