Hmm, that's not an easy question. But perhaps I can say some things about it.

Do you need a fully optoisolated design? If not, it is much more easy to connect PIN1 to digital ground (and transciever ground). Then you can simpliy connect the transciever with the MCU. DO NOT CONNECT DIGITAL GROUND (PIN1) TO CHASSIS! If you do, you might get ground loop problems. Usually I build my circuits like this and do not get any problems.

Does it need to be a MAX485? For example you can use a LTC1535. Then it is much easier to build a fully optoisolated circuit. If you use this IC, you can connect digital (logic) ground to cassis, without getting any problems. If you want to do it with the MAX485, you should look at the datasheet of the LTC1535 to get an idea why (and how) to make a isolation.

If you want to use the MAX485 and want to get an optoisolatet circuit, you need two power supplys: one for the mcu and one for the MAX485. They must be fully isolated (transformator), for example a second 7805 is NOT enough. They must NOT have a common GND! Then connect the MAX485-GND with PIN1. The data connection between the MAX485 and the MCU must go through an optocoupler.

Sorry for the late answer, but i was not here on this webpage for a long long time.