Minecraft – What’s the difference of the two systems


I've lately been working on a piston door in Minecraft. It is almost finished and the only part that I have to add is the redstone signal elevator, but then it does something that leaves me confused.
Here are two picture of the systems, which normally should be giving the same result.

The first one is the one which works…

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And the second one won't work.

enter image description here

Both are powered by a button (it is necessary).

Here's the question: Is this a bug or it is normal? And if yes, normal, why?

Best Answer

Ok, so finally I found the reason by myself.

Before I start to explain, I forgot to say that that machine was supposed to do: displace or drag back the block in a location, and to avoid the observer block doing an entire mess, we have to retract the pistons in a specific order.

To make them retract in a specific order, I had to use a comparator. The thing is that the comparator compare which side's signal is the strongest, and if they are the same, it will let the signal pass, which is EXACTLY what I DON'T want.

So what I had to do is simply to switch the comparator from compare to sustract mode, since 15-15=0.

PS: Because the problem is solved, I won't put a world download.