What indicates a device is active?

What indicates a device is active?

• A. There are no markings on the device.
• B. It uses open-collector outputs.
• C. Terminals labeled 4-20 mA output (+/−).
• D. It has a label 'Current sourcing required'.

Answer: (C)

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.

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.