[MC-12464] Powering Adjacent Dispensers or Droppers from above fails

2 (or more) adjacent dispensers or droppers powered from above work correctly once, and subsequently only 1 fires.

What I expected to happen was...:
Both dispensers would fire whenever powered.

What actually happened was...:
Both dispensers fire the first time, but then afterwards only 1 dispenser fires. Breaking and replacing repeaters resets the behavior.

Steps to Reproduce:
Place 2 dispensers next to each other, with solid blocks above them. Run repeaters into the blocks above, and power those with redstone dust powered by a button.

This behavior is not what happened in 1.4.7 and is new to 1.5 This does NOT seem to be a duplicate of any piston issue. When replacing the dispensers with pistons, both pistons fire correctly.

When powered similarly from below, this problem does not occur, only when powered from above. I have also been able to duplicate this without the repeaters, running redstone dust on the blocks above the dispensers.

Related issues

duplicates

MC-11193 The order in which powerable blocks (e.g. redstone dust blocks) along a wire are powered or de-powered is not clearly defined and causes a non-deterministic behavior for redstone contraptions

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Comments

migrated 2013-03-20T18:33:34Z

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migrated 2013-03-21T00:22:39Z

Confirmed in 1.5.1. Originally they both shot, but soon only one was shooting. Spamming the button seems to be able to switch between which one fires. I suspect the cause is a combination of MC-11193 and MC-108.

migrated 2013-03-22T06:11:10Z

Ok, I can see now how this might be similar to MC-108 (a block above one dispenser is at an upper diagonal to another dispenser), But pistons are unaffected by this particular setup.

I have found that by removing the upper blocks the repeaters will correctly fire the dispensers. This is new to me, but apparently this also worked in 1.4.7 I don't understand how the repeaters power the dispensers, the air block powers the dispenser? (a similar setup will NOT light up Lamps). Having air blocks or transparent blocks above the dispensers is a workaround for my particular redstone project, but not for anyone needing a signal from that upper block.

migrated 2013-04-02T17:34:55Z

I encountered a similiar issue in a different setup in 1.5.1.
My setup (see image 2013-04-02_19.21.50.png) seems a bit simpler, as it containt only one dispenser. It works for the first button press and subsequent presses does not activate the dispenser. Block update near dispenser resets the setup (allows the dispenser to fire once).
Also with some similiar setups direction was important - eg. 2 directions worked fine and other two did not.