v0.34:Minecart logic

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The addition of minecarts to Dwarf Fortress has opened up new and exciting logic and computing options for the ambitious fortress manager. Minecart-based logic gates and memory cells are easy to build (arguably easier than fluid logic systems), they are easy to reconfigure, and react quickly.

Techniques and Circuits

There exist a great number of different techniques by which a minecart can receive input, compute, and deliver output. This article does not aim for a comprehensive list of techniques and circuits; the interested reader is encouraged to investigate further. The following examples were chosen to demonstrate both a variety of techniques and a few commonly used gates.

Key

Adequately diagramming minecart logic devices can be difficult; each tile on each z-level might need to display up to four slices (track, ramp, furniture, minecart) that can lay on top of each other. Ramps are displayed on the furniture layer for the sake of simplicity, and some slices may be omitted when unnecessary. Components of each lower slice are displayed on the higher slice when unchanged by new components to give the reader a sense of placement. Wall O is typically displayed only where it is essential to the operation of the circuit. Unengraved floor , is sometimes needed for other components, but of course can be smoothed as desired. Track direction is laid out with ║═╗╝╚╔ and ends in a tile with ╨╥╡╞. Minecarts ■ are accelerated by rollers to the east ╟ west ╢ north ╧ or south ╤ and decelerated by track stops ≡. Rollers are controlled via gear assemblies, either engaged ☼ or disengaged ☼, typically connected to sufficient power P. Pressure plates ^ provide output and, in some cases, modulate the circuit itself; in such cases, they are typically colored to make it clear to which components they are linked. Up ▲ and down ▼ ramps may be necessary to travel z-levels or alter minecart velocity; they may be roofed or covered with empty space . in some views. Doors ┼, hatches ¢, and retractable bridges ╬ are commonly used to control the path of minecarts. Where necessary, clarification can be found in the descriptions of each circuit.

Power to signal

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In this simplest of all designs, the output plate sends an on signal when the gear assemblies ☼ are powered P. When power is lost, the minecart settles onto either the northern or southern roller spaces, and the output plate sends an off signal.

This device is very general purpose. Left as an exercise for the reader, alternate construction can result in a repeater or edge detection.

Newton's Cradle Memory

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TinyPirate's Newton's Cradle memory cell is notable both for it's tiny footprint and for demonstrating an important principle of minecarts. When the southern gear assembly ☼ is briefly engaged, the southern roller ╧ becomes powered, launching the southern minecart ■ onto the output plate ^. But rather than continuing past the output plate, the southern minecart collides with the northern minecart ■, sending it onto the northern (unpowered) roller. When the northern gear assembly is briefly engaged, the situation reverses: the northern minecart knocks the southern minecart off of the output plate.

Roller derailer NOR

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In this gate, a minecart entering from the south is diverted by W->E rollers ╟ and ╟ onto an alternate pathway whenever either is active (that is, when neither has received an on signal). A slight redesign could easily evaluate power rather than signal; small changes would make this an OR, AND, NAND, or NOT gate.

Bridge drop/bridge derailer AND

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While impractical, this AND gate demonstrates a few useful principles. If the green retractable bridge ╬ is closed, a minecart entering from the north travels over it, eventually dropping one z-level at the southernmost ramp before continuing southward. If that bridge is open but the cyan retractable bridge ╬ is closed, the minecart drops into the northern ramp and continues straight south, since the bridge covers the track. Only should both bridges be open will the minecart drop, then divert onto the eastern path, triggering its output plate ^.

Note that using bridges in this manner can be very tricky. When bridges change state, minecarts traveling over them are liable to be thrown. Bridges can still be useful in some circuits where the player is certain of the timing involved. For instance, in this circuit, if the player knows that the green bridge and the cyan bridge will only ever change state at the same time, and the minecart travelling the circuit rests atop the green bridge rather than entering the circuit at unpredictable times, the above circuit is safe.

Also note that should the cyan input change very soon after the green input, there is the potential for this circuit to output when the inputs are not actually true at the same exact instant. This problem is common in AND gates. Should this be important to your design, it is possible to work around it, paradoxically, with another AND gate, by ANDing your inputs with a regularly pulsed clock signal. The bridge-drop (or its superior variant, the hatch-drop), linked to a clock, can be used to create circuits where all of your inputs change state at the same time.

Potential as an independent logic discipline

Minecarts can also be set in motion by ramps and switched between different paths by buildings, opening the path for a powerless logic discipline. The basic binary logic gates can be built in this fashion and combined to perform other operations like counting or basic algebra. The circuits tend to look quite complicated, especially if they stretch over multiple levels.

This kind of minecart logic is primarily an alternative to creature logic. Since minecarts move relatively quickly and completely deterministically, simple minecart logic gates can be relatively small and quick. Since a minecart only reacts to the conditions of its current tile and the tile it tries to move into, creature logic will have an advantage when looking at multiple and long logic paths, where a creature instantly detects and chooses the open path, while the minecart has to check every tile and building separately.

For signal generation, memory cells, repeaters and adders, this kind of minecart logic offers a variety of options.

See Also

• BloodBeard's Minecart Dwarfputing Ideas thread.
• TinyPirate's Minecart Logic 101 instructional video.
• Powerless logic based on hatch-switched minecarts. Logic gates built under this design doctrine.