Mega7800 Adapter, Sega Light Phasers & The QuadTari

[+ZeroPage Homebrew]

Heya @SainT!

 

I have an interesting edge case for you. There's a game, Ducks Away, in development that allows you to connect four Light Guns or Light Phasers to the Atari 7800 through the QuadTari and you can have a four player light gun game! Amazing!

 

Everything looked like it was working fine at first when were playing yesterday on ZPH but the two guns hooked up through the Mega7800 Adapters were not getting any points, no matter what we tried. The screen was flashing when we pulled the trigger but they weren't hitting the ducks.

 

WATCH THE GAMEPLAY HERE

 

I did some troubleshooting and the Mega7800 adapters work fine without the QuadTari but not when they're hooked up through the QuadTari, no matter what ports I put it in. The interesting thing is, my janky Sega->Atari adapter, made using the instructions here, for one of my light guns works perfectly through with and without the QuadTari.

 

In summary:

Mega7800 + QuadTari = Trigger Working but no Hit Detection

Mega7800 = Works Perfect

Sega -> Atari Adapter + QuadTari = Works Perfect

Sega -> Atari Adapter = Works Perfect

 

Any ideas?

 

- James

[SainT]

Interesting, was the light on the adapter remaining yellow?

 

My guess is it may be using the directions as outputs to handle the 4 player input which is then putting the Mega7800 into 8 button output mode and screwing up the light gun.

[+ZeroPage Homebrew]

The adapter had no problem with the yellow light coming when we aimed it at the CRT and the yellow light stayed on the whole time.

 

- James

[+SmittyB]

Detecting the QuadTari works so that if INPT0 is clear and INPT1 is set then it's plugged into the left port, and likewise if INPT2 is clear and INPT3 is set then it's plugged into the right port.
Setting bit 7 of VBLANK to dump them to ground enables reading of controllers 3 and 4 so during normal gameplay that bit is toggled each frame.

Unrelated to the QuadTari but I'm also toggling bit 6 of VBLANK during the gun's hit detection to latch the fire button as the light sensor is equivalent to a button press. The gun's trigger is equivalent to holding up when released, and releasing up when it's pulled. I auto-detect the presence of a lightgun by whether up is held shortly after the game starts. (James let me know to delay this a bit to give the Mega7800 time to kick in its appropriate mode).
Oh and I'm disabling the 7800's 2 button mode as the first thing I do because that just breaks compatibility with every other controller and I'd love to know what GCC were thinking.

[SainT]

Interesting, I don't really see any reason it shouldn't work as long as the INPT0/1 aren't passed through from the attached controller, as they will still be being pulled high / driven low which may be affecting the reading. That said, though, if the trigger is working then it's definitely reading from each controller. The only difference, as you say, between trigger and light sensor is that it's either UP or the FIRE button being used. The light sensor signal is piped directly to the fire output, so I don't see how that would really be different to UP... 

 

Short of getting a QuadTri and having a play, I'm not sure what to suggest.

[+ZeroPage Homebrew]

Heya @gauauu, any insight on what might be happening here with the combination of the QuadTari, Mega7800 and Light Phasers?

 

- James