Chapter 7
Most RS-485 lines use an end termination consisting of a resistor equal to the
characteristic impedance connected across the differential lines at or just
beyond the farthest receiver. Figure 7-3 illustrates.
For a cable with a characteristic impedance of 120Ω, the termination is 120Ω
across lines A and B, just beyond the A and B pins at the farthest receiver.
When two or more drivers share a pair of wires, each end of the line has a termi-
nation resistor equal to the characteristic impedance. No matter how many
nodes are in the network, there should be no more than two terminations.Figure 7-3: A parallel end termination requires a resistor across the differential lines at
or just beyond the last receiver. A two-way interface has a resistor at each end of the
line.