The noisy device has one of those RFID 13.56MHz modules with onboard etched antennas. I put a lot of caps to keep the power as clean as possible, but I never realized the ground plane was so noisy until I tried to send serial data to another device. By only connecting the two ground planes it drives the second device insane.

There could be other things to cause it other than a badly design ground plane. Its sounds like the typical problem when you use a non isolated power supply. Which one way to get rid of it is a .1uf capacitor with a 100 ohm resistor in series. But without inspecting the circuit, if you are trying to use a RS232 driver with a single supply it does this. They require +5 and -5 supplies.

 

Exactly what serial interface are you using?
There are bi-directional isolated couplers that may work for you, such as this for the I²C protocol..

The standard cable that I'm using has only 2 wires. I would need 4 wires to use this component, 2 for the serial and two for the power.

 

There could be other things to cause it other than a badly design ground plane. Its sounds like the typical problem when you use a non isolated power supply. Which one way to get rid of it is a .1uf capacitor with a 100 ohm resistor in series. But without inspecting the circuit, if you are trying to use a RS232 driver with a single supply it does this. They require +5 and -5 supplies.

The power supply comes from the USB port in the PC. The noisy device gets the power from the USB cable. I have added many caps to the design.

 

Of course it didn't work -- you were trying to send an electrical signal on a single wire with no reference.

View attachment 320889
Here's a crude sketch of how to do it.

The one on the left is the one you can change, the one of the right is the one you can't. You bring over your two signal lines (TX and RX, assuming your communications are full duplex) and the ground from the right side.

You connect these to the appropriate side of the optocouplers, using an isolated ground plane if needed.

If your comms are half duplex, then that makes things a bit trickier, but there are optocouplers specifically designed for this.

What protocol are you using and what is the baud rate?

Thanks. That's closer to what I need to do except that the cable I'm using has 2 wires only. I'm using standard asynchonous serial data with one wire being the ground plane and the other one being the ones and zeroes. The baud rate is 9600.

 

I just started a new thread as a continuation to this one. Now, I'm trying to reduce the ground noise levels. Thank you for your help.

 

The power supply comes from the USB port in the PC. The noisy device gets the power from the USB cable. I have added many caps to the design.

That is going to marginally work for you.
The problem derives from using USB power which some of the time, the voltage regulator in the usb UART chip is going to filter out. In those cases where there is excessive noise, the solution people use is an isolated USB hub. Because there is too much noise on the ground of the computer And that ground will ground loop across devices.

 

That is going to marginally work for you.
The problem derives from using USB power which some of the time, the voltage regulator in the usb UART chip is going to filter out. In those cases where there is excessive noise, the solution people use is an isolated USB hub. Because there is too much noise on the ground of the computer And that ground will ground loop across devices.

Thanks for your reply. Using a USB hub is not possible as this unit gets connected to the PC USB port directly. I cleaned up, filtered out, the power the best I could. Please, take a look a this thread.
https://forum.allaboutcircuits.com/threads/reducing-ground-noise.200658/