I need to communicate with a piece of hardware on an elevator car. I have a few twisted copper pairs available, but it's not CAT5, so Ethernet is out. Instead I'd like to use RS485.
I've been reading and ground level difference can really mess up 485, I'm also passing power along with the RS485. ( So 6 conductors: A,B,X,Y,5V and GND ). Since the unit in the elevator will draw power from the copper pairs is ground level difference still an issue? Do I need to go full out on the isolated 485 or is that overkill?
note1: These cables can be as long as twice the height of a building ~100-200FT
note2: Communication speed is not critical, it can go slow.
note3: LDO takes 5V to board 3.3v so we have some room for voltage drop on wire.
Best Answer
If you can guaranteed the grounds are going to be close (within +/-7V peak) under all possible conditions, then no isolation is required.
Personally, I would go for isolation without question. What happens when lightning strikes nearby? I foresee a bunch of RS-485 transceivers getting replaced and an unhappy customer.