Ground potential differences (GPDs) between remote bus nodes exist because bus nodes receive their local supply from different locations in the electrical installation. The RS-485 standard defines a transceiver’s total common-mode voltage range (VCM) as the sum of the driver output offset (VOC), the ground potential difference (VGPD), and the coupled noise from external sources (Vn) into the cable: VCM = VOC + VGPD + Vn. The standard specifies this range with 7V above Vcc (5V + 7V = 12V) and 7V below GND (0V – 7V = -7V), thus yielding a VCM range from -7V to +12V. The standard also demands reliable data transmission for a maximum ground potential difference of VGPD = ±7V. Assuming a symmetrical 5V driver design, the driver output common-mode will be VOC = Vcc/2 = 2.5V. The sum of these two common-mode voltages then provides sufficient headroom for any coupled noise of up to Vn = ±2.5V to yield the maximum common-mode voltage range. An example using the equation above: VCM+ = 2.5V + 7V +2.5V = 12V and VCM- = 2.5V + (-7V) + (-2.5V) = -7V. GPDs of ±7V and coupled noise of ±2.5V represent huge values for a well-balanced data link. The ability to operate reliably under these challenging conditions has made RS-485 the industrial workhorse amongst interfaces operating in harsh environments. Note: the ground potential difference between remote nodes is an AC voltage that swings at the 3rd harmonic of the 60Hz mains supply.