KL4021 | BECKHOFF | 1-Channel Analog Output Terminal 4-20 mA

Product Overview

The KL4021 (KL4021) is a precision 1-channel analog output terminal designed for the Beckhoff Bus Terminal system, delivering a standardized 4 to 20 mA current signal. Engineered for high-reliability process control, this module is the industry standard for interfacing with proportional valves, dampers, and frequency inverters in water treatment, HVAC, and chemical processing plants. With its 12-bit resolution and a low output error of less than ±0.1 percent, the KL4021 ensures stable and repeatable actuator positioning. Its robust design is further validated by extensive marine (DNV GL) and hazardous area (ATEX/IECEx) certifications, making it a versatile choice for both onshore industrial sites and offshore installations.

Technical Configuration

The hardware architecture of the KL4021 is optimized for the K-bus protocol, featuring a single-ended 1-wire connection technology. The terminal draws its 24 VDC supply via the internal power contacts, though it optionally supports 15 VDC operation when used with a KL9515 power supply terminal for specific energy-efficient or legacy requirements. The module provides 500 V electrical isolation between the internal K-bus logic and the field signal voltage, safeguarding the central controller from field-side electrical surges. The process image is presented as a 1 x 16-bit data value, and the unit is designed for "plug-and-play" operation, requiring no complex hardware addressing or manual configuration settings.

Technical Specifications

Technical FAQs

Can I use the KL4021 for a 0-20 mA application?

The KL4021 is hardware-optimized for 4-20 mA to provide "live-zero" monitoring. For applications strictly requiring 0-20 mA, you should select the KL4011. Using the KL4021 for 0-20 mA would result in the inability to output signals below 4 mA.

What is the maximum distance the current signal can travel?

Because the 4-20 mA signal is a current loop, it is highly resistant to voltage drops and EMI. The distance is primarily limited by the total loop resistance; as long as the total resistance (wire + actuator) remains under 500 Ohms, the terminal will maintain its specified accuracy over several hundred meters.

How do the 12-bit resolution and 16-bit process image interact?

The 12-bit output value is mapped into the 16-bit data word of the K-bus. Typically, the value is left-aligned. In your PLC program, you will scale your process variable (e.g., 0-100%) to the corresponding digital range (typically 0 to 32767 or 4095 depending on configuration) to drive the output.

Engineering & Installation Guide

• Shielding for Precision: Although 4-20 mA is robust, for the highest accuracy (especially in maritime or Ex zones), use shielded twisted-pair (STP) cables. Connect the shield at only one end—typically at the control cabinet's earth bar—to prevent ground loops from introducing signal offsets.

• Load Matching: Verify that your actuator's input impedance does not exceed 500 Ohms. If the load is too high, the terminal will be unable to drive the full 20 mA, resulting in a "saturated" output that does not match your PLC command.

• Power Supply Stability: While the terminal can run on 24 VDC via power contacts, ensure your power supply has sufficient headroom. The KL4021 consumes roughly 30 mA plus the actual loop current (up to 20 mA). In a large terminal strand, calculate the total current to ensure the bus coupler’s power contacts (max 10 A) are not overloaded.

• Hazardous Area Mounting: For ATEX/IECEx Zone 2 compliance, the KL4021 must be installed in an enclosure providing at least IP54 protection. Ensure that all wiring is secured against accidental disconnection, as per the "Ex nA" and "Ex ec" safety requirements.