Change sign

Assuming "MyValue" stores the value to be written...

if ( MyValue >= 0 )
{
  // Make the motor go forward here
  analogWrite( pwmPin, MyValue );
}
else
{
  // Make the motor go backware here
  analogWrite( pwmPin, [glow]-[/glow]MyValue );
}

@AlphaBeta: 15 seconds apart. I'd say that's close enough for ... JINX!