Table of Contents

Technical details

VPE connects to MPF using gRPC, which is a high-performance, low-latency RPC framework. When using MPF, VPE acts as a hardware controller. MPF does not "know" there is no real playfield.

When VPE starts up, it will:

  1. Launch MPF as a Python process
  2. Create a gRPC client
  3. Connect to MPF
  4. Inform MPF about the starting states of all switches
  5. Query the machine description and check whether it matches the description saved earlier
  6. Tell MPF to start the game

At runtime:

  • VPE sends an update to MPF whenever a switch is opened or closed
  • MPF sends commands to VPE to control coils, lights, and displays
  • The MpfGamelogicEngine executes those commands

When VPE shuts down, it will:

  1. Notify MPF that it is shutting down
  2. Disconnect from MPF and shut down the client
  3. Wait until MPF shuts down (or kill the process if it does not shut down within one second)

Included MPF binaries

VPEs MPF integration comes with a custom prebuilt version of MPF that is slightly different from the official version. VPEs version of MPF supports a Ping RPC that allows VPE to check whether MPF is ready without any side effects (like starting the game). This way, the game can start as soon as MPF is ready, regardless of how long MPF takes to start up. We have proposed this change to the MPF developers, but as of February 2025 they have not yet gotten around to including it in the official version. Therefore, you will need to set the Startup Behaviour of the MpfGamelogicEngine to Delay Connection if you want to use any official version of MPF.

Hardware rules

VPE supports MPF's hardware rules, which are dynamic connections between coils and switches that are handled by the hardware controller boards in real pinball machines in order to reduce latency, but they are useful for VPE as well. For example, when the player presses the flipper button on the keyboard, VPE already knows that the flipper coil should be activated and does not need to wait for a response from MPF, because MPF has previously told VPE that the flipper switch should activate the flipper coil. When the table goes into attract mode, MPF removes the rule and the flippers no longer work. This means that even though MPF runs in a separate process, using it to control your VPE table does not increase latency.

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