communication bet two remote slave and one dcs

This is a dcs system .IA & B are remote I/O's acting as slave to main Dcs.The Distance bet A & B is 200m & Distance between Main Dcs & this two slaves is approx.1 km.All communication based on ethernet .converter is used to convert ethernet to optical fiber link & vice versa,for both I/O's.
my question is can we avoid this 1 km optical link from remote I/O' to dcs & establish the communication between Remote I/O'(B) & dcs by using RS 485 link connecting direct to switch of Remote I/O' (A).By using this ethernet switch (of A) is possible to establish comm???

Also plz suggest if any extra requirements /module require to establish comm..