Communication issue RS485

Dear all

I have used Arduino UNO 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.

which one is better to use to make solve this kind of problems.

I have seen similar happenings. After a lot of time when I connected all the grounds everything worked OK. All my units were isolated from the mains via there power supply. You may also want to look at the termination, it should only be at the physical ends or last module on the bus.

Fine explanation, keywords are common Gnd and single bus terminator :slight_smile:

I have used terminating resistor 120E across non isolated version.

I have gone through few forum , kindly let me know

expert suggests no Gnd is necessary for non-isolated RS485 module.
for Isolated RS485 module gnd must be connected to maintain output potential from -7V to 12V DC

I have tested unit all non isolated and isolated unit gnd common. it worked fine for 1 hour but after 1 hour
when communication checked its giving time out error.

when i isolated product , only non- isolated products are working well. where as isolated RS485 modules are not even working.

it suggested that always use isolated gnd RS485 module since it give galvanized isolation and these device are better in noise compensation.

grounding of isolated module help to keep voltage level across A&B -7V to 12V DC in field condition provided ground is given

i have question

1)when to use isolated -gnd and non isolated gnd RS485 module concept in product??

2)if i used non isolated gnd , if other manufacturer used isolated -gnd how we should take care of
grounding

Have you removed all bus terminators from your modules?

Bus terminators introduce Vcc and Gnd connections which can make trouble with multiple power supplies.

DrDiettrich:
Have you removed all bus terminators from your modules?

Bus terminators introduce Vcc and Gnd connections which can make trouble with multiple power supplies.

Can you guide me which are bus terminator do u means to say R14???

Bus terminators include all R and C connected to the A and B lines.