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Autofire Coils

Related Config File Sections:

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.

First, some background...

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:

  1. The slingshot switch is activated.
  2. The hardware controller debounces that switch.
  3. The hardware controller sends a notification that the slingshot switch changed state to your Python game code via USB.
  4. Something in your code says, "if the slingshot switch is activated, fire the slingshot coil."
  5. The Python game code sends the "fire the slingshot coil" command to the hardware controller via USB.
  6. That command is queued on the USB bus and transmitted.
  7. 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. :)

How MPF interacts with autofire rules

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.

How MPF handles autofire rules

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.

Debounce and Recycle in Autofire Coils

In MPF you can configure debounce for each switch and recycle for each coil. If you do that MPF will respect that configuration for autofire hardware rules. However, if you do not configure it (or set debounce to auto) MPF will try to select a reasonable default. For autofire coils it selects debounce quick if you either did not specify debounce or set it to auto. Recycle will be set to true if you do not specify it.

In some platforms MPF might reconfigure your switch debounce settings when activating the hardware rules (if the platform does not allow separate settings). This happens when debounce is set to auto (or unspecified) as switches are then automatically configured as debounce normal and then reconfigured as quick when the rule is send to the hardware (if the platform only supports one configuration at a time).

You can overwrite both settings using switch_overwrite and/or coil_overwrite in your autofire_coils section.

Monitorable Properties

For dynamic values and conditional events, the prefix for autofire coils is device.autofires.(name).


: Boolean (true/false) which shows whether this autofire coil is enabled.

Fully working basic example

Let's learn by example. Though the following example is a fully working minimal set for the Cobra controller, it is as well helpful to understand the concept more if you use a different set of hardware. For this example to work physically, the Cobra board needs to have the micro controllers powered up only. No need for a high voltage power supply, neither for any coil. The config.yaml below is the only configuration file you need in your project. The config file is fully valid for the Cobra board connected to a Linux PC running MPF. If you have a Cobra board but run Windows or macOS you have to change the ports. If you run a completely different hardware you have to adapt the hardware section.


   platform: opp
   driverboards: gen2

   ports: /dev/ttyACM0, /dev/ttyACM1

playfield:    #playfield must exist for autofire coils
   tags: default
   default_source_device: bd_plunger   #value must be set, default "none" not allowed when having autofire coils

   ball_capacity: 1
   mechanical_eject: true

   number: 0-0-11

   number: 0-0-16

   coil: c_my_coil
   switch: s_my_switch
   enable_events: simulate_start
   disable_events: simulate_stop

   event: simulate_start
   event: simulate_stop

Now run mpf both to start above example. The Cobra board has a little LED next the coil output which will light up yellow when the coil is activated, see the Cobra board documentation for details. Now press the connected switch, you will see that the LED will not light up since the coil has not been activated. Press key 1 and afterwards press again the switch, this time you will see the LED light up for a short time. After you pressed the key 2, the LED won't light up anymore when the switch is activated, because you deactivated the coil.

A few comments on the above example:

  • The playfield is needed even in this basic example, in a real setup you have it anyways.
  • Coils are enabled by MPF upon the ball_started event and disabled by the events ball_will_end, service_mode_entered. In our basic example we don't have these events, thus added our own events when the keys are pressed. In a real pinball most likely you won't have these additional events.
  • Both, coil and switch, need to be controlled by the same micro controller for autofire coils, as you can see both number value starts with 0. If you would use different values MPF will throw an exception once the coil is being enabled, but not directly at startup. The error message is Config File Error in OPP: Invalid switch being configured for driver. Driver = 1-0-1 Switch = 0-0-16. Driver and switch have to be on the same board.
  • The auto fire rules are stored in the micro controller. If you execute the above example, then change the coil to another coil (on micro controller 0) and run it again. Now the switch will then trigger both coils. If you do these kind of changes you want to power down the micro controllers to have a fresh start and avoid strange behavior.


: The autofire coils can be configured to enable or disable based on other events.

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