Here there is the reader/writer on k'nex chains. A turing machine is not only a finite state machine, it must also be able to change its state depending on what it reads over a chain, and it must also be able to write the letter it read. Here, it is possible to read and write over 1-bit chain The reading is done by a différential with middle gears. The writing is done by a cylinder, which turns the letter read.