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I've found that when I use Arduino Nano connected with USB everything is working great, and when I tryed to use it with USB to TTL connector I've understood that I can only read data. I tryed to connect GND to RST as it was on some picture in Internet, and it works! Now I can send data through my USB to TTL (and through another UART device). So I can't uderstand, why should I do it? Why can't I just send data through Rx Arduino pin? Why do I need an extra action? Is it related with Arduino microcontroller?

Даниил Поляков
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The key here is to remember the simple rules:

  1. One device can send to many devices
  2. One device can receive only from one device

In your situation you have three devices that are communicating:

  • The USB to UART chip
  • The on-board ATMega, and
  • The off-board ATMega connected to the TX/RX pins.

All three of those are connected together through the TX and RX pins. This means that:

  • The UART chip sends to the on-board ATMega, and
  • The UART chip sends to the off-board ATMega

This fits with rule 1 above: one device can talk to many devices.

But:

  • The UART chip receives from the on-board ATMega, and
  • The UART chip receives from the off-board ATMega

This breaks rule 2 above. You can't have two devices sending to one receiver.

By connecting the RESET pin to GND you are effectively turning off the on-board ATMega. It's like you removed it from the board. It's not allowed to do anything - including communicating with the UART chip.

So that fixes rule 2 above, because now you only have one device sending to the UART chip, not two.

Majenko
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  • Thanks! That helped me to understand more! By the way, is it the best way I can itteract with another controller or there are another ones less difficult (Software Serial?) ? – Даниил Поляков Nov 02 '19 at 16:59
  • It depends what and how you want to communicate. I2C is a popular choice for sending data from one MCU to another. – Majenko Nov 02 '19 at 17:11