Adding a GameGun mode switch to the Mega Drive Justifier adaptor

Thursday, 30th June 2022

The light gun adaptor for the Mega Drive and Mega-CD that I'm working on now has a mode switch that allows you to choose between "Justifier" and "GameGun" modes. The circuit is starting to look a little messy:

Adaptor from a Light Phaser or Justifier to Justifier or GameGun
Light Phaser/Justifier to Justifier/GameGun

I have not fully tested the diagram or circuit in practice, but my initial tests are encouraging. The main change is an extra multiplexer that changes the source of four output pins on the adaptor:

Output Justifier GameGun
TH Latched light gun 1 or 2 sensor Raw light gun 1 sensor
DATA0 Light gun 1 or 2 trigger Light gun 1 trigger
DATA2 Raw light gun 1 or 2 sensor Logic high
DATA3 Logic low Light gun 1 start

The DATA1 pin continues to be connected to the Justifier start button output in this arrangement which is not correct but there aren't enough multiplexers available to also switch that, and in my testing it doesn't seem to affect the detection or handling of the adaptor as a GameGun.

At this point I think I need to build the circuit again on some prototyping board using the diagram as a reference (and not the breadboard prototype) to ensure that the diagram is correct. If that still works I can redesign the PCB and get some new boards manufactured to fully test the circuit's performance in all the Mega Drive and Mega-CD games once more.

The circuit constructed on a breadboard

Hopefully these PCBs will work a little better than the first batch now that I've made all of these improvements to the circuit!

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