Getting garbage after connecting TX to board

Hi guys,
Basicllly I have RTL8196D based board with ttl header on board, Im using arduino uno as usb to serial converter and RX working fine, Im receiving "log" messages from board but as soon as I connect TX to board Im getting garbage values on arduino serial monitor, board rate is 38400. So is it up to arduino do I need usb to serial and how could I further troubleshoot this

TTL at 5 V, or is it serial at 3.3 V, 1.3 V, or ??

I'd also check that there are proper pull-up/down resistors on the serial port.

I just mesured it and TX is at 3V, and RX at -3.3v. How much ohm resistor should I use on tx and rx pull-up/down ?

Huh?? Negative voltage seems very strange. Reversed leads on your meter??

The value of the resistor(s) would be dependent on what is already in the circuit both on device as well as on your adapter.

Yes when I reversed leads and it mesure 3.3v, I think this should be fine without pullup/down resistors since its ttl at 3.3v ?

I also tried arduino sketch SerialPassthrough to avoid connecting reset to gnd but its same deal, as soon as I conect TX to arduino garbage and questions marks are shown on serial monitor, if anyone need it here is sketch

  created 23 May 2016
  by Erik Nyquist

void setup() {


void loop() {
  if (Serial.available()) {      // If anything comes in Serial (USB),
    Serial.write(;   // read it and send it out Serial1 (pins 0 & 1)

  if (Serial.available()) {     // If anything comes in Serial1 (pins 0 & 1)
    Serial.write(;   // read it and send it out Serial (USB)

Silly question but are you sure about 38400? 115200 is pretty typical these days... Docs?

Issue may be Linux is set differently than bootloader...

Yes it works with 115200 as well as 38400 I tested on both speeds same behaviour

You could try connecting ttl-usb to PC instead of arduino for further troubleshooring.