When testing RS-485 wiring, the voltage between RS-485+ and RS-485- should be between which range?

• A. -3 V to -5 V
• B. 3 VDC to 5 VDC
• C. 0 V to 1 V
• D. 10 V to 12 V

Answer: (B)

RS-485 uses a differential pair, so the signal is the voltage difference between the plus and minus lines. A healthy, actively driven RS-485 bus typically shows about 3 to 5 volts of differential swing. This level gives strong recognition by the receivers even with cable losses and noise, while staying within what the driver can safely deliver on a 5V supply. If the differential is much smaller (0 to 1 V), the receiver may misread data; voltages in the tens of volts or negative polarities aren’t expected on a standard RS-485 link and could indicate a fault or wrong configuration. The sign of the differential just tells you which line is high; the important factor for testing is the magnitude, aiming for a 3–5 V differential.