Continuing the discussion from Arduino Nano refusing to communicate to UART devices without a computer:
On genuine Arduino Nano board UART works if the GND pin 29 is connected along with TX RX.
other GND pins does not work for some reason, looks like its to do with USB GND connection . Pin 2 and Pin 4 GND are connected to USB GND unlike PIN 29 GND
Nano does not have a pin 29.
I am not quite convinced about that. Pin 2 is RXD. On the official Nano schematic, USB ground is connected to all other grounds (pins 4 and 29).
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Ok after further test, UART on Arduino Nano worked by adding a 10K ohm pulldown resistor on RX.
After adding 10K ohm resistor across RX0 (GP1) to GND seems to have resolved, I am now able to receive data on UART
There must be something wrong with your board as you should NOT need any external resistors for proper UART communications.
Sorry on the Pin numbers, I meant the ground pins next to RST pin.
yes I agree with you, I should not need a pull down resistor on UART on the contrary it should be pull up which I do not understand. Some work without the resistor and some work with the resistor and these are brand new boards which I do not understand. It is like voltage level difference on each board.
You should NOT need ANY resistors. You are doing something else wrong somewhere else
Thanks for your input. In theory I should not need any resistor as you say but on some boards if I do not I get garbage data out from rx line. It is like I am receiving noise. When I add the resistor I can see readable data coming in.
RX is an input NOT an output.
sorry I meant is what I receive on Rx I tried to print them on serial monitor and they are garbage data.
The nano is 5V. Is your RX/TX data 5V TTL?
Thanks for your reply. Appreciated.
yes it is 5V TTL. Like I said before out of the 4 brand new genuine arduino boards, one works without any resistors out of the box but other 3 only works if I add a pull down on Rx pin value of 1K2 resistor which does not make sense. They seem to show garbage data.
So most of them do not work, I also tested on few dfRobot arduino nano board (their board layout is bit different than original) and they all work without any resistors as expected. So I am kind of lost what is the difference on builds.
Is it picking up noise?
It is not an issue to add a pull down resistor to make it work, but why should it need anyway and if I add the resistor I an unable to reprogram Nano on usb because of the pull down resistor which is understandable as it expects it to be high idle state I assume.
Then it certainly seems like some of the boards are damaged. I would return them for a refund.