[Solved] Problems with Custom Protocol over Half Duplex Asynchronous

J Quest Asks: Problems with Custom Protocol over Half Duplex Asynchronous
I’m facing some issues regarding Writing and Receiving data from another device. Unfortunately there is not a lot of documentation available about the device I’m trying to communicate with. All the documentation I received is the following:

Code:
Wiring
- Ground
- Single wire async communication

Byte Format
- Asynchronous 
- Half duplex
- 38.4kbps +/- 1.75%
- 1 start bit
- 8 data bits
- lsb first
- 1 parity bit (even)
- 2 stops bits

Checksum
- The last byte of the packet contains the packet checksum, which is calculates by summing the previous bytes of the packet and then 1's complementing the answer.

Startup Packet
- 1 Byte for Header (HEX 53, ASCII S)
- 1 Byte for Checksum
- Total 2 Bytes send @38.4kbps (0.624ms)

- The Startup response should me executed every 10ms until there is a valid response

Startup Response
- 1 Byte for Header (HEX 73, ASCII s)
- 1 Byte for settings (0-7 bits)
- 1 Byte for settings (0-7 bits)
- 1 Byte for Checksum
- Total 4 Bytes send @38.4kbps (1.248ms)

Based on the information above, I’ve setup my Arduino MEGA 2560 with the schematic and code below. Every 10ms I send the Startup Package, but I don’t receive a single response. I also tried multiple Checksum Calculations but none of them are working. I expect the is probably an issue with Checksum, Baudrate or Wiring. But not sure where too look anymore.

My current Test Code:

Code:
int incomingBytes[2];
int index = 0;
//byte dataArraySendStartup[] = {0x53, 0x53};
unsigned long newmicros, oldmicros;
const long DELAY = 9996;   // 10 ms;


void setup() {
  Serial.begin(500000);
  Serial3.begin(38400, SERIAL_8E2);
  oldmicros = micros();
}



void loop() {

  //Send serial Bytes in code block below
  newmicros = micros();
  if (newmicros - oldmicros >= DELAY) {
    if (newmicros - oldmicros != DELAY){
        oldmicrosByte = micros();
        Serial3.write(0x53);
        Serial3.write(0x3A);
        newmicrosByte = micros();
        Serial.println(newmicros - oldmicros);

        oldmicros = newmicros;
    }
   
  }


  //Write serial Bytes to Serial Monitor
  if(Serial3.available()>0){
    incomingBytes[index] = Serial3.read();    
    Serial.println(incomingBytes[index], HEX);
    index++;
  }

  //Reset the incoming bytes
  if(index == 2) {
    index = 0;  // reset index so we don't overwrite next time
  }  
}

My current Schematic:
enter image description here

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