User:Munin295/Quasiconnectivity

Quasiconnectivity is a feature of pistons, dispensers, and droppers that allows them to be activated by anything that would activate the space above them, no matter what actually occupies that space (stone, glass, air, etc.).

Activation
Before discussing how components can be activated by quasiconnectivity, let's review the normal methods of activating mechanism components.

Normal activation
A mechanism component is a block which affects the environment and can be controlled by redstone (e.g., pistons, redstone lamps, doors, etc.). Causing a mechanism to change its state is called "activation".

There are a number of ways to activate mechanism components: /* image of mechanisms powered by power components */ /* image of mechanisms powered by transmission components */ /* image of mechanisms activated by power components */ /* image of mechanisms activated by powered blocks */
 * 1) Anything that would power a block in that position will activate a mechanism, including:
 * 2) * correctly positioned active power components (a redstone torch underneath the mechanism, a pressure plate above the mechanism, etc.), and
 * 1) * correctly configured powered transmission components (e.g., powered redstone wire configured to point at the mechanism, or a powered repeater or comparator facing the mechanism). In addition, if redstone wire can be placed on the mechanism (including all solid opaque mechanisms, and hoppers), then powering the wire will activate the mechanism.
 * 1) Any active power component adjacent to (above, below, or next to) the mechanism component will activate it, even if that wouldn't normally power a block in that position (e.g., an active redstone torch next to, or above, the mechanism).
 * 1) A powered solid opaque block adjacent to (above, below, or next to) the mechanism component will activate it.

The last two points are why it's useful to use different words for powering mechanisms and activating mechanisms -- anything that will power a mechanism will activate it, but not everything that will activate it will power it (in the sense that you could get a redstone signal from it with a repeater or something).

The difference between powering and activation also becomes important when the mechanism is itself a solid opaque block (e.g., command blocks, dispensers, droppers, note blocks, and redstone lamps). If a solid opaque mechanism is powered, it will also activate any adjacent mechanisms (according to the third method of activation above) -- but if it's activated by a method that doesn't power it, it won't activate adjacent mechanisms. /* image of solid opaque mechanisms powered and activated */

Before we move on to activation by quasiconnectivity, think about how these rules would be applied to a door. A door is a mechanism component (it affects the environment by providing or blocking access, and can be controlled by redstone), but unlike other mechanism components it occupies two spaces: a top space and a bottom space. Anything that activates the top space of a door also affects the bottom space, and vice versa. /* image of activated doors */

Activation by quasiconnectivity
For the purposes of activation, all pistons, dispensers, and droppers are considered to be two blocks tall (like a door), no matter what direction they're actually facing, and no matter what is actually in the space above them.

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The catch
The catch is that when redstone components change their state, they only notify other redstone components to change their state up to two blocks away -- but quasiconnectivity makes it possible to put a piston, dispenser, or dropper into a state where it should be activated from three blocks away, but because the notifications don't reach that far, the mechanism doesn't activate until it's "woken up" by something else.

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Uses
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