What it is and what it does
The Siemens 3RK1908-1AA00-0BP0 is a SIMATIC 3DI/LC control module — a 4-channel digital input accessory that plugs directly onto motor starters in the 3RK1 family. Inputs 1 through 3 are parameterizable for flexible signal mapping; input 4 is dedicated to local control (LC), typically wired to a local pushbutton or selector switch for manual override at the motor.
Key ratings and what they mean for fit
Four digital inputs, Type 1 per IEC 61131, so they accept standard 24 VDC sensor signals from proximity switches, limit switches, or pushbuttons. Input current at signal <1> is typically 9 mA — light enough on the sensor supply, which is rated for 0.1 A maximum at 60 °C. That 0.1 A limit means you can power one or two small sensors from the module's own supply, but for a bank of photoelectric eyes you'll want an external 24 VDC supply. The module detects short circuits on the sensor wiring and reports a diagnostic — a useful check when a cable gets pinched in a panel or crushed under a cable tray. Input filtering is fixed: signal transition times run 10 to 15 ms for both <0> to <1> and <1> to <0> at rated input voltage. That's typical for a motor-starter-mounted DI module — fast enough for interlocking and run-permissive signals, but not for high-speed counting or encoder pulses. If you need sub-millisecond response, this isn't the right module.
Where it goes and how it mounts
This module is designed to snap onto a motor starter — no DIN rail required, no separate enclosure mount. It occupies 30 mm width, 54.5 mm height, and 42.3 mm depth, so it adds minimal footprint to the starter assembly. Mounting position is flexible: vertical, horizontal, or flat, which helps when the panel layout is tight or the starter is oriented oddly. IP20 protection means it's for inside a cabinet only — no washdown, no dust-laden environment without an enclosure.
Wiring and environment
Wire termination accepts 0.2 to 1.5 mm² solid or stranded, or 0.25 to 1.5 mm² finely stranded with ferrule. Use a standard 0.6 mm x 3.5 mm slotted screwdriver. Sensor cable runs up to 30 m — fine for local sensors on the same machine frame; beyond that, watch for voltage drop and noise pickup. The module is rated for operating temperatures from -25 to +60 °C, storage/transport from -40 to +70 °C, and 10 to 95 % relative humidity (non-condensing). Shock resistance is 6g for 11 ms; vibration resistance is 15 mm at 6 Hz, then 2g up to 500 Hz — typical for industrial machinery mounted on a panel or subframe.