That looks really interesting. Ive only read the first page so far but it seems to be making sense. From what i can see it looks like only 1 device can be sending at any one time but it can stop sending and allow another device to send instead. I think thats why there are 4 pins on the MAX485 chip for data. 2 of them look like they switch between sender and receiver. It sounds a bit like DMX and would explain why manuals for DMX lights say you can damage them by having 2 devices set to master at the same time.

I dont *need* bi-directional communication but it would be nice. There will be a master box which takes an input from RS232 or USB and converts it to RS485. Everything else will simply listen but it would be nice to add functionality for each device to become a master if the main box fails or the PC crashes.

I will continue reading about it tomorrow when i have more time.

Thanx for that. Even just the first page is a huge help