Konami Justifier adaptor compatibility woes with the Mega-CD and older Mega Drive

Friday, 24th June 2022

Work has slowly continued in the background on the Mega Drive Justifier adaptor, as I'd love to be able to turn this into something I can build and sell to people who'd want one if they were unable to build one themselves. As the whole reason for the project is to save money, I'd need to be able to do this cheaply. The parts aren't expensive, but hand-wiring a board is very time consuming, so I thought I should design a PCB. The last time I designed a PCB was the best part of twenty years ago, so I'm pretty rusty (and I've never made a multi-layer board before), but I gave it my best shot in KiCad and sent off the designs to JLCPCB. Ten days later the boards turned up and I soldered the parts on:

The prototype circuit constructed on a PCB

I was pretty happy with the way the board turned out and was delighted to find it still worked just as well as the breadboard prototype described in the previous journal entry.

A couple of 10K resistors on the Mega Drive side of the circuit have been removed and a couple of 1K resistors have been added between ground and TR on the gun input side. The 10K resistors were connected to high-impedance logic inputs anyway so were unneccessary. TR is usually an output from the controller but here it's used to select which gun - blue or pink - is selected, so in case a Justifier is plugged into the adaptor TR is held low to select the blue gun (the pink gun uses a different physical connector so cannot be attached to this adaptor). Using a 1K resistor may prevent damage to other controllers that try to drive TR high. The updated circuit is shown below:

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

In the process of updating that diagram I noticed a mistake in my PCB layout - pins 8 and 9 on the gun inputs are swapped, so in my case TR is connected directly to ground but the gun's GND is connected via a 1K resistor. This may be cause for concern and I've fixed it for the next PCB revision but I think I have more serious issues to contend with first!

A Mega-CD console underneath a Mega Drive

I need to be able to properly test these circuits, and the majority of games that support the Justifier light gun are not for the Mega Drive, they are for the Mega-CD. I didn't own one of these until recently, as they are ferociously expensive, but if I am to sell a product to others I have to make sure it's fit for purpose.

First, the good news: I tried the following Mega-CD games and they worked with the adaptor described above:

  • Lethal Enforcers
  • Lethal Enforcers II: Gun Fighters
  • Snatcher

The Lethal Enforcers games shouldn't be too surprising as they are not considerably different from their Mega Drive cartridge counterparts, chiefly upgraded with a CD audio soundtrack and many more sound effects. Snatcher was also developed by Konami, like the Lethal Enforcers games and the Justifier gun itself so that was not too surprising. However, the following games developed by American Laser Games do not work at all:

  • Mad Dog McCree
  • Mad Dog II: The Lost Gold
  • Crime Patrol
  • Who Shot Johnny Rock?

These games all support a number of different light guns but when started with the adaptor plugged in the best result I've got is to detect it as a "Gamegun" or "Other gun", not as a Justifier. If I plug in a real Justifier then that works, so there's definitely something up with how my circuit works. If I plug in a Justifier then swap to my adaptor I can sometimes get to the calibration screen, at which point the game either fails to respond to trigger pulls, hangs completely until the adaptor is unplugged or – if it does see the trigger pull - fail to register any shots on screen. Very disappointing, but this isn't even the worst of it…

The Mega-CD I bought came with a Mega Drive. I already have a Mega Drive (and Mega Drive II) and the adaptor works well with both of them whether playing cartridge or Mega-CD games. It doesn't work properly at all on the Mega Drive that was included with the Mega-CD. Most games don't recognise the adaptor as a Justifier at all, and if the game does happen to detect it (which it does intermittently) then it won't react to the Start button or trigger pulls in menus. I say in menus as weirdly if I get into a game of Lethal Enforcers II pulling the trigger does flash the screen (as if firing a shot) but no hits are registered and when game over occurs pulling the trigger does nothing (normally it advances the counter). Normally in the menus it advances to the next menu item, too, but not here – you can only fire blanks!

This is all rather strange. I've tried a few points around the circuit with a logic probe and it seems to be reacting as it should but the games are definitely not happy about the adaptor on that particular Mega Drive. The real Justifier works absolutely fine on it, though! I haven't yet had the opportunity to dig through activity logs with a logic analyser to see where the difference is creeping in, but thought I should mention this to save disappointment in anyone who was planning on building these projects.

This is unrelated to the pin 8/9 swap described above, incidentally – I get the exact same behaviour in my PCB prototype adaptor, the adaptor on a breadboard I used to design it and the adaptor I built in this earlier journal post. All three boards work fine on my other Mega Drive and Mega Drive II, just not the Mega Drive included with the Mega-CD.

The only difference I can see is that this new Mega Drive is actually an old Mega Drive – it's one of the earlier models with an EXT port and the "High Definition Graphics · Stereo Sound" label around the cartridge slot, whereas my previous Mega Drive is a slightly later revision without the port and text (but not so new that it suffers from poor sound quality).

I will continue probing, and update here when I've found the solution. On the plus side, the Menacer adaptor I'd developed works fine on the Mega-CD and its accompanying Mega Drive, so this weird behavior is mercifully not shared by that adaptor!

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