Cannot exchange data between nodemcu and arduino mega

[morcillo]

Hello, I'm trying to exchange data via UART between an arduino mega and a nodemcu (programming both using arduino IDE).

I lowered the voltage from the arduino to 3V3 and I'm able to receive data in arduino mega from nodemcu. The problem is that I can't receive data in nodemcu.

When they are disconnected I can clearly see the arduino mega sending my data, but as soon as I connect to my RX pin in nodemcu, the signal gets pulled down to 0V and there is no more data going through.

Does anyone have any idea as to why this happens?

I tried using Seria.swap and I can still see the signal clearly going from nodemcu to arduino mega but the same problem happens with the Rx pin.

[pratik]

What firmware is your NodeMCU running?
You must enable the UART0 Tx and Rx pin functionality on the ESP8266 if you are using your own ESP8266 program. In this case it seems like your NodeMCU Rx pin is configured as a GPIO and is putting out a low state. That may be why the UART signal is possibly being pulled low.

Also, make sure that your UART signal voltages are 3V3 compatible, and not 5V. The ESP8266 is supposed to run at 3V3 (including IOs). It can handle 5V in open drain mode but anyway, better to err on the safe side.