well one more way to do it
A slightly different solution which does have the advantage of avoiding earth loop problems would be to use optical isolation.So you have no electriccal connect between the two systems and they can run on completely isolated power supplies.
Also true for the 485 driver solution above, the problem with opto-isolation in the simple form is that it does not provide a differential interconnect which - as noted - is bad in very noisy environments. Of course, if you want belt, suspenders and a piece of rope, use opto isolated 485 drivers.

It a more elegant solution than trying to interface different voltages which never works very satisfactualy
I have never had a problem when using level converters such as the 4009/4010

Erik