Pro Micro as a USB UART TTL / LPG cable

So hello there guys. As the title suggests, I’m trying to make an LPG cable and I was planning on using a Pro Micro as a USB UART. Now I do realize that the AtMega32U4 does not convert RX and TX natively so I tried using the following code:

void setup() {
  while (! Serial);

void loop() {
  if (Serial.available()) {
  if (Serial1.available()) {

And that did not work. Of course I should mention that the code worked properly to my knowledge but the software for the LPG did not detect my cable. I tried different baud rates to no avail.

And this is the adapter I put together for the car side.

I have not used the 12V pin of the LPG for obvious reasons but as you’ll notice, my LPG system seems to use one pin for RX/TX so I was forced to just use one cable and split it among the RX and TX of the Pro Micro I also connected GND and tested all leads with a multimeter.

Now I’m wondering whether the RX/TX split is an issue here or not. To my knowledge, my board is 3.3V if that helps.

I wired it with a split for RX/TX since I’ve found guides for my LPG that use a regular USB UART but for 5V it’s recommended to use a single 180 Ohm resistor for each serial line.

Oh man, well i think you should for sure put a current limiting resistor in the data line coming from the LPG system, and probably a voltage divider, problem is that because of the joined RX-TX lines the voltage divider is not going to work as you want. The LPG data system probably runs on 5v, but you will have to make sure of that. What is the kind of data you are expecting to receive and what do you need to send ?
The biggest issue is that if your LPG unit sends at the same time as the Arduino… well that is the main concern.

Well, I did check the data line and it gave on average 1.5 volts. I will check with the resistor I guess. But later since shops won’t be open for a few days. Holidays are coming over here.

I’m expecting it to basically act as a COM serial cable, as a bridge between my car and laptop. The software I’ll use is from Fobos Gas.

Here’s what the suggested wiring is for a standard USB UART:


Notice: the 180 Ohm resistors were recommended only if the UART is of 5V instead of 3.3V.

Well in that case it should be ok, you should see at least the echo back on the TX line

I just downloaded an older version of my LPG software from 2018. And my contraption is showing up in settings as a COM option which could be very good news. I even see the board flashing whenever I query for a connection.

I’ll try again tomorrow with the older software since it’s late and if it works, I’ll feel like a god.
I’ll keep you posted.

Welp. Didn’t work, I’m stuck on trying to connect. Board is detected in software and it is receiving some sort of info but I’m getting no connection.

Maybe I’ll try a Leonardo or dedicated UART TTL drive. The 180 Ohm resistors are still on the table.

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