Circuit for ground isolation for RX TX

Thread Starter

ammarqs153

Joined Feb 15, 2016
14
Hello,
I need assistance in designing a ground isolation circuit for RX TX that I am feeding to an arduino board.
I have a third party controller that is powered by 110 VAC and the Neutral is common with the ground on the RX/TX. I am using that Ground (neutral) with my RX/TX to connect that with the arduino but i know its not safe, and that is why I need a circuit that can provide ground isolation for my RX/TX that is going to the arduino.
FYI: my baud rate is 115200. I understand that i have to use a optocoupler, but how and where, thats the question.
I dont have any schematics because there arent any, its just interfacing.
Thanks.
 

Thread Starter

ammarqs153

Joined Feb 15, 2016
14
I know, but unfortunately that is the case. This is Schneider's Zelio PLC. the 120VAC version, the neutral is grounded to the board 0V. So the ATMEGA controller GND and everything else that is labelled GND is shorted with the Neutral.
 

djsfantasi

Joined Apr 11, 2010
9,156
so neutral is connected to ground. Ground here means 0V..
There are actually many types of ground. Earth ground, circuit ground, analog ground and digital ground are a few common ones.

It’s likely (but not definitely) that the ground on your Arduino is NOT connected to neutral. How are you powering your Arduino? How many volts, connected where on the Arduino and what is supplying the low voltage DC voltage required by the Arduino?

I suspect that the RX/TX ground is already isolated from any mains voltage.
 

Thread Starter

ammarqs153

Joined Feb 15, 2016
14
Assuming the Arduino has its own power supply, the best option is probably to use an opto-isolator in each data line.
Thats exactly what my question is. But i want to know how to choose the right opto and what circuitry goes with it? i understand that the rx/tx baud rate affects in choosing the right opto keeping in mind the switching time, but then again, something that i need help with.
 

Thread Starter

ammarqs153

Joined Feb 15, 2016
14
There are actually many types of ground. Earth ground, circuit ground, analog ground and digital ground are a few common ones.

It’s likely (but not definitely) that the ground on your Arduino is NOT connected to neutral. How are you powering your Arduino? How many volts, connected where on the Arduino and what is supplying the low voltage DC voltage required by the Arduino?

I suspect that the RX/TX ground is already isolated from any mains voltage.
The GND on the arduino is connected to the GND on the Zelio board, and that GND is connected to the Neutral.
My Arduino is powered externally with 5 VDC. I have to have common GND for both Arduino and Zelio so that they can talk over RX/TX.
 

Jon Chandler

Joined Jun 12, 2008
1,029
Here's a circuit I used for a USB interface to a ancient piece of FAA technology. I wanted to be certain a laptop connected to it wouldn't fry, but much more importantly, I wanted to absolutely certain that some glitch wouldn't trip the nav aid off-line or somehow damage it.

This circuit worked great at USB speeds, and completely electrically–isolated one side from the other. Note that the power supplies for each side are separate.

In your case, you probably won't want the CH340 UART-USB chip; just connect Tx and Rx appropriately. Add a MAX232 chip if needed at the non-Arduino end if it's using RS-232 levels. Screenshot_20211112-093947_Edge.jpg
 

Ian0

Joined Aug 7, 2020
9,671
I've isolated data at 250kbaud with the H11L1, which can be driven by HC/TTL logic and, with a pull-up resistor, will drive TTL/HC logic. 250k baud is about the top if its range, so it should do 115200 baud.

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