The Hagelin B-21 makes an interesting use of half rotors for encipherment. It uses a 25-letter alphabet, and the row and column co-ordinates of each letter are enciphered separately.
For enciphering, co-ordinate signals entered on the spindle side of a half-rotor with ten positions. Essentially, it worked like two separate half-rotors with five inputs and outputs: the odd contacts were wired one way to the five contact strips on the output side, and the even contacts were wired another way.
The following illustration:
shows how such a half rotor works: the five contact strips on the spindle side are shown in yellow. Five wires in blue proceed to the rotor side; then these five wires are connected in a random order to one group of five rotor contacts by green wires, and to another group by purple wires.
The B-21 had pinwheels of sizes 17, 19, 21, and 23. These advanced one step for each letter enciphered. An OR (not an XOR), between two pinwheels caused each half rotor to move one step if the result was a 1.
The B-211, in addition to printing its output on a strip of paper, had two plugboards with five plugs and five sockets to scramble the five outputs taken from the half rotors.
The arrangement of the B-211 is illustrated below:
The French military used a variation of the B-21 which had a second pair of half rotors. The extra pair of half-rotors had fifteen rather than ten positions.
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