There are several methods of terminating data lines. A resistor is added in parallel
with the receiver’s “A” and “B” lines in order to match the data line
characteristic impedance specified by the cable manufacture (120 W is a
common value). This value describes the intrinsic impedance of the
transmission line and is not a function of the line length. A terminating resistor
of less than 90 W should not be used. Termination resistors should be placed
only at the extreme ends of the data line, and no more than two terminations
should be placed in any system that does not use repeaters.