16 directions

[tz101]

Forgive me if this seems like a dumb question. I have two Intellivision model I consoles, but have never had the controllers apart. So, I got to wondering exactly how Mattel achieved 16 directions with the disc pad. Knowing that the disc rotates makes it even more mysterious. Are there 16 separate digital contact points on the controller PCB? Hitting on any one of the 16 makes the sprite travel in that direction?

[doppel]

I think it might be more accurate to state that there are sixteen different bit states that the disc pad can invoke. There are five bits connected to the disc, and the positions at which they're connected differs. Whether or not the game actually responds to 16 different directions depends on the game itself, how it was programmed, whether or not the astrological sign of the programmer was in Uranus, etc. You end up with 1 to 3 closed switches (which clears the bit on the I/O port it's connected to) depending on the direction pushed, and there are sixteen unique states. Some of the bits are also connected to the keypad and they also map to other bits, and those other bits are mapped to the buttons on the sides. And that's about as technical as I can get without actually revealing which bits are mapped where and whathaveyou.

 

And the disc doesn't actually rotate, that's just the outside.

Edited September 27, 2010 by doppel

[jaybird3rd]

Forgive me if this seems like a dumb question. I have two Intellivision model I consoles, but have never had the controllers apart. So, I got to wondering exactly how Mattel achieved 16 directions with the disc pad. Knowing that the disc rotates makes it even more mysterious. Are there 16 separate digital contact points on the controller PCB? Hitting on any one of the 16 makes the sprite travel in that direction?

There are no moving parts inside the Intellivision (or Aquarius) hand controllers, aside for the disc itself; they're built using printed circuits on mylar sheets, the same way most keyboards are made today. The areas directly underneath the disc are covered with a ring of conductive pads in two layers, sandwiched together. As you press the disc in a particular direction, it closes the gap between the layers, the pads touch, the circuit closes, and the direction is registered. The keypad and action buttons work in much the same way. It's actually a bit more complicated than that because there aren't actually separate pairs of pads for each direction, but that gives you the general idea.

 

Because the design wasn't based on physical switches (the way Atari joysticks are), going from eight directions to sixteen was just a matter of adding extra contact points. If a game only needed four or eight directions, it would simply ignore the others. For games that did benefit from all sixteen directions, the design worked pretty well; the control was actually quite smooth.

 

And the disc actually did rotate inside the hand controller; it's a solid piece of plastic with a metal (or, for the Intellivision II and Aquarius, hard black plastic) plate on top of it to make it more durable. You can spin it around in circles, or you can rock it back and forth on its center with two fingers. These are the easiest ways to use it; trying to push it around with your thumb like a gamepad will only fatigue your hands.

[intvnut]

I think it might be more accurate to state that there are sixteen different bit states that the disc pad can invoke. There are five bits connected to the disc, and the positions at which they're connected differs. Whether or not the game actually responds to 16 different directions depends on the game itself, how it was programmed, whether or not the astrological sign of the programmer was in Uranus, etc. You end up with 1 to 3 closed switches (which clears the bit on the I/O port it's connected to) depending on the direction pushed, and there are sixteen unique states. Some of the bits are also connected to the keypad and they also map to other bits, and those other bits are mapped to the buttons on the sides. And that's about as technical as I can get without actually revealing which bits are mapped where and whathaveyou.

 

Here's a (perhaps ugly) diagram I drew up a long time ago showing how the bits are connected for the Intellivision controller pad:

 

 

The neat thing about it is that if you ignore 'bit 4', the encoding is pretty much the same as an 8-direction controller. The additional bit is what gives the 16 directions.

 

The other neat thing is that it's set up similar to a "Gray Code", in that only 1 bit changes between any two adjacent positions. This is helps keep the control smooth as you roll through several positions. As you roll from one direction to the next, only one bit switches, and so only one bit should be noisy. The other bits stay put. If more than one bit needed to switch, you risk having an intermediate state that looks rather different from either your old direction or the new direction you're moving to.