Most 485 tranciever chips similar to 75176 permit the activating and deactivating of the transmiter using a pin on the chip.
Usually you can tie the receive and transmit enables together ( they have opposite active polarities)and activate them for the mode ( talk or listen) that you need.
You will also need 3 high speed opto isolators (6n136 or 6n137 as an example) one for the received data, one for the transmitted data and one to control the direction enable pin on the tranceiver.
In addition you will need a separate isolated power supply for the 485 tranceiver and opto isolators.
Hope this gets you started....