I have no idea what a CJB306 is, but in a situation like yours it is trivial to use 3 RS-485 repeaters at the central station to form a nice 4-subnet system. The central RS-485 is only a few feet long and works at minimum stress. Each of the 3 "home runs" is a simple point-to-point, which is the most robust, most tolerant form of RS-485.
Find a nice application note covering isolated RS-485 repeaters here:
- LynnL, www.digi.com
A very poor substitute for the truth, but the only one
discovered to date.