HART Protocol and 4–20 mA Loop Communication Fundamentals Practice Test 2026 - Free Practice Questions and Study Guide

The key idea is that an active transmitter directly drives the loop current to convey the measurement. When a device has terminals labeled 4-20 mA output, it is designed to generate and modulate that loop current, making it an active source in the loop. The other signs aren’t as direct: open-collector outputs are digital switches, not an analog current signal; a label saying you need current sourcing suggests the device expects an external source rather than producing the loop current itself; and no markings give no clear indication of activity. So, the presence of 4-20 mA output terminals is the clearest indicator that the device is actively driving the loop.