A diagram of a loopback-rotor construction; in this simplified example, two rotors with nine contacts each are used. There are six inputs and outputs, leaving three loop-back wires.
The HX-63 was an advanced rotor machine designed by Crypto AG, who started the design in 1952. The machine had nine rotors, each with 41 contacts. There were 26 keyboard inputs and outputs, leaving 15 wires to "loop back" through the rotors via a different path. Moreover, each rotor wire could be selected from one of two paths. The movement of the rotors was irregular and controlled by switches. There were two plugboards with the machine; one to scramble the input, and one for the loop-back wires. The machine could be set up in around 10600 different configurations.
Only twelve of these machines were manufactured, and it was used by only one department of the French government.