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PROFIBUS-DP adopted much of the RS485 standard, but added a few more restrictions. First, 120Ω shielded twisted pair cables have become the normal recommendation for RS485 systems. PROFIBUS-DP, however, recommends 150Ω shielded twisted pair, necessitating the use of different and, in most cases, new cables. To minimize signal reflections, RS485 installations typically use a single 120Ω termination resistor at both ends of the bus, whereas PROFIBUS-DP recommends a 172Ω termination network at both ends of the bus. In the figure on this slide, users can see that two 390Ω bus biasing resistors are used in conjunction with the 220Ω termination resistor for PROFIBUS-DP; the effective differential resistance of this termination network is 171Ω, which is not a perfect match for the 150Ω cable, resulting in a slightly underdamped network. In most general purpose RS485 transceivers, pin A is the non-inverting receiver input (and non-inverting driver output) and pin B is the inverting receiver input (and inverting driver output) relative to the receiver output and driver input. However, the PROFIBUS standard describes the bus polarity in such a way that pins B and A are exactly the opposite of this.
