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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.

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  • \$\begingroup\$ What is X and Y used for? \$\endgroup\$
    – Andy aka
    Commented Jan 31, 2014 at 15:05
  • \$\begingroup\$ full duplex rx and tx pairs. \$\endgroup\$
    – Ron
    Commented Jan 31, 2014 at 15:07
  • \$\begingroup\$ How can you do isolation and power? \$\endgroup\$
    – pjc50
    Commented Jan 31, 2014 at 15:29
  • \$\begingroup\$ @pjc50: transformers are really good at transmitting power across an isolation barrier. For example, MAX1480B isolated power and data for RS-485; Linear LTM2881 isolated power and signal for RS-485; Maxim app note 3754; Analog ADM3252E; Analog ADuM5000 + ADuM4160; etc. \$\endgroup\$
    – davidcary
    Commented May 10, 2014 at 0:33

2 Answers 2

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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.

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  • \$\begingroup\$ I don't like unhappy customers :) \$\endgroup\$
    – Ron
    Commented Jan 31, 2014 at 15:07
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I'd consider using radio transmitters. I've used 433MHz off-the-shelf cheap radios for sending data down 200 feet of tunnels and along about the same distance in railway carriages and they worked fine with less than 1mW output power.

Your data rate isn't very great (you said) so sending data half-duplex shouldn't really be a problem either. This just leaves you with getting power to the device and if there is some local power in the "lift" you can use then maybe drop the cable idea entirely (that's got to save a few dollars!).

If you need higher data rates, there are plenty of cheap transceivers capable of sending data at well-over 100kbps. Transmission frequency depends on the country you are in but the 2.45GHz "worldwide" band may be a little too high in frequency for this to work effectively.

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