Controlling these from a serial port is definitely going to cause you to have a burned up processor and/or port. They aren't meant to do PWM nor can they supply enough current for a motor.
Use an Arduino to handle the PWM as they have built in libraries and tons of examples along with motor driver shields that are meant to do motor control. You can use the serial (pins 0 & 1 on arduino) along with a serial logic level converter to step it to ~12v then you can use a Com port along with an SIO and your own protocol to go between them.
There are tons of online resources on how to do this. Including on Arduino's web site and within their software.
