How to connect Arduino to Raspberry through USB-TTL (called like this in electronics shops)?

Question

I want to connect an Arduino to Raspberry Pi through a USB-TTL converter like this one:

My doubt would be about logic voltage. I know that Arduino works with 5V logic, while Raspberry Pi does with 3.3V and is non-tolerant. So, how should the wiring be done?

I'm not intending to connect any weird stuff to the Raspberry. To the Raspberry Pi is only going to go the usb-end of the adapter.

My adapter does not have a serial number to identify it, but its main chip does, it is this:

https://www.sparkfun.com/datasheets/IC/cp2102.pdf

I just want to avoid damaging the Pi through USB. I don't know if supplying 5V through it might damage it, though.

Basically just want to know if should be this:

Or this:

References I used for documenting:

• https://deepbluembedded.com/usb-ttl-converter-tutorial-pc-control-for-microcontrollers/
• https://www.instructables.com/How-to-Access-Headless-Raspberry-Pi-Using-USB-to-T/

Answer 1

First that device is NOT TTL (it would be CMOS).

No one can tell what it does from a picture. You need to check the specifications. Many (most?) such devices actually output 3.3V logic.

IMPORTANT DO NOT connect ANY power pins 3.3 or 5V to the Pi!

Answer 2

Connect like this:

Arduino - USB Serial
Txd     - Rxd
Rxd     - Txd
GND     - GND

Do NOT connect any power pins.

Answer 3

Well, after getting better documented and performing tests, I got the right wiring for avoid damaging the Raspberry Pi and getting the desired serial communication as expected.

USB standard gives 5V max output (considering the possible threshold) which by design would not damage the Pi. Which is not the case for GPIOs, where if you would like to do something similar, you should use a level shifter since those are not 5V tolerant.

Here is the right wiring: