KL1704 by Beckhoff

The KL1704 is a digital input terminal manufactured by Beckhoff within the KL Analog Input Modules series. It accepts line-level AC signals and forwards their logical states to the controller, allowing limit switches, contactors, or sensors to be monitored in automated machinery. The unit groups four channels in a 12 mm housing, saving cabinet width when multiple inputs must be added. Its variable installation position allows the device to fit horizontally or vertically on a DIN rail without derating, providing flexibility in tightly arranged panels.

The module operates directly on a nominal supply of 120/230 VAC, so each channel can handle standard mains voltages without external transformers. It has 4 independent input circuits, enabling the acquisition of four discrete field signals through the internal bus. Wiring follows a 2-wire scheme in which the phase and neutral conductors terminate on spring contacts, simplifying connections to the module. A switching time of 10 ms indicates the minimum pulse width that the module will recognize, making it suitable for mechanical contacts or slow optocouplers. The inputs have resistive and capacitive characteristics, allowing signals from resistive and low-capacitance sources to be evaluated. Galvanic separation between the AC side and the logic plane is verified for one minute with a test voltage of 3750 VAC, protecting the controller from transient overvoltage.

The internal backplane remains isolated from the main terminals by a K-bus barrier rated at 500 V, so the module can share a power rail with low-voltage I/O components without leakage. The IP20 housing provides protection against accidental finger contact once the unit is installed in a terminal strip. When mapped into the PLC, the device occupies a process image width of 4 input bits, and this fixed length supports deterministic addressing in standard Beckhoff libraries. Channel states are shown through front-mounted LED indicators, allowing wiring and signal presence to be checked without connecting diagnostic tools.