Negative voltage on TX

I was wondering if anyone knows what is happening in my circuit (and how to prevent it), I believe I’ve managed to burn one of my Serial pin due to this.

Check the attached picture. Hopefully it’s self explanatory.

While the Electron is starting up, there is a delay where I do certain things before performing digitalWrite(A4, HIGH);
before that code is ran, it seems like TX’s voltage is in a negative voltage (I’ve measure both -11v and -9v) which might be doing something strange with the device connected in the serial pins and that device might be sending something to RX in the wrong voltage as well. I think that might have burned my serial ports. Once A4 is set to high the voltage is on the positive side (around 2.3v).

Any one has any idea on why is this happening and how to prevent it? Should I be putting a pull down resistor somewhere? would that create a voltage divider?

Thanks!

PS: Voltage regulator is L7805CV

Do you have anything external connected to that pin? It sounds like you have a serial device connected externally and wired to the wrong pin. True serial RS232 is ±3 to 15 volts where as the Tx pin is 0 to 3.3 volts.

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@Iv4n, schematics would be good.