I'm confused about what the DMX512-A standard recommends in terms of the receiver isolation circuit. I'm working on a receiver circuit for a prototype light fixture using the MAX485 chip. I'm wondering if I understand things correctly. This is what I think I should do. The receiver chip's ground is connected directly to Pin1/shield of the DMX cable. This connection acts as the ground for the receiver. The receiver receives 5V power, but it's completely isolated from the rest of the microprocessor and other circuitry power. The receiver's signal output is then fed to the MPU (for processing)through some sort of opto-isolator circuitry. The receiver's ground (a.k.a. Pin1/shield) is also completely isolated (>22Mohm resistance) from the other circuitry's digital ground and the chassis and earth grounds. The chassis, earth, and digital grounds are all connected as one node.

Does this make sense? Do I understand this right? Is it a bad idea to connect earth and chassis ground to digital ground? I've heard about a lot of weird DMX ground loop problems and I've even seen a few weird things in my own testing, so I'm obviously trying avoid all of that.

Any and all feedback from you pros would be great. Thanks in advance.

Brian