In this section, I'll explore the concept of backpressure. After all, you must understand simple backpressure before you can recreate backpressure with combinators.
When buffers fill and refuse more input, it is called backpressure. In the above video, the backpressure of copper plates travels up the belt and reaches the furnace, preventing the right yellow inserter from outputting onto the belt.
Backpressure continues upstream throughout the factory. The furnaces stop taking coal and ore once they buffer 100 copper plates. With copper ore backpressured, even the drills will stop. The whole factory will enter idle: shutting down production and using significantly less electricity. Despite many beginner's initial instincts, backpressure is a good thing!
Speedrunning may initially seem incompatible with backpressure. Speedrunners want to maximize production of important material like green circuits or solid fuel. Nonetheless, speedrunners can still benefit from a backpressure based factory by building arrays of storage chests where production should be forced to continue. Most normal players will not have much benefit from large scale buffering however.
Backpressure based factories stop using resources automatically if there's nothing consuming those resources. Why make more copper than needed? Why drill for more copper ore if no one is consuming copper plates? What begins as a problem becomes the core strategy of seasoned Factorio players. Look at this second example.
This setup is iron starved and copper rich. The two assembly machines in the southwest need iron plates, gears, and green circuits but the furnaces typically can't keep up with all of the iron demand. But as green circuits and gear assembly lines lock up to backpressure, eventually iron plates are finally delivered to the southwest assembly machines.
A savvy player would have increased iron production somewhere along the way. But by watching the long term trend of this setup, I hoped to demonstrate how proper backpressure design can ensure the eventual completion of any assembly line.
Most items stack in groups of 50, although iron and copper plates stack to groups of 100. Some items, like chemical plants, stack in groups as small as 10. Green circuits have the largest stack at 200.
Backpressure must still be handled on the output, as the final output to the iron chest is a huge buffer by default. Fortunately any chest can be used to limit buffering. Click on the red X button to limit automatic insertion to a certain number of stacks. With backpressure handled at the tail-end of this assembly line, we can be assured that all resources will be always diverted to some location that needs them.
Now that we know the basics of backpressure, we can use combinator circuits to modify, or even create, backpressure across the factory. Creating backpressure in the case of trains and active provider chests in the logistic network can be helpful for the endgame player. But before getting there, the simplest of combinator circuits should first be introduced.