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To avoid noise from the external system I tried to isolate the internal completely. So I added DC-DC isolated converter with signal isolation. I have attached the picture. Still when reading data and communicating with VFD it receives data (sometimes garbage) and the PC cursor shows unusual behavior which gets data from the FTDI(FT232R) connected to it.

What might be the reason of getting this noise because the internal system is fully isolated with the RS485 system.System block diagram and connection

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  • \$\begingroup\$ I looked up the RT9080, and its PSRR is pretty good at lower frequencies, but falls off above a few kHz. You could try stacking some smaller MLCC or film caps on the input and output to filter some of that noise better. Something like a .1u and .01u. \$\endgroup\$ – Stiddily May 3 at 12:34
  • \$\begingroup\$ Ok sir, I will test it, is there any chances of reverse leakage current from R1SE-0505 to the system or from the SMPS(LRS-35-12) which I'm using for power supply. What are the worst possible way? Because both of these converters are isolated. \$\endgroup\$ – PCBLearner182 May 3 at 13:18
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There could be a few reasons here. The disturbance may even be radiated rather than conducted on A and B from your circuit's point of view.

As a simple place to start; your drawing doesn't seem to show a shared ground connection from the isolated side of the RS485 to the remote terminal. Without that you are in the realms of 'might work' and you may be exceeding the common-mode performance of your transceiver. An alternative (worse) approach to a shared isolated ground reference is to have 'leak' resistors from A and B to your local signal ground, fail-safe biasing resistors can create the same result too.

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  • \$\begingroup\$ In this picture I have not mentioned about the bias resistor as it is connected in the main schematic. The value of the resistor is 560 ohm, connected from A to 3.3V(pulled up) and other one connected from B to GND_iso(pulled low). \$\endgroup\$ – PCBLearner182 May 3 at 13:12
  • \$\begingroup\$ @PCBLearner182 Do you still need assistance on this? I would strongly recommend a shared isolated-ground if you haven't tried that already. \$\endgroup\$ – Cursorkeys May 8 at 19:50
  • \$\begingroup\$ Yes I need assistance on it because I'm not clear about this issue.The system is already has a DC-DC isolated converter. The external input side has a isolated ground as shown in the figure \$\endgroup\$ – PCBLearner182 May 13 at 11:17
  • \$\begingroup\$ Just to double-check, along with joining A and B together between your two devices you are also joining the two isolated grounds together? \$\endgroup\$ – Cursorkeys May 13 at 11:29
  • \$\begingroup\$ Other device is a high frequency drive (VFD) output which I connected to the A, B and GND_ISO. \$\endgroup\$ – PCBLearner182 May 13 at 11:49
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Isolation is not a cure for all noise problems, as Cursorkeys says the noise you see might very well be radiated.

I think you'll have to try and see what works for you.

This resource helped me when I had to deal with issues similar to yours: https://support.fccps.cz/download/adv/frr/RS485/RS485.html

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