Control DFRobot Devastator Tank via a Romeo BLE (Uno equivalent) and the Arduino IDE Serial Monitor.

SerialControl.ino

/*
 * SerialControl:   Control a DFRobot Devastator tank via a Romeo BLE
 *                  (Uno equivalent) and the Arduino IDE Serial Monitor.
 *                  Based on the DFRobot Devastator Tank Mobile Platform sample
 *                  code written by Phoebe.
 * 
 *            
 * Version 0.1, August, 2018
 * Copyright 2018 David Such
 * 
 */

#define NO_DATA -1

const byte M1_DIRECTION = 4;    // M1 Direction Control
const byte M1_SPEED = 5;        // M1 Speed Control
const byte M2_SPEED = 6;        // M2 Speed Control
const byte M2_DIRECTION = 7;    // M2 Direction Control

// Motor Control
//////////////////////////////////////////////////////////////////////////////////////////////////////////
// Functions

void stop()                    
{
  digitalWrite(M1_SPEED, LOW); 
  digitalWrite(M1_DIRECTION, LOW);    
  digitalWrite(M2_SPEED, LOW);   
  digitalWrite(M2_DIRECTION, LOW);    
}   

void forward(int m1DutyCycle, int m2DutyCycle)          
{
  analogWrite (M1_SPEED, m1DutyCycle);   
  digitalWrite(M1_DIRECTION, HIGH);    
  analogWrite (M2_SPEED, m2DutyCycle);    
  digitalWrite(M2_DIRECTION, HIGH);
}  

void back (int m1DutyCycle, int m2DutyCycle)          
{
  analogWrite (M1_SPEED, m1DutyCycle);
  digitalWrite(M1_DIRECTION, LOW);   
  analogWrite (M2_SPEED, m2DutyCycle);    
  digitalWrite(M2_DIRECTION, LOW);
}

void left (int m1DutyCycle, int m2DutyCycle)            
{
  analogWrite (M1_SPEED, m1DutyCycle);
  digitalWrite(M1_DIRECTION, LOW);    
  analogWrite (M2_SPEED, m2DutyCycle);    
  digitalWrite(M2_DIRECTION, HIGH);
}
void right (int m1DutyCycle, int m2DutyCycle)            
{
  analogWrite (M1_SPEED, m1DutyCycle);
  digitalWrite(M1_DIRECTION, HIGH);    
  analogWrite (M2_SPEED, m2DutyCycle);    
  digitalWrite(M2_DIRECTION, LOW);
}

//////////////////////////////////////////////////////////////////////////////////////////////////////////

void setup(void) 
{ 
  // Serial Monitor
  Serial.begin(115200);
  Serial.println("Romeo BLE Device");
  Serial.println("Initializing...");
  Serial.println("Serial connected - awaiting command (w, a, s, d, t, x): ");

  // PIN Definitions
  pinMode(M1_DIRECTION, OUTPUT); 
  pinMode(M1_SPEED, OUTPUT);
  pinMode(M2_DIRECTION, OUTPUT); 
  pinMode(M2_SPEED, OUTPUT);
     
  //  Initial State - stopped
  digitalWrite(M1_SPEED, LOW);   
  digitalWrite(M2_SPEED, LOW); 
} 
 
void loop(void) 
{
  if (Serial.available()) {
    // Get the number of bytes (characters) available for reading from the serial port. 
    // This is data that's already arrived and stored in the serial receive buffer (which holds 64 bytes).
    char incomingByte = Serial.read();

    if (incomingByte != NO_DATA)
    {
      switch(incomingByte)
      {
      case 'w':
        Serial.println("Forward Maximum Speed.");
        forward(255, 255);
        break;
      case 's':
        Serial.println("Backward Maximum Speed.");
        back(255, 255);  
        break;
      case 'a':
        Serial.println("Turning Left.");
        left(100, 100);        
        break;       
      case 'd':
        Serial.println("Turning Right.");
        right(100, 100);
        break;
      case 't':
        Serial.println("Hello");
        break;
      case 'x':
        Serial.println("Stopped.");
        stop();
        break;
      default:
        Serial.println("Unknown Command...");
        Serial.println("Valid Commands are: w = forward, a = left, s = back, d = right, t = test message, x = stop.");
        break;
      }
    }
    else stop();  
  }
}