title: Post Streamlined: Need PLC Project Advice: Arcade Button Pre-Encoder, Remapping+Turbo+Slow Functions
body: <p>I'm working on an arcade cabinet that will be able to play various video game consoles (real hardware, not emulated.)  There will be a PC inside to run a selection menu.  I'll have to write that myself. I'll also need program a PLC which will do various things like control the relays which switch audio/video/controls between the PC and the various consoles, etc. I'll need help with those two tasks in time, but they are not what I'm working on right now.</p>
  
  <p>What I'm working on as a starting point has to do with the controller encoding. Basically, the controls for each player consist of a few buttons and a joystick. These use momentary, normally-open contact switches, one for each button, and one for each cardinal direction on the joystick. Pressing the button, or joystick direction, closes the switch. The state of the buttons is then communicated to the console by an encoder. </p>
  
  <p>The encoder has a connection for each button and joystick direction which is connected to 5 volts ("high") through a pull-up resistor. When a button or direction is pressed, a connection to ground is made through the momentary switch. When the encoder reads ground ("low") on a button connection, it knows that a button has been pressed and it communicates this to the console.</p>
  
  <p>I already have all this working with the various consoles, but I've thought of some features that would be nice to add. This is where my current task comes in.</p>
  
  <p>The first feature is button remapping. Some of these games were designed with controllers in mind, so when you use them with an arcade control panel, some of the buttons may not be where you want them. Some games allow buttons to be remapped via software, but others do not. My idea is to add a PLC in between the joystick and buttons and the encoder. I'll call this PLC a "pre-encoder."</p>
  
  <p>The pre-encoder would read the states of the buttons on some input pins, then write these states back to some output pins, relaying them to the encoder. The advantage is that its programming could associate any input pin with any output pin, effectively remapping the buttons. Whenever a console is selected via the computer's menu, a button-mapping profile associated with a particular game could be selected as well, and forwarded to the pre-encoder.</p>
  
  <p>Of course, the pre-encoder's routine which reads the buttons and relays their states to the encoder must repeat very quickly for smooth control. These games will be running at about 50 to 60Hz, meaning a new a video frame every 16.67ms or less. Ideally, the pre-encoder will be able to repeat this routine many, many times per frame to ensure the absolute minimum input lag. I want to ensure that the code and hardware selection is optimized to run as fast as possible.</p>
  
  <p>The second feature is turbo buttons. Some games, especially arcade games, require a fire button to be pressed repeatedly every time you want to fire your gun, or your ship's cannons, etc, even if you have unlimited ammo. This seems unnecessary, and it will tire your fingers out pretty quickly. A turbo button is one that can be held down continuously, yet the game is being told that you are rapidly pressing and releasing it. This could be done in software for anything running on the PC, or with an analog solution like a 555 timer, but the best method is to synchronize the turbo button timing with the video refresh rate. By feeding the vertical sync pulse from the PC or video game console's video output to a PLC, it will know exactly how often a frame of video is rendered. Turbo button timing can then be controlled by defining, in numbers of frames, the periods when the button should be pressed and released. Timing information could also be included with the game-specific button profiles.</p>
  
  <p>The third feature is slow buttons. Actually, this would probably only be applied to the joystick, but I'm referring to the switches for its cardinal directions as buttons. In certain games (it will probably only be used in shmups) it is sometimes needed to move your character (ship/plane) through very tight spaces. If movement is too fast in response to even minimal joystick input, you may go too far and crash. The idea is that, while a slow activation button is held, the joystick will be made less responsive by rapidly activating and deactivating it in the same manner as the turbo buttons.</p>
  
  <p>I'm not sure if I want the pre-encoder itself to be watching the vertical sync pulse or if it will slow it down too much. My current thinking is that a seperate PLC will be responsible for general management of the cab itself; watching the "on" button, switching relays, communicating directly with the PC, watching the vertical sync pulse, etc. This will free up the pre-encoder to run more quickly.</p>
  
  <p>Here is some example "code" for the pre-encoder. Obviously, it's just a rough outline of what I have in mind, as I don't even know what language it will be. This example assumes that a dedicated PLC will be used just as the pre-encoder. A separate PLC will be responsible for watching the vertical sync pulse, in addition to other tasks, like getting a game profile from the computer and passing some of that info to the pre-encoder. That PLC will know what the frame timing should be for turbo and slow functions, it will count frames, and during frames when turbo buttons should be disabled, it outputs high to a pin on the pre-encoder PCB, letting it know to disable turbo buttons. During frames when it should be enabled, it outputs low to that pin. Same idea with the slow buttons. There is also a pin which the pre-encoder checks at the end of its routine, so it can be told to stop and await a different game profile.</p>
  
  <pre><code>get info from other PLC (which got it from the computer, from a user-selected game profile):
  array containing list of turbo buttons (buttons are identified by what input pin they are connected to)
  array containing list of slow buttons (will probably only be the joystick directions, if any)
  array containing list of slow activation buttons (should normally be only one button, if any)
  array containing list of normal buttons (not turbo or slow)
  array containing which output pin to use for each button (this determines remapping)
  
  Begin Loop
  
    if turbo pin is high
  
       for each turbo button
          output pin = high
       next
  
    else
  
       for each turbo button
          output pin = input pin
       next
  
    end if
  
    if slow pin is high and slow activation button is pressed
  
       for each slow button
          output pin = high
       next
  
    else
  
       for each slow button
          output pin = input pin
       next
  
    end if
  
    for each normal button
       output pin = input pin
    next
  
  Restart Loop unless stop pin is low
  </code></pre>
  
  <p>If you've read all this, thank you for your time. So (finally), here are my questions:</p>
  
  <ol>
  <li><p>What are your overall thoughts; on my idea in general, feasibility, etc.?</p></li>
  <li><p>What kind of PLC should I use for the pre-encoder? I was originally thinking of trying an Arduino, but my reading indicates that it will be much too slow, due to its use of high-level programming libraries. I don't have a problem building my own board around another PLC.</p></li>
  <li><p>What language should I use to program the PLC? I don't mind learning a new language. There's no time limit on this project, and I'll put it in whatever it takes to get the pre-encoder running as fast as possible.</p></li>
  <li><p>What will I need to flash my program onto the PLC?</p></li>
  <li><p>At run-time, how should these PLC's communicate with each other, and with the PC?</p></li>
  <li><p>Am I asking in the right place; right forum, right section, etc.? Anywhere else I should ask?</p></li>
  </ol>
  
  <p>Awaiting your response eagerly,</p>
  
  <p>-Rob</p>