Hi!
In all the applications of RS 485 we put in serial on each line +TR /-TR a 220 ohm resistors with zener diode and diode for protections and then a 120 ohm resistor is parallel to the outside cable. Normaly we connect togheder on 100 m of cable 7-8 units o more. The the data line characteristic impedance of 120 ohm is as declared impedance not resistance.
Regards Clemente