RS485 Communication issue,

Thread Starter


Joined Feb 22, 2013
Dear all

I have used PIC18F45k40 device for RS485 communication. I have 2 ckts one is isolated and another non isolated RS485 design.

I have schematic & placement for reference.
I have connected multiple slave device independently isolated and non isolated product , its working well. There is no issue with poll mismatch

Now i am combining isolated and non isolated module in parallel.during this stage, after hour isolated module stop communicating but isolated version product working well.

difference i found in both device i found is voltage level during operation isolated version if measure voltage across A& B w.r.t gnd is 7V-9V respectively i.e for ADM2483(Isolated ground)

where as SN75176 non isolated IC voltage level across A &B w.r.t ground is 2.5V-5V respectively.

i would like to why my communication of isolated version get disturbed when non isolated version is introduced.

1)which one is better for application for isolated and non isolated ground
2)what should i do if my system having unit with isolated and non isolated ground.


Delta prime

Joined Nov 15, 2019
Hello there:)
I remember a mountain top RF repeater site Public safety LAPD I was installing motorolas APCO 25 interoperability controllers."A pipe dream! "where a low-cost PLC was connected by non-isolated modems to a DCS 2Km away. A lightning discharge caused a common mode surge which destroyed both modems and the PLC's partially isolated communication port -- destroyed meaning black,charred components. Yet the PLC itself was unaffected and continued it's control without interruption
I can think of 5 possible resolutions to your predicament. A little more information is needed. is your computer and control device in the same room and both plugged into power points in the same circuit of the power distribution system?