Half Duplex to Full Duplex

Hello Experts,

I am an undergrade student using the arduino as my micro for my project. I am trying to update a PID routine using the APC220 RF modules. The problem i am encountering is that these RF’s are sending the data, and when i try to send the constants back using the arduino terminal is not responding. Do you think i need to implement a communcation protocol or is there a way a can go around this problem? Please find attached a very simple code i am using (which worked Once) to test the communication. Using the current code i am only recieving data, while any effords done to send numbers from the arduino terminal where useless.

char chara = 0;
float number = 0;
unsigned long PrevTime =0;
double delta_t = 25;
int CounterRecieve = 0;
int CounterSend = 0;

void setup()
{
  Serial3.begin(19200);
}


void loop()
{
  while ((millis() - PrevTime) < delta_t) {}
  PrevTime = millis();
  
  CounterRecieve++;
  CounterSend++;
  
  if(CounterSend >= 4)
  {
    //if(Serial3.available() <=0)
    CounterSend = 0;
    Serial3.println("Sending");
  }
  
  if(CounterRecieve >= 20)
  {  
    CounterRecieve = 0;    
    chara = Serial3.read();
        
    if(chara == 'p')
    {
      while (Serial3.available() <= 0);
      number = Serial3.parseFloat() ;
      
      Serial3.print("I received: ");
      Serial3.println(number);
      
    }
    
    else if(chara == 'i')
    {
      while (Serial3.available() <= 0);
      number = Serial3.parseFloat() ;
      
      Serial3.print("I received: ");
      Serial3.println(number);
      
    }
    
    else if(chara == 'd')
    {
      while (Serial3.available() <= 0);
      number = Serial3.parseFloat() ;
      
      Serial3.print("I received: ");
      Serial3.println(number);
      
    }
      
    else
    Serial3.println("No Valid Data");
  }
  
}

Where's the link to the hardware used?