Hey All-
I'm probably way over thinking this but I need some validation. I have a set up when I'm trying to read a rs485 sensor in a high noise environment.

The setup is a standard AC/DC power supply powering a modbus master device, then running to a dc/dc converter and rs485 opto-isolator then to the sensor. On the sensor side, there is no connection to earth ground to avoid ground potential differences between the modbus master and the sensor. My question is, with no connection to earth ground and both my power and rs485 lines isolated, and the long cable run, should I be worried about my signal/power lines floating to a voltage higher than the sensor can handle (around 30VDC).

 

Make sure you use shielded data cable, and connect the shields to the RS485 signal ground at each end of each cable.
As you have isolation, do not connect the 2 sides shields together.
We has an early installation give us funny results as the RS485 signal ground was not connected through, and the common mode voltage was exceeded.
Connecting the signal grounds at both ends together fixed the problem.
Have you seen the Mornsun isolated RS485 (and others) drivers?
https://www.mornsun-power.com/html/products/48/rs-485-transceiver-module.html

 

With galvanic isolation you can still use ISO grounds/shields on individual isolated segments without system current loops.
https://en.wikipedia.org/wiki/Isolated_ground

https://e2e.ti.com/blogs_/b/analogw...-questions-about-isolated-rs-485-transceivers
https://www.analog.com/media/en/reference-design-documentation/design-notes/dn228f.pdf