ESP8266 Community Forum

I tested NodeMCU using on board USBTTL converter through AT commands in Hyper Terminal. It works well. I want to interface it to Microocntroller. To test communication over TXD0 and RXD0, I connected external USBTTL converter and run Hyper Terminal in Laptop. When, Power is turned on or Rest button is pressed, the "Ready" is sent over TXD to Laptop and displayed in Hyper Terminal. But, when, AT is typed in Hyper Terminal and Enter Key is pressed, there is no response from NodeMCU. Does, on board USBTTL interfere the communication with external USBTTL ? How, to resolve this issue ?

Hello, you may find the following links useful. You can crosscheck if you're missing any point:
https://www.instructables.com/Get-Start ... s-NodeMCU/
https://arduino.stackexchange.com/quest ... mmand-mode

Hello, I tested the UART waveform output from USB TTL Converter with/without connecting to Rx PIN of NodeMCU DSO. When, not connected, the UART pattern appears very well. When, connected, the signla alwyas stays at Logic High (3 V). It means that the NodeMCU is forcing High logic level and external USB TTL is not capable to drive it low due to strong High Pull by USB TTL provided on board of NodeMCU. Same behaviour was observed when, instead of external USB TTL converter, I sent the AT command from a 3 V microcontroler. Then, I added the circuit (see attached image) between USBTTL or 3V Microcontroller and NodeMCU V3.WIth this modification, the NodeMCU responds the AT Commands. So, we need either to add buffer or devise method to ensure weak pull up by onboard USB TTL converter.

Good to know that it is working now. Thanks for sharing with us.