Detecting short wires ou open wires on RS422/485

Hi, I want to know how can I detect if a pair of wires is shorted or
open in a RS422/485 environment. The MAX3097 and MAX3098 RS422/485
receiver have this option, but Maxim IC are a bit expensive. I read
in a maxim application note that if A\ is open, it will be at the same
level as A (same thing if this two wires are shorted). So if (Va -
Va\) < 400 mV (or 200 mV) the ALARM pin is set to true (false
otherwise).