Using a seperate thread to control motors

Question

Hi am new to Java and am struggling with a probelm.
The application is to control 4 motors connected to the parallel port. When the user presses a button the motor is set to a certain speed (byte output to port). Once the button is released the port outputs to stop the motors, i also have it working so multiple motors can be driven at the same time (multiple buttons held down) i have the code working ok, but at present the program just outputs a byte of data when its worked out what buttons are pressed/released.
The problem is i want to drive one of the motors using a crude form of PWM (output an 'on' byte, wait 1 second, output an 'off' byte, wait one second, ect)
This is clearly going to have to be performed in a loop that is basically an infinite loop untill the key is released. However the program also need to be listening for other key inputs to turn on other motors.
I think the best way to do it is to create a thread for the PWM that sleeps, allowing the program to do other things, and breaks out when it recieves an interrupt (triggered by another key press), however i am have major difficultly converting the code. at present the code looks like this:
private void DebugKeyReleased(java.awt.event.KeyEvent evt) {                                 
int c;
c = evt.getKeyCode();
switch (c)
case 83:     Control(100);
               break;
     case 68:     Control(80);
               break;
     case 65:     Control(40);
               break;
     case 87:     Control(20);
               break;
private void DebugKeyPressed(java.awt.event.KeyEvent evt) {                                
int c;
c = evt.getKeyCode();
switch (c)
case 83:     Control(1);
               break;
case 68:     Control(8);
               break;
case 65:     Control(4);
               break;
case 87:     Control(2);
               break;
private void Control(int direction){
switch (direction)
     case 1:      forwards = 1;
               break;
     case 100:     forwards = 0;
break;
     case 2: backwards = 1;
               break;
     case 20: backwards = 0;
break;
     case 4:          right = 1;
               break;
     case 40: right = 0;
break;
     case 8:          left = 1;
               break;
     case 80:     left = 0;
break;
     global_direction = forwards*1+(backwards*2)+(left*8)+(right*4);
if(global_direction == 0)
moving = "Stopped";
short stop = 255;
lpt.output(Addr,stop);
     else if(global_direction == 1)
moving = "Backwards";
short Back = 239;
lpt.output(Addr,Back);
ECT, there are alot more if statements that account for all possibilities.
i want the backwards and forwards cases to enter loops that output data on and off for certain time periods.
can anybody suggest the best way to convert the code?
thanks for any info / help
cheers John

Answer 1

A finite state machine is an abstraction in which you have a finite collection of states, and a set of transitions between states. One state is active at a time. So a traffic light might have red, green, and yellow states, and transitions between green to yellow, yellow to red, and red to green. There can be multiple transitions out of a single state.
So for your app, each state might be a kind of signal or a kind of sequences of signals set to the motors, and the transitions would be taken when you need to change the signals. You could send the key events and time events to whatever state currently is active. That active state would then decide what transition to take based on the event.
I'm sure there will be an entry in Wikipedia about it.
If the code works, that's great, but (1) it's obviously not working entirely because you're posting here, and (2) code like this can get hairy very fast and thus hard to maintain and thus bug-prone. So if you can simplify it, it will only help.
If you want to use a thread, you can. You basically have that now. But instead of Thread.interruped as you have it now, you could just have the key press event change the value of the flag to one that will stop the loop.
(for example, you'd have a "running" boolean field, a "while(running) {" in your run() method, an the event can change the value of "running" from true to false to cause the thread to stop.