RS485 signal transmission
Martin,

I'm by no means an expert at RS485, but I have run into some of the same problems you've had with independent power supplies on multi-drop RS485 networks.

Most frequently, in spite of claiming fail-safe operation, they don't play nice with ground loops/DC offsets.  For me, in the past, the most direct route is to use fail-safe biasing on the two lines.  E.g. .

By the way: Most of my learning has come from others here as well as some pretty good application notes from Texas Instruments (PDF), Maxim (HTML), and Analog Devices (PDF).

Hope to hear if this gets you in business.

Josh