Okay...

So a while back, I was playing in an emulator, and I set my NES retrousb joypad adapter to control player one and player 2 inputs simultaneously. It's a quick and dirty hack, but can have interesting results in certain two-player games. For intance, in Tengen tetris, you can play two identical games of Tetris side-by-side at once, and it even gives you the identical pieces. I think in BattleToads, both players quickly combine into a single fighter with double attack power. It may also have unintended consequences or unusual effects in other multiplayer games as well.

So someone posted a thread about connecting two NES Advantages to their console at once, and I remembered the selector switch on the NES Advantage that allows you to choose between player one and two. It always annoyed me because I could never figure out which plug was which, hence I would have to keep flicking the toggle switch.

So I decided to modify the NES Advantage so that it would play both players simultaneously. After studying the motherboard closely, I determined that the selector switch automatically switches the joypad out (P0) line between player 1 and player 2 inputs on the NES. Since this is an output and it isn't connected to anything else but the select switch (it essentially is left "floating" when the switch is set to the other player), I decided it was safe to solder a jumper wire between the P0 output lines to mod the Advantage into fooling the NES and controlling both players at once!

Well, the behavior isn't what I expected. I booted the PowerPak and loaded "NES_Test" to investigate what is going on.

With the Player Select set to "1":

"A" button activates "A" on Player 1 joypad and activates all buttons simultaneously on Player 2.

B, Start, Select, and the D-pad operate normally on Player 1 and do nothing on Player 2.

With the Player Select set to "2":

"A" button activates "A" on Player 2 joypad and activates all buttons simultaneously on Player 1.

B, Start, Select, and the D-pad operate normally on Player 2 and do nothing on Player 1.

(For some reason, the "NES_TEST" program doesn't seem to read data from Start and Select on Player 2. I verified this with a known good controller.)

Either way, this behavior is very bizarre. I also noticed earlier while I was probing the Advantage montherboard, that the Player Select switch is a DPDT switch which also selects the clock signal input as well as the P0 output. Originally I assumed this was just redundant design. The ground, latch, and VCC from both controllers are all tied together on the Advantage motherboard. My only conclusion is that the Player 1 / Player 2 Clock signals must somehow operate independantly of each other. I get the impression that since the "Clock" signals are being generated independantly, it would be a bad idea to short them together.

Basically I want to know if there is any way to mod the NES Advantage to operate both players 1 & 2 simultaneously, short of adding an extra 4021 chip to the schematic, which IMO isn't worth the effort. Otherwise, I'll just cut off the jumper I made and reseal the Advantage, and it will be back like it was before as if none of this voodoo ever happened to start with.