RS485 Rx signal connected with two drivers
U27a receive and U29a receive are isolated by R123 680 ohms. MPU should have control but this addition implies that both receivers may be active at the same time. In that case U27a will always dominate over U29a. If U27a is NOT in receive mode (it goes to high impedance if RE\ is high) then R123 allows data from U29a to be read. A simple and crude priority switch.

The resistor is there to prevent a short if both receivers drive the same wire with no current limiting, which is what R123 is really there for.