Adding input support for the Konami Justifier to the Light Phaser to Justifier Mega Drive light gun adaptor

Tuesday, 22nd March 2022

The Sega Light Phaser to Konami Justifier adaptor project that I've spoken about in the past (and the circuit of which can be found on the "product" page here) has had some interest from people who've asked me to sell them one. I've been somewhat hesitant to get into this as I am unable to test most Justifier games as I only have a Mega Drive (and can't afford a Mega CD, which most of the games require) and wouldn't want to sell people something that doesn't work! I own a Blaze "Guncon Adaptor" for the PlayStation which allows you to use a "normal" gun (e.g. Konami Hyper Blaster) to play Guncon/G-Con 45 light gun games, however it only works with some titles (e.g. Time Crisis) – I haven't been able to get it to work with any PlayStation 2 titles, for example, and it malfunctions with some non-Namco PlayStation games too. This was a bit disappointing to me, and I don't want to be a source of disappointment in others…

However, since originally developing the project I did manage to get a pretty good deal on one Konami Justifier so I thought it would be a good opportunity to at least compare my design to the real thing. In my experience the adaptor performs just as well in-game beyond the inconvenience of the Light Phasers not having a Start button on the gun, relying instead on a button on the adaptor itself. This did make me think about the opportunity to plug a Justifier into the adaptor and use it instead of a Light Phaser. This may sound like a daft idea – why do you need an adaptor to play Justifier games with a Justifier? – but this would allow you to use two player 1 (blue) Justifiers to play two-player games, as the pink player 2 Justifier is considerably more expensive than the blue player 1 Justifier.

Updated circuit for the Light Phaser to Justifier adaptor with Justifier support
Light Phaser/Justifier to Justifier adaptor

The updated circuit is shown above, though I must stress that at this point I haven't properly checked the diagram! I've built the circuit as a prototype and then drew the diagram based on the prototype, normally I'd then build another version of the circuit from the diagram to double check I'd drawn it correctly.

The type of gun is identified by DATA3 pin, which is soldered directly to GND in the Justifier gun but not connected to anything in the Light Phaser. This is pulled high with a 10K resistor and connected to the select pin of a pair of multiplexers that switches between 0 ("Justifier") and 1 ("Light Phaser") modes. The rightmost multiplexer selects the source of the "trigger" signal - in "0/Justifier" mode this comes from DATA0, in "1/Light Phaser" mode it comes from TL. The leftmost multiplexer is used to pass the TL signal back from the console to the light gun, as the Justifier requires this pin to be pulsed to reset the gun each frame. In "1/Light Phaser" mode the multiplexer's input is pulled high so it always outputs a high level and the reversed diode on the output prevents this from interfering with the input signal from the gun's trigger. In "0/Justifier" mode the signal comes from the console's TL output, with the reversed diode allowing the console to pull the gun's TL low if required.

The start button is a bit simpler, as the Light Phaser lacks a button of its own – this is just connected to DATA1 from the gun, with a reversed diode allowing the gun to pull the signal low but preventing the button built into the adaptor from shorting the output directly to ground if the gun was driving it high.

One difference between the adaptor circuit and the real Justifier is the need to reset the gun each frame. The real Justifier latches the state of TH – when the gun sees light from the CRT TV, TH goes low and will remain low until the gun is reset by pulsing TL. The Light Phaser does not do this, TH only goes low briefly whilst it can see light from the TV. This is why the circuit needed to be adapted to allow the console to reset the gun via TL. I am not sure if this difference in behaviour has any impact on real games but it doesn't affect Lethal Enforcers or Lethal Enforcers II, the two games I have to test with. If it did cause issues then a flip-flop could be added to the TH path through the adaptor to replicate this latching behaviour, but it seems like quite a lot of added complexity for something that may not be required.

The prototype circuit on breadboard

The photo above shows the current prototype of the updated circuit. I am currently using two additional 74HC157 multiplexer chips to handle the two guns but I think that I should be able to replace them with a single 74LS153 chip – some are currently in the post on their way over to me for further testing. A short video of the adaptor in action can be found on my Twitter account.

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