Zapper

The Zapper is a light gun, often associated with the game "Duck Hunt". It reads light from a CRT SDTV and sends the brightness of the area where it is pointed on the controller port.

The Zapper can be used in either controller port, though most games will only allow you to use it in controller port #2, leaving port #1 for a standard controller used for navigating through options, moving the view, changing weapons, etc.

The Famicom Zapper is logically compatible, but can only be plugged into the Famicom expansion port and so only read from $4017. The Vs. System Zapper is not compatible - see below.

Output ($4016/$4017 read)

7  bit  0
---- ----
xxxT Wxxx
   | |
   | +---- Light sense (0: detected; 1: not detected)
   +------ Trigger (0: released; 1: pulled)

The proper way to implement Zapper support in a game is as follows:

1. Each frame, check if the player has pulled the trigger. Remain in this step until the trigger is pulled.
2. During vertical blanking, verify that the light gun is not detecting light, to ensure that the player is actually pointing the gun at the screen. If bit 3 is false during vblank, a standard controller is probably plugged in. Do this near the end of your vertical blank code to let the light "drain out" if the gun happens to be pointed near the bottom of the screen. If you are using sprite 0 hit, a good time to read it is right after the sprite 0 hit flag turns off.
3. Optional: Turn the entire screen white or display white boxes on all targets, and use timed code to wait for the Zapper to start detecting light in order to see how far down the screen the Zapper is pointed. This can narrow the set of targets that must be checked in the next step.
4. For each target the player may hit, display a black screen with a single white box at the target's location. Wait the entire frame to check if white is ever detected. The sensor may turn on and off in 10 to 25 scanlines, so continue to check throughout a whole frame. If any of the targets is hit, register a 'hit' within the game; if not, move on to the next target or, if there are no additional targets, register a 'miss' within the game
5. Restore the screen to its original state.

The Vs. System Zapper behaves mostly the same, though its interface is different - rather than reading the register once to get the two status values in bits 3 and 4, it is strobed and read 8 times, just like a standard controller, such that the 5th read (bit 4 = 0x10, "Up") is always 1, the 7th read (bit 6 = 0x40, "Left") returns the "Light sense" status, the 8th read (bit 7 = 0x80, "Right") returns the "Trigger" status, and all other reads return 0.