Open Pinball Device is a new kind of USB HID interface designed specifically for virtual pinball cabinet I/O controllers.
OPD is essentially brand new as of this writing (September 2024, during the development phase of Visual Pinball 10.8.1), and it's still in the experimental stages. Its goal is to provide a better alternative to the traditional joystick/game controller emulation that all pin cab I/O controllers up to now have used. "Better" in two main ways...
First, pretending to be a joystick can sometimes create compatibility problems for people who play other kinds of video games on their PCs apart from pinball simulators. This isn't really the fault of the other games, either: USB HID devices are supposed to use interface abstractions that match the physical functions they perform, and pin cab I/O controllers that pretend to be joysticks are in flagrant violation of that principle. So the first benefit of using an interface that's explicitly for pinball devices is that it won't confuse non-pinball video games that are looking for joysticks.
The second benefit is that it can reduce the amount of manual configuration work required to set up a pinball simulator like VP, because a simulator can know for sure that an Open Pinball Device is actually a pin cab controller, and not some random joystick. It can also know that the nudge input from the device is actually a nudge input, the plunger input is actually a plunger input, and so on, without requiring the user to map all of that out via a dialog box.
As of this writing, the only pin cab I/O controller that implements the Open Pinball Device interface is Pinscape Pico, which is still in development.
The interface is fully documented in a public, open-source specification (http://mjrnet.org/pinscape/OpenPinballDevice/OpenPinballDeviceHID.htm), so the hope is that other open-source and commercial devices adopt it and that it eventually becomes widely implemented in devices. Write to your device developer and ask them to add support today!
An Open Pinball Device can supply nudge, plunger, and button inputs to the PC. VP lets you configure any combination of these as Open Pinball Device inputs. You can freely mix inputs from OPD controllers with inputs from traditional joystick/game pad interfaces.
For the "axis" inputs - plunger and nudge - you connect an axis to an Open Pinball Device by selecting OpenPinDev in the drop-down list for the axis. For example, if you want VP to read nudge input from your Open Pinball Device, set the Nudge X and Y axis drop lists to OpenPinDev. When you assign an axis to OpenPinDev, there's no need to select which axis to assign (the way that you have to select among X, Y, RX, RY, etc for a joystick), since the pinball device labels each of its inputs with the specific pin cab function it performs.
The Open Pinball Device interface defines two kinds of buttons: "pinball" buttons and "generic" buttons.
The "pinball" buttons are a set of pre-assigned buttons that cover the specific special pinball functions that are common to most pinball simulators, such as "Start", "Left Flipper", and "Coin In". Consult your I/O controller's documentation for a list of available buttons and instructions on how to assign physical buttons to the pinball functions. For a physical button assigned to an OPD pinball button function, there's nothing to configure in VP, because VP knows exactly which VP function to invoke when you push that button.
The "generic" buttons are an extra set of up to 32 buttons that are simply given numbers, #1 to #32, with no special pre-assigned meanings. In VP, these work exactly like joystick buttons. In the VP Keys dialog, you assign these using the same numbering assignments as joystick buttons. Note that VP doesn't have a way to distinguish between "Joystick button #1" and "Open Pinball Device button #1", so if you're wiring your button inputs through multiple physical devices, you'll have to take care to assign a unique number to each button across your whole system.
VP lets you assign each type of input independently, so you can freely mix Open Pinball Devices with other devices that implement the traditional joystick interface.
For example, suppose that your plunger is connected to an Open Pinball Device unit, but your accelerometer is connected to a separate device that only has a joystick interface. In the VP Keys setup dialog, you'd assign the Plunger Position and Plunger Speed inputs to OpenPinDev, and assign the Nudge X/Y axes to X and Y, or whichever joystick axes that your other device uses for its accelerometer reports.
If you have multiple Open Pinball Device controllers in your system, Visual Pinball automatically combines their inputs by making the simple assumption that there's only one instance of each physical sensor present in the system: one accelerometer, one plunger sensor. With this assumption in mind, VP simply takes input for a given sensor from whichever controller reports non-zero input for that sensor. This is done for each sensor individually, so you can have the accelerometer on one controller, and the plunger on a separate controller, and VP will correctly combine the inputs across the two controllers.
This assumption is based on the idea that you might need multiple controller boards in your system, but there's no reason you'd ever need two nudge accelerometers in the same pinball cabinet. There's simply no good use case for multiple nudge inputs, since they'd all be picking up the exact same motion, making the second and additional units uselessly redundant. If you're using two pinball control boards that both come with accelerometers as standard features, you should simply disable the accelerometer on the second controller, since it's not needed. (This doesn't mean that you can't have additional accelerometers for other purposes besides pinball nudge inputs. But those will presumably be attached to other kinds of controllers that don't act as Open Pinball Devices, so they won't create any confusion for VP.)
If this assumption turns out to be wrong, the effect it has is that VP arbitrarily chooses input from one of the sensors. So it still works, but the results might be erratic. If you really do have multiple accelerometers in your system, you should be sure that your controllers are configured so that only one of them is actually reporting accelerometer input on the Open Pinball Device interface.
Button inputs from multiple controllers are combined simply by treating a given button as ON if any controller reports that it's on. That is, all of the buttons are logically "ORed" together. So if, for some reason, you have two separate physical buttons that you assign as "Start" buttons on two separate controllers, pushing either button will activate the "Start" function.