SN65HVD72: Communication modes

Both sides should have 120 (there should be only two sides with 120 ohm resistors, if you have more receivers any that sit in the middle don't need the termination resistor).

If you are using an RS422 set up, you can use one 100 ohm resistor. (Rs422 is usually meant to be used in one direction communication as point to point communication, seems like you aren't using this if you're sending a message and waiting for a reply).

The important thing is the termination resistor matches the characteristic impedance of the cable you choose.

The external fail safe bias (the pull up and pull down resistors) are generally only used on one side. You can use them on two sides if you really want.

Dharmesh Joshi said:

I would also like to know if there are any disadvantages to enabling the receiver when transmitting a message. I presume we would get a copy of the message we transmit. My configuration is for my main PCB to transmit a message to the secondary PCB and wait for its instant reply.

No real disadvantage, the receiver network is already loading the bus. disabling it just disables the TTL output. 

Yes, you would expect to see what you transmit back on the Rout pin.

-Bobby