What indicates a device is passive?

• A. Terminals labeled 4-20 mA output.
• B. Labels like 'Current sourcing required' or open-collector outputs.
• C. Has its own power supply to drive current.
• D. Uses internal solar power.

Answer: (B)

In a 4–20 mA loop, a passive device does not generate or source the loop current; it only interacts with the loop using the power provided by the loop itself (often sinking current or relying on an external pull-up). The best indicator of this is labeling that says “Current sourcing required” or features like open-collector outputs. An open-collector output can only pull the line toward ground and relies on an external source to set the high level, meaning the device itself isn’t supplying the loop current. In contrast, options that show the device actively drives current, has its own power supply, or uses internal solar power indicate an active device, not a passive one.