Minecraft – Why does this redstone pulser circuit produce this result

minecraft-java-editionminecraft-redstone

I was playing around with a self-referential comparator pulser and came across something I don't understand.

In short, it's a comparator in subtraction mode, with the output of the comparator fed back into the side input of the same comparator along the edges of a 3×3 square.

The premise of using a self-referential comparator to make a pulser makes sense to me, and if I add repeaters to this design it works exactly as I'd expect. Without repeaters, something very odd happens.

The comparator produces a steady on signal, but only close to its output. As you move further away from the comparator, it begins producing rapid pulses, which degrades to no signal at all further way. If I increase the signal going into the comparator, the same basic pattern happens but at different distances from the output.

To me, what seems to be happening is:

  1. The blocks closest to the comparator output are getting a pulse signal where the ON pulse is long enough that the off pulses don't register [1-1-1-1-1-1-1-0-1-1-1-1-1-1-1-0-1-1-1-1-1-1-1-0 etc]

  2. The blocks furthest away are getting the opposite, a pulse signal where the ON is so short that it's not long enough to turn the blocks on [0-0-0-0-0-0-0-1-0-0-0-0-0-0-0-1-0-0-0-0-0-0-0-1]

  3. The blocks in the middle are getting a pulse signal that is able to be processed in a meaningful way [1-1-0-0-1-1-0-0-1-1-0-0-1-1-0-0-1-1-0-0-1-1-0-0]

  4. Increasing/decreasing the signal strength scales the lengths of these output sections accordingly

Assuming the above is accurate (it may very well not be, I don't think I'll ever truly understand redstone circuits) I really do not understand why distance from the pulser affects the rate of the pulse signal.

Best Answer

A comparator has 2 modes

Compare mode (torch unlit):

  • Check if the signal coming in from the back is greater than or equal to the signal strongest signal from either side.
    • If yes, emit the power of the back side to the front side
    • If no, emit no power to the front

Subtract mode (torch lit):

  • Check if the signal coming in from the back is greater than or equal to the signal strongest signal from either side.
    • If yes, emit the difference in power from the sides vs the back side to the front side
    • If no, emit no power to the front

In your case, you made a loop of 7 redstone wires long to the side of the comparator.

Lets assume you just placed a lever behind the back of the comparator, and that you just turned the lever on. (this is like your last example)

  1. The comparator (compare mode) now receives a signal of 15 from the back, and 0 from the sides. It will emit a signal 15 the next tick.
  2. The comparator (compare mode) now receives a signal of 15 from the back, and 9 (15 - 7 + 1) from the sides. It will emit a signal 15 the next tick.

As you can see, a comparator in compare mode is "stable", lets toggle the mode

  1. The comparator (subtract mode) now receives a signal of 15 from the back, and 9 (15 - 7 + 1) from the sides. It will emit a signal 6 (15 - 9) the next tick.
  2. The comparator (subtract mode) now receives a signal of 15 from the back, and 0 (6 - 7 + 1) from the sides. It will emit a signal 15 (15 - 0) the next tick.
  3. The comparator (subtract mode) now receives a signal of 15 from the back, and 9 (15 - 7 + 1) from the sides. It will emit a signal 6 (15 - 9) the next tick.
  4. The comparator (subtract mode) now receives a signal of 15 from the back, and 0 (6 - 7 + 1) from the sides. It will emit a signal 15 (15 - 0) the next tick.

This doesn't look very stable at all...

Using the values, we can also explain what the trap doors are doing.

  • The trap doors in the green region are between 0-6 tiles away from the comparator, meaning they always see the full signal
  • The trap doors in the yellow region are between 6-15 tiles away from the comparator, meaning they see the signal going on and off quickly
  • The trap doors in the red region are between further than 15 tiles away from the comparator, meaning they never see the signal

The lamps take 2 ticks to turn off, meaning they don't react quickly enough for the fast clock.

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