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An autofire coil in MPF is used for “instant response” type devices (like pop bumpers and slingshots) where you want a switch activation to trigger a coil as close to instantaneous as possible.
The Mission Pinball Framework is based on Python. Running a “real” pinball machine means you have some kind of computer-like board running Python (Mini ITX x86 computer, Raspberry Pi 3, etc.) which runs your game, controls the display, and plays your sounds. That computer connects to your hardware controller (P-ROC, FAST, etc.) to interface with your actual pinball machine components (switches, coils, lights, motors, LEDs…).
There are several types of devices in a pinball machine that you want to react “instantly.” For example, when a switch in a slingshot or pop bumper is activated, you want the coil to fire as fast as possible. When the player pushes a flipper button, you want that flipper to fire instantly, and when the player releases the flipper button, you want the machine to cut power to that flipper coil instantly. Unfortunately if you think about what the flow chart of activity looks like for that to happen, there are a lot of steps. (And it’s certainly not instant.) For example, imagine what happens when a ball hits a slingshot:
- The slingshot switch is activated.
- The hardware controller debounces that switch.
- The hardware controller sends a notification that the slingshot switch changed state to your Python game code via USB.
- Something in your code says, “if the slingshot switch is activated, fire the slingshot coil.”
- The Python game code sends the “fire the slingshot coil” command to the hardware controller via USB.
- That command is queued on the USB bus and transmitted.
- The hardware controller fires the slingshot coil.
Wow! That’s a lot of steps just to fire a coil when a switch is hit! Unfortunately the entire process of all this going from the hardware to the computer to the game code to the hardware to the coil takes some time—-maybe 10ms or so. But with a fast moving pinball you might find that it’s not fast enough. (What if your game code was in the middle of updating a bunch of lights and that delayed it another 5ms?) You might find that by the time your game code gets around to firing the coil it’s too late. In effect your slingshot firing has lag and might miss the ball altogether. Not good!
Fortunately the people who designed the hardware controllers know this, so they have options where “autofire” or “trigger” rules can be written into the hardware controller which the hardware controller can handle on its own. In the Mission Pinball Framework, we call these types of rules “Autofire” rules, because we specify that a coil fires automatically based on some switch event without any involvement of our host computer or the Python game code.
To use an autofire rule, you specify the name of a switch, the state of the switch (whether it goes active or inactive), the name of a coil or driver, and what you want that coil to do. (Turn on, turn off, pulse for a certain number of milliseconds, receive a pwm pulse pattern, etc.)
So for example, if you want to configure a slingshot, you might use a rule on your hardware controller which says, “when switch left_slingshot goes active, fire coil left_slingshot_coil for 30ms.” Or you might have a rule which says, “When switch right_flipper becomes inactive, cut power to the coil called right_flipper_hold.
You can set any combination of rules you want onto a hardware controller. In fact, MPF will use several individual rules on the same set of switches and coils to do what might seem like simple things. For example, think about what rules you’d need for a dual-wound (power and hold windings) flipper coil:
- When the flipper button becomes active, enable the power coil.
- When the flipper button becomes active, enable the hold coil.
- When the EOS switch becomes active, disable the power coil.
- When the flipper button becomes inactive, disable the hold coil.
- When the flipper button becomes inactive, disable the power coil. (We need this one to “cancel” the flip action if the player releases the flipper button before the flipper hits the EOS switch at the top of its stroke.)
- If the flipper button is active and the EOS switch becomes inactive, enable the power coil. (This causes the flipper to go back to the “up” position if for some reason it comes down when the player is holding the flipper button.)
Now look at that above list. That’s six rules just for one flipper! If you have four flippers in your game, you’ll have 24 autofire rules just to get your flippers set up!
Fortunately MPF makes this easy and hides the complexity from you. :)
The hardware controllers in your pinball machine have no concept of what your game code is doing at any given time. (Actually they don’t even know what a “game” is, or really what a “pinball machine” is.) They just know that they have rules programmed into them, and those rules specify what instantaneous actions they should take based on certain switches changing state. So your game code can overwrite rules at any time (and as often as you want) to overwrite existing rules with new actions. For example, if your player tilts the machine, then you need to disable the flippers. To do so you would overwrite the above six rules with the following:
- When the flipper button becomes active, do nothing.
- When the flipper button becomes inactive, do nothing.
- When the EOS switch becomes active, do nothing.
- When the EOS switch becomes inactive, do nothing.
And just like that, your flippers are disabled! You can also see how you can use these autofire rules to do all sorts of fun things, like reversing the flippers (so the right button controls the left flipper and vice versa), or making “no hold” flippers, or inverting the flipper buttons so pushing them in disables the flippers and letting go enables them. :)
The final thing that’s important to know about these autofire rules you program into your hardware controller is that they do not prevent the hardware controller from doing everything else it might do. For example, if you have a pop bumper then you will probably install an autofire onto your hardware controller that causes the pop bumper coil to fire instantly to knock the ball away.
When that rule is installed, the hardware controller will do two things when the pop bumper switch is activated. First, it will fire the coil, but second, it will also notify MPF that the pop bumper switch was hit (since it notifies your game of any switch that was hit). Then your game code can respond how you want, perhaps by scoring some points and playing a sound effect. When this happens, technically speaking they won’t happen at the same time. The hardware controller will probably fire the coil in under 1ms, and it might take your game code 5 or 10ms to add the score and play the sound. But that’s fine. 10ms is still 1/100th of a second and no human player is going to notice that delay. (Heck, the speed of sound is so slow it takes another 1/100th of a sound for the sound wave to travel from your machine’s speaker in the back box to the player’s ear!)
The point is that just because you install autofire rules doesn’t mean you can’t also service those switches in your game code. It’s just that you end up dividing the duties-—the hardware controller handles the coil responses on its own, and you handle audio and scoring in your game code.
Oh, by the way, it’s not like you need to use these autofire rules for all your coil activity. Most things like ejecting balls, resetting drop targets, and firing your plunger can all be handled in your game code because in those cases you don’t care about the extra 1/100th of a second delay. You only need autofire rules for things you want to happen instantly, which is usually only pop bumpers, slingshots, and flippers.
Now that you just read 1500 words on how autofire rules work, the good news is that you don’t really have to worry about these details of them when using the Mission Pinball Framework. In MPF, you use the configuration files to setup devices like pop bumpers, slingshots, and flippers, and the framework handles all the autofire hardware rule programming based on the switches and coils you specify in your config files.
In fact the framework automatically creates lists of your devices and gives them enable() and disable() methods, so rather than having to know all the intricacies of all those different rules, enabling your flippers is as simple as self.flippers.enable(). Nice! (But if you dig through the source code you’ll see that the framework uses all these rules behind the scenes.)
You can also configure autofire coils manually for simpler things like pop bumpers and slingshots. See the autofire_coils: section of the configuration file reference for details.