MateDealer | MateDealer , a open source MDB cashless device implementation | Mobile library

I am trying to communicate with a Vending machine using MDB Protocol with the help of Arduino Mega and I am partially successful in doing so.

The circuit that I am using is attached here

While the circuit diagram is shown below

The Arduino Mega is communicating with laptop via Serial 1 and the vending machine communication is done via Serial 2 of Arduino Mega.
USART is being use for communication.

The strange issue that I am facing is, the communication with the vending machine starts sending data only when brown colored wires A and B are initially not connected(which are connecting Arduino TX to Vending Machine RX). Once I see the data coming from Vending Machine, I short wires A and B and Arduino Starts sending data to the vending machine too.
But if Wires A and B are connected on the start of a vending machine, the vending machine doesn't send any data to Arduino and hence no communication takes place. I want to make it work without manually connecting wire A and B every time.

I have tried replacing Optocoupler PC817C with

• PC814
• 4N35
• 6N137 High Speed Optocoupler

I tried replacing Hex Inverter with

• 74HC04AP

I tried putting a mechanical and then solid-state relay between wires A and B and tried connecting them via external input but nothing happens
I tired adding an optocoupler as a switch between wires A and B and tried controlling it via firmware.

I even tried replacing the hex inverter with hex buffer HCF4050BE and a NOT Gate but regardless of any combination of mentioned components the issue remains the same, it only works if Wires A and B are initially not connected and then shorted once data starts coming from the vending machine.

I am using MateDealer's code.
I want to automate this manual process and need a simple solution for that.