Differential driver output voltage (VOD) is perhaps the most misinterpreted or most intentionally ignored specification in PROFIBUS-DP’s physical layer. RS485 specifies that the VOD between the A and B lines shall be 1.5V to 5V, peak differential, measured at the driver terminals with a 54Ω resistor between A and B. However, PROFIBUS-DP specifies that the VOD shall be 4V to 7V, peak-to-peak differential, measured at the far end of the cable, with termination at each end. Clearly, these requirements are quite different, so be wary of the all too common “PROFIBUS” compatible RS485 transceiver that only specifies a VOD minimum value (for example, 2.1V) without a maximum value. The best way to ensure PROFIBUS-DP VOD compliance is to test the transceiver with a PROFIBUS load. The figure shows how the LTC2876 and LTC2877 are tested with a PROFIBUS-DP load and some series resistance to simulate cable losses, where the driver output voltage (the blue curve) is generated from measurements taken at the “end of the cable” (A’ and B’) to ensure the PROFIBUS-DP specification is truly met.