Constructing a prototype of the updated Justifier adaptor for the Mega Drive

Wednesday, 6th July 2022

The latching behaviour of the light sensor output TH on the Justifier had caused one complication in the design of the adaptor, described in this earlier post in detail, and that is that there needed to be some way for the console to output the state of the TL back to the gun to reset TH each frame when TL goes high. The American Laser Games don't reset the gun in this fashion, but they don't need to as they use DATA2 for the light sensor output from the gun instead of the latching TH. The circuit had been adapted to tap into this unlatched signal on DATA2 when a Justifier as attached, but there was also some additional complications in the circuitry to ensure that when the adaptor was in "GameGun" mode it would still hold TL low to ensure that any connected Justifier was enabled as it couldn't rely on the game doing so.

Unfortunately, with all these complications I found that Lethal Enforcers no longer worked reliably, for example pulling the trigger would advance two items in the menu instead of one. I think all of the additional routes for the signal to work its way through the circuit – as well as the crude diode logic used to avoid having to add even more logic chips – was causing timing issues. Now that I know it's possible to tap directly into DATA2 to get the unlatched light gun signal from the Justifier it removes the need to reset the gun every frame, which allows for somewhat simpler diagram:

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

Compared to the previous circuit you'll notice the TL signal coming back from the console to the gun inputs has been removed along with the supporting diode logic near the mode switch. Diodes are used between DATA3 and TL on the gun inputs to ensure that if DATA3 is low (which is used to identify a Justifier) it'll also pull down TL to keep the adaptor enabled. (TL is used for the trigger input when a Light Phaser is connected so can't be held low permanently). The 470Ω resistor is used to limit the current in case a different type of controller that is driving TL high and DATA3 is connected – protection against that sort of thing is also why TR has a 1KΩ resistor on it and the TH output sports a 470Ω resistor.

After making these changes, I redrew the circuit in KiCad in preparation of a new PCB design. The above diagram is clearer on how the logic elements are laid out, but at least with the KiCad diagram I can check the correctness of the PCB before spending any money on manufacturing. This of course assumes that the translation from breadboard→logic diagram→wiring diagram is correct too! To assist in this, I printed off the KiCad diagrams and then rebuilt the circuit on some prototyping board, connecting pin to pin using the numbers assigned in the wiring diagram:

The circuit constructed on a prototype board

It's not pretty, but it works! Unfortunately the particular prototyping board I used was of very low quality, with the solder preferring to make bridges than solder to the pads (which themselves don't take much heat to loosen and fall off the board – don't even think about reworking any mistakes!) and the insulation on the wire I'm using seems to have a pretty low melting point, making it shrink back from solder joints and form short circuits very easily. In other words, the less said about the bottom of the board, the better:

The underside of the circuit constructed on a prototype board showing the wiring detail

However, the important thing is that it works, and now that I've been able to test it out in all of the released Mega-CD games I feel much more confident about redesigning the PCB.

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