What are the two wires in a 2-wire loop?

• A. Ground and signal
• B. Data and power
• C. Positive and negative of 12 V
• D. Power/signal (+24 V) and signal return

Answer: (D)

In a two-wire loop, power and the measurement signal travel on the same two conductors. The loop is powered from a +24 V supply on one wire, and the transmitter modulates the current (4–20 mA) that flows through the loop. That same current returns via the other wire back to the supply. So the two wires are the power path and the signal path, with the signal being the loop current. This is why describing them as Power/signal (+24 V) and signal return is the correct idea. The other options imply a separate ground, separate data line, or a nonstandard voltage, which doesn’t match how a 2-wire, 4–20 mA loop actually carries the signal.