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2.3 Jumper Settings
For RS-422/485 serial communications, when an electrical signal travels
through two different resistance junctions in a transmission line, the impedance
mismatch will sometimes cause signal reflection. Signal reflection causes
signal distortion, which in turn will contribute communication errors. The
solution to this problem is to establish the same impedance at the end of the
lines by terminating them with resistors.
Ideally, the two ends of the cable will have a termination resistor connected
across the two wires. Without termination resistors, reflections of fast driver
edges can cause multiple data edges that can cause data corruption.
Termination resistors also reduce electrical noise sensitivity due to the lower
impedance, and bias resistors (120 ohms for twisted pairs) are required. The
value of each termination resistor should be equal to the cable impedance.
