The mysteries of the American Laser Games GameGun

Tuesday, 28th June 2022

When experimenting with the American Laser Games titles to try to get my Light Phaser-to-Justifier adaptor working properly I was surprised to see the circuit was initially detected as a GameGun. This was not a light gun I was familiar with so went to read about it on Sega Retro which turned up a photo of the gun:

The American Laser Games GameGun
The American Laser Games GameGun – photo from Sega Retro by user Black Squirrel

This gun is from American Laser Games themselves and is labelled "Sega CD Compatible" as it was apparently based on their similar gun for the 3DO. Sega Retro does also mention the following:

The GameGun [...] is also compatible with games designed for both the Sega Menacer and Konami Justifier. The GameGun is the only widely produced light gun to substitute the Menacer, and is therefore the only smaller alternative to an accessory often criticised for its cumbersome size.

This does sound like an intriguing device! It must pack some fairly sophisticated technology to be compatible with all three light gun protocols, considering how different they all are from each other. I'm not entirely sure how that would practically work, either, as there doesn't seem to be a mode switch or the additional buttons found on the Menacer. Maybe they're on the other side of the gun in the photo…

Fortunately, nesdev.org forum user nintendo2600 owns an ALG GameGun and posted some photos of the internals, which forum user TmEE used to derive a circuit diagram:

The American Laser Games GameGun
The American Laser Games GameGun circuit diagram reverse engineered by TmEE on nesdev.org

This circuit is very simple, only passing on the amplified light sensor signal to the TH output, the state of the trigger button to DATA0 (pin 1) and the state of the start button to DATA3 (pin 4). The GameGun appears to lack any sort of circuitry to generate a controller device ID, which on the Mega Drive is handled by setting the DATA0 to DATA3 pins to various "invalid" combinations depending on the state of TH as an input (the DATA pins are mapped to the d-pad in a standard controller, and an "invalid" combination would be for example reporting that up and down were held simultaneously – this is covered in somewhat more detail in this earlier journal post.

The Menacer and Justifier both contain the appropriate logic to generate a valid controller device ID, and so games supporting those guns check to see whether such a gun returns a valid device ID before allowing you to use the respective gun. Without that logic in place, the GameGun will not be detected as a Justifier or Menacer and so is unsupported in these games. I'm not sure where the source of the information that the GameGun is compatible with games that require a Menacer or Justifier originates, but the circuit doesn't back that up and it would also explain why people who own this gun have not been able to get it working in games that do not explicitly support the GameGun.

The inability to detect a GameGun is also why the American Laser Games default to assuming you have one plugged in, even if you don't have anything plugged into the controller port at all! Its simple design also makes a passive adaptor from the Master System Light Phaser to the GameGun possible:

Adaptor from a Light Phaser to GameGun
Light Phaser to GameGun adaptor

A passive Justifier-to-GameGun adaptor should also be possible, as well as a universal adaptor that could handle both guns. Interestingly the two games I was having compatibility difficulties with before – Mad Dog II and Crime Patrol – work with such a simple adaptor and a Light Phaser, even though the repeated triggering from the Light Phaser causes problems when it's converted to a Justifier using the circuit described in my previous post. Maybe a better solution to the compatibility issues would be to provide a Justifier/GameGun switch on the adaptor…

Even if I can get the Justifier adaptor working completely in Justifier mode in Mad Dog II and Crime Patrol a GameGun mode may still be handy as Corpse Killer on the Mega-CD supports the GameGun but not support the Justifier.

The above video shows the simple passive adaptor working in Mad Dog II: The Lost Gold. A big thank you to TmEE for reverse engineering the GameGun and to nintendo2600 for posting the photos of the insides of their controller!

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