If a Hopper pipe is routed over a Chest where items drop into it, and a Hopper pipe is routed under the Chest to catch the items passing through the Chest, sometimes the items pass through the chest without triggering a Redstone Comparator that is attached to the Chest.
How to reproduce (see screenshot for clarification):
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Place a Chest and put a Hopper over it. Put a Redstone Comparator against the Chest so it turns ON when an item is placed in the Chest. Put a Hopper under the Chest, and the item will drop into it and turn OFF the Redstone Comparator. Add several Hoppers against the bottom Hopper to create a pipe so the item ends up in the last Hopper.
If you place an item in the upper Hopper, sometimes the Redstone Comparator will flash indicating something passed through the Chest before the item drops into the Hopper below it, and sometimes it won't. The item will always pass through to the last Hopper.
If the Redstone Comparator does NOT flash when an item is placed in the upper Hopper, try putting an item directly into the Chest and the Redstone Comparator will most likely flash. After that, the Redstone Comparator will most likely flash when an item is put into the upper Hopper. But if you despawn the chunk where this structure is located (go far away and come back), the Redstone Comparator will not flash until the Chest is accessed directly again.
Gotchas:
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Sometimes it works as built and sometimes it does not, apparently location has something to do with whether it works or not. You will have to build several structures until you reproduce the bug.
Sometimes when it doesn't work, directly accessing the chest still won't make the Redstone Comparator work.
If you build it and it doesn't work, and then it works after directly accessing the chest, teleport far away to despawn the chunk and teleport back, and it won't work again until the chest is accessed again.
I've been able to reliably reproduce the bug by building the structure where the Redstone Comparator is placed 'west' of the Chest.
I've tried replacing Chests with Dispensers, and the structure operates with the same bugs; if it worked with the Chest, it will work with the Dispenser. If it didn't work with the Chest, then it won't with the Dispenser. The only difference is that it will NOT work if you first access the Dispenser and drop an item directly into it.
I've been able to reliably reproduce three different scenarios, and it all depends solely on where the structure is built: 1) It works as expected. 2) It doesn't work until the chest is accessed first, then it works fine until the chunk is despawned. 3) It doesn't work at all, nothing makes the Redstone Comparator work.
Linked issues
is duplicated by
MC-8631
Hopper Loop starts skipping Hoppers
MC-8803
Unpredictable Dropper flow
MC-8895
Comparators do not detect items passing through droppers for 1-tick
MC-9351
Inconsistent comparators
MC-9379
Hopper Inventory bug in a hopper line
MC-9560
Timing issues with hoppers
MC-10226
please delete this
MC-12010
hopper towers skipping hoppers
MC-26778
Comparator Does Not Output From Items Moving Through Stacked Hoppers
MC-49881
Hoppers transport items down to quick
MC-55282
A Chest Does not activate a comparator when a hopper is Directly under it
MC-152188
Comparators do not give an output from a hopper that receives and immediately feeds into another container.
MC-152953
No comparator signal from items moving through hoppers
Attachments
Comments
I get this too - I set up a group of furnaces that are logs via a hopper loop, and one of the furnaces is always skipped entirely.
I attached two screenshots of a simple system to replicate the bug:
There are four furnaces in a 2x2 square. Above them, four hoppers feeding into the furnaces. Above those hoppers are four more hoppers, arranged in a loop (travelling counter-clockwise). The chest feeds into an arbitrary hopper. What should happen is that if all four furnaces have no logs in them, logs from the chest should flow into all four furnaces one at a time, first filling up the furnace, then filling up the hopper that feeds directly into the furnace (assuming the wiki is correct). Instead, one of the furnaces in the square is skipped (in my images, it's the second in the loop). I wasn't able to show it in the images, but if you look at the inventory of a hopper in the loop, you can see logs flashing through it - unlike a normal hopper feed, where one item remains in the hopper consistently. This doesn't occur in any of the other hoppers (the one directly under the chest, or the four that feed directly into the furnaces). If you use a comparator to check the capacity of one of the hoppers in the loop, it flashes on and off. (For me) this seems to occur primarily with hoppers that are pointing sideways into a block with inventory, but also have a hopper underneath them. Unloading and reloading the chunk doesn't do anything to help, though - nor does checking any of the inventories involved.
Confirmed.
This may have been fixed along with MC-9687. Can anyone else confirm?
I tested this with 13w09c, and now a comparator connected to a chest does not work at all, no matter what the configuration or orientation of the comparator... Looks like there's more bug-stomping to do.
Confirmed. Tested with chests, hoppers, and dispensers. The items don't seem to stay in the container long enough for the attached comparator to register it's presence.
I got the same thing in the pre-release with a vertical hopper setup.
It seems like what Michael Turner says but it gets weirder.
I got a vertical setup with 6 hoppers high, the bottom one is being checked by the comparator.
1 tower is always sensing the items, but the tower 2 blocks to the side and 1 up does not detect it.
But when i place the items in the 1 of the 3 hoppers above, it will detect. from the 4th it will just "blink" the items in the GUI but like Michael sad seems not to update the comparator.
Still exists in 1.5. A horizontal row of hoppers with a comparator attached to each one blinks each comparator as expected, but a vertical row of containers (not limited to hoppers) does not. This implies that the delay that occurs when a hopper is putting something into a container is long enough to set off the comparator but if another hopper is also pulling from that container then the delay is too short.
Confirmed in 1.5.1.
qmagnet
My bug got posted as a duplicate of this one. I'm not sure if it's the same problem. But posted a picture to explain.
Comparator does not output a signal from the stacked hopper while items are travelling through unless a redstone signal is active near it but not powering the hopper.
The left side has no output from the comparator while the items are moving. The right side does receive output because of an indirect redstone signal. This make no sense - almost as if the redstone is slowing the item down enough for the comparator to make a signal. Note that the redstone signal does not close the hopper.
Further testing shows you also need to more than 1 hopper below the powered block for the item to pass.
qmagnet
Could you update the current version to 14w02c? My specific issue posted on MC-26778 (which I still believe is not a duplicate of this) shows this better. This has not been fixed. A comparator cannot output a signal from a vertical hopper stack unless a a redstone powered block is placed below the comparator.
Still a concern in 14w08a. Probably relates to MC-11193.
marcono1234
Relates to MC-75571
galaxy_2alex
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. It will be reopened as soon as the requested information has been delivered.
NeunEinser
Can confirm for 1.12
Confirmed for 1.13.1.
Confirmed for 1.13.2-pre1.
Please, if you don't mind, I'd like to be the reporter of this ticket. I'll update it accordingly.
Confirmed for 1.13.2-pre2.
This bug still exists in 1.14.4 (vanilla)
I have reproduction in my minecraft server.
With a chest minecart unloading into a vertical line of hoppers, an item passes through the hopper connected to a comparitor for what seems like 0-1 ticks - it barely even flashes in the inventory when viewing the hopper's contents. The comparitor does NOT light up unless the item is in the hopper for a longer time (e.g. pausing/locking the hopper)
Tldr the comparitor doesn't output signal when an item is in hopper for very small amount of time.
The weird thing is, when this was built it worked fine. I assume at some point the chunk was unloaded and when reloaded it was broken. Destroying the comparitor and replacing it does fix the issue. The root cause may be the item being in hopper for too short duration, it seems far too short for normal hopper behavior.
I found this issue with a horizontal line of hoppers. I placed a comparator against one and the system would output a short redstone signal when an item passed through the hopper. This worked as expected until I relogged from the server, after which the comparator never sent out a redstone signal until I destroyed the hopper it was attached to and replaced it. Again it worked until the chunk was unloaded.