Cannot let motor go backwards

I'm programming to TREX motor controller. Somehow I cannot make the motor go backward. However, my code is pretty much the same as the sample code in the tutorial. Could someone help me out? Thanks a lot.

The code is like below, when the inputSpeed less than 0, it should set lmdirpin and rmdirpin to 0, and therefore go backwards, however, the code doesn't work and I cannot debug it out. And I'm sure the pin numbers are right. Can any one help me to figure it out. Thanks for the help.

int motorControl(int ifForward){
  // Increase or decrease speed as requested
  if(ifForward){
    inputSpeed += 10;
  }
  else{
    inputSpeed -= 10;
  }
 
  // Be sure speed stays within range
  if(inputSpeed > 255){
    inputSpeed = 255;
  }
  else if(inputSpeed < -255){
    inputSpeed = -255;
  }
  digitalWrite(lmdirpin,inputSpeed>0);                          // Set left motor direction, doesn't work       
  digitalWrite(rmdirpin,inputSpeed>0);                         // Set right motor direction, doesnt' work
  analogWrite (lmpwmpin,abs(inputSpeed));               // set left PWM to absolute value of left speed - if brake is engaged then PWM
  analogWrite (rmpwmpin,abs(inputSpeed));               // set right PWM to absolute value of right speed - if brake is engaged then
  delayMicroseconds(50);                                            // limit full power to 50uS
}