A.1.2 Cable TerminationThe characte

A.1.2 Cable Termination
The charactenstic impedance of twisted pair cable is a function of frequency,
wire size and type as well as the kind of insulating materials employed. For
example, the characteristic impedance of average 24 AWG. copper conductor,
plastic insulated unshielded twisted-pair cable, to a 100-kHz sine wave will be
on the orderof loon.
In general, reliable operation of the balanced interface circuit is not particularly
sensitive to the presence or absence of the cable termination at lower speeds
(below 200 kbitls) or at any speed where the signal rise time at the load end of
the cable is greater than 4 times the one-way propagation delay time of the
cable.
At other speeds and distances cable termination is required. Two options exist
with option 1 being more common, and option 2 ideal for use on low speed
control lines. Option I is a resistor ranging from 90  to 150 () and Is connected
across the cable at the load end. Ideally. Rl is selected to match the
characteristic impedance of the cable (Zo) to minimize reflections. Option 2 Is a
series connection of a capacitor and a resistor and is connected across the
cable at the load end. The resistor should be in the range of 90  to 150 fl arid
s selected to match the charactensbc impedance of the cable (Zo).