Look guys,
My schematic use 2 transistor and 3 Rs per tx line. You dont need any other ic as converter or similar.
I am mounting 4 PIC forming a network, 1 master and 13 slaves. Tx from master will be conected to all Rx of slaves, then all Tx from slaves will be conected to master Rx.

Anyway, looks like demon have resolve this...

/jose p