The FANUC backplanes for Series 15 control systems and onwards. The back plane pcb is the board which connects all of the processing boards of the Fanuc 15 together on a BUS. Later systems such as the 16, 18, 21 and so on use yellow system cases with each and everyone having the backplane board at the back of the case which the PSU, main cpu, and other processing or modular plugin’s insert into to communicate to the main CPU. Normally backplanes do not actively process on the 15, 16, 18, 21; which is left for the later 15i/16i/18i/21i which utilise a main board or master, etc.