About
The Great Ball Contraption Standard is a simple set of rules designed to make individual ball contraptions work seamlessly together without pre-planning or modification. The standard is located at the Team Hassenplug website
Rules
This is a faithful representation of the canonical rules for a Type 1 GBC module originally posted by Steve Hassenplug. The changes are mostly grammatical.
Each module should have an "in-basket", and will move balls to the next module's in-basket, which must be directly in line.
The in-basket should be 10 studs by 10 studs (outside dimension) with an 8-stud by 8-stud opening, and should be 10 bricks tall.
The front of the basket should be no more than 32 studs from the back of the module. This will allow all modules to be lined up against a wall. The back of the module can be closer to the basket, but not farther.
The in-basket should be located on the left side of the module, and output should go to the right.
There are no size limits beyond those listed.
Each module should be able to accept balls at an average rate of 1 ball per second. Balls can be passed continuously, or in a batch. A batch should not exceed 30 balls.
Speaking about GBCs, Steve Hassenplug said "It can be really simple, or devilishly complex as time allows!" The same can be said about the GBC standard. Years of experience have shown that while the simple rules make creating a compliant module straightforward, they are not in themselves sufficient to guarantee compatibility between modules. The following discussion attempts to fill some of the gaps, thereby improving the odds of inter-module compatibility.
Rule 1
Each module should have an "in-basket" and will move balls to the next module's in-basket, which must be directly in line.
Rule 2
The in-basket should be 10 studs by 10 studs (outside dimension) with an 8-stud by 8-stud opening, and should be 10 bricks tall.
The in-basket must not be any higher than 10 bricks tall. Making it lower can help make it more compatible with modules that have a too low output, but might also raise an issue of balls bouncing out of the basket. The 8 by 8 opening is assumed to be centered within the 10 by 10 outside dimension.
Tip
It is a good practice to output balls at about one brick above the maximum basket height and keep your in-basket at the maximum height. This compensates for uneven surfaces on which your module might find itself and for modules to either side which might push the compatibility limits.
The 8x8-stud opening is only the minimum size. Making it larger can be useful in large displays as it can act as a "speed matching buffer" where extra balls can be collected or staged as need be.
Some argue that the rules imply building on a baseplate and having an in-basket whose top layer is a row of bricks with the studs exposed. This would imply the actual maximum height is a baseplate plus 10 bricks, plus the studs on the top row of bricks. Others would argue that the baseplate should be thought of as the surface from which to measure from.
Rule 3
The front of the basket should be no more than 32 studs from the back of the module. This will allow all modules to be lined up against a wall. The back of the module can be closer to the basket, but not farther.
In practice GBCs are rarely lined up against a wall, but some include trains or other mechanism to transport the balls behind the modules. Since GBCs require frequent intervention, people monitoring the GBC usually stand behind the GBC to allow better access and to avoid blocking the view of the public. The 32-stud maximum "setback" is used for lining up the ball path, while allowing the whole module to vary in depth.
Rule 4
The in-basket should be located on the left side of the module, and output should go to the right.
The effect of this rule is to provide a standard reference frame for a module. If you are facing a module such that its in-basket is on your left and the output is on your right, then the front of the module is nearest you and the back is furthest from you.
Rule 5
There are no size limits, beyond those listed.
While the standard does not enforce any other limits, there are often practical limits like the area available for the module in the layout, the ability to transport the module, or parts available to build the module. Most public displays take place on a string of standard-sized tables (in the US they are generally 30 inches deep). That depth can be further limited if the organizers run a module with a train track behind the other modules (see rule #3). Most modules with a depth of one or two baseplates would have no problem, but if you intend on joining a public display your design should take into account their requirements.
Rule 6
Each module should be able to accept balls at an average rate of 1 ball per second. Balls can be passed continuously, or in a batch. A batch should not exceed 30 balls.
The period over which to average is assumed to be 30 seconds, as a shorter period would exceed the rate of 1 ball per second and a longer period would not be an issue.
Tip
Going slower than the standard means your module would eventually flood as balls continue to enter at 1 ball per second. Going faster will not cause any problems assuming your module does not receive balls faster than 1 ball per second.
The 30-ball batch size implies that the in-basket should have a capacity of at least 30 balls.
Breaking the Rules
There are some instances[2] when breaking the standard above (in certain ways) is actually conducive to a smoothly running Contraption.
Turns
If every module has its output directly in line with its input, we rely on skinny modules to be able to make turns. If there are insufficient such modules, constructing a closed loop becomes problematic. Consequently, it is useful to have some modules be able to deliver balls not necessarily in line with its input.
Recirculate
Sometimes a module needs a little bit of maintenance, which requires halting the flow of balls to that module. Rather than shutting down a whole section of Contraption, it is useful to have a module upstream that can (temporarily) recirculate balls. That is, send balls to its own input instead of the input of the next module.
In all these cases, it is important that the module can still be used in a standard-compliant way. In other words, a module may be able to make a left turn in the ball path, but it should not require a left turn.
Types
The original rules talk about a Type 1, which is discussed above, and a subtype of Type 1 called Type 1b. The Type 1b has additional size restrictions (32 studs x 32 studs with the in-basket being at the front of the module) and is intended to work in a non-ad-hoc layout, specifically within a LEGO train display. It turns out that the Type 1 works pretty well with trains also, but will require more planning by the layout organizer and often require additional space to be filled.
There is no Type 1a nor Type 2