Building a NeoGeo Multiplayer Bus Adapter – Deterministic Lockstep Prototype - Page 1

NeoGeo Multiplayer Bus – Hardware Adapter Project

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This diagram shows the architecture of the NeoGeo Multiplayer Bus project.

The goal is to connect multiple NeoGeo systems and compatible platforms through a deterministic synchronization bus.
The system allows multiple machines to run the same game while verifying that both machines stay in an identical state.

The prototype synchronization system is already running between two instances of MAME using deterministic frame hashing.

A demonstration video shows two synchronized systems running Metal Slug while exchanging frame hashes to verify identical emulator state.

Existing Hardware

The Usb-Adapters already exists and are currently in use.

It is based on ESP32 hardware and acts as a communication node for the multiplayer bus.

This hardware already allows communication between devices through:
USB
WiFi
wired serial connections

Target Hardware

The adapter on the right side represents the next step of the project.
This adapter is designed to connect directly to the cartridge slot of a
Neo Geo AES
or
Neo Geo MVS
system.

The goal is to create a dedicated hardware interface that allows real NeoGeo systems to communicate through the multiplayer bus.



Supported Platforms

The architecture is designed to work with multiple platforms:
original NeoGeo hardware
arcade cabinets
emulators such as MAME
FPGA systems such as MiSTer FPGA

Future expansion could allow additional retro platforms to participate in the bus network.

Why Funding Is Needed

The existing prototype demonstrates that deterministic synchronization between systems is possible.
However, the cartridge-slot hardware adapter shown on the right side still needs to be developed and manufactured as a working prototype.

Funding will be used for:

PCB development
prototype manufacturing
hardware testing on real NeoGeo systems
firmware integration with the existing bus system

Project Goal

The long-term goal of the project is to build a multiplayer synchronization bus for retro hardware, allowing multiple systems to run the same game while remaining deterministic and synchronized.

If you find this project interesting and want to help bringing the hardware adapter to life, you can support development on Ko-Fi:

https://ko-fi.com/macdaxter

Donations will be used for prototype PCB manufacturing and hardware testing.

Nice idea! So with this device you could play online with your *Actual* Neo Geo AES?

In the video, it looks like each side has to pause every now and again - I assume to keep synced. is there a way to make that less obvious? e.g to sync more frequently?

Yes, that is exactly the long-term goal of the project.

The idea is that an actual Neo Geo AES or MVS system could connect to the multiplayer bus through the cartridge-slot adapter and synchronize with other systems.
Those systems could be other original Neo Geo machines, arcade cabinets, or emulator instances such as MAME.

In the current prototype the synchronization is intentionally very strict.
Both systems exchange frame hashes and if the local system gets too far ahead it temporarily waits for the other side. That is what causes the small pauses you can see in the video.

This behaviour is mainly a safety mechanism during development to guarantee deterministic synchronization while testing.

In the final implementation the synchronization will work differently:

inputs will be exchanged before each frame is executed
both machines will run in deterministic lockstep
small timing differences will be absorbed by short input buffers rather than visible pauses

With that approach the gameplay should appear completely smooth, even over network connections.

The video was mainly meant to demonstrate that both machines remain in an identical state for thousands of frames, which is the key requirement for deterministic multiplayer on retro hardware.

The hardware adapter is the next step to bring the same mechanism to real Neo Geo systems.

Ah great to hear it will be smooth for the final version. I always thought it would be good to hav a way to connect 2 Neo Geo consoles online

Keep going with it!