does not define the protocols for data transmissions but Rs232 does
RS-232 does not define the protocol.

Michael your suggestion is definately solves the collision problem
Collision on RS-485 makes the circuit something other than RS-485. RS-485 is designed for collision avoidance, a misapplication of RS-485 the J1708 works on collision detection.

If you want to design with collision detection you will experience transcievers fighting each other and getting too hot to handle. Thus J1708 install 47 ohm resistors in series with the A and B outputs and lose a good deal of the noise rejection inherent to RS-485.
Also J1708 for the above reasons is ground referenced and lose the advantage of true differential inherent to RS-485.

Enough, so, in short use collision avoidance, stay away from collision detection. If the above is not enough, here is more; the software for collision detection is much more complex than the software for collision avoidance.

Erik