Lindy RS-422/485 - Manual (Page 11)

6.1 Jumper Settings
These cards feature built-in termination resistors for each serial port. You can
modify the jumper settings (short the pins) to avoid impedance mismatch
problems when operating under multi-drop transmission.
When an electrical signal travels through two resistance junctions in a
transmission line, the impedance mismatch will sometimes cause signal
reflection. Signal reflection causes signal distortion, which in turn contributes to
communication errors. The solution to this problem is to establish the same
impedance at the line ends as in the line itself by terminating them with
resistors. It is normally sufficient when the value of the termination resistors
equals the characteristic impedance of the transmission line. LINDY
RS-422/485 serial boards feature built-in termination resistors to prevent these
problems.
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