The cipher text characters read from the paper tape are

switched either from the near or far readers into these

panels at the top on the back of the R rack.

There are five panels each processing one of the five

tracks from the tape.

The panels produce both the direct signal and the delta,

the difference between adjacent characters.

These output signals switch between + and – 80 volts,

the standard logic levels used throughout Colossus.

Both direct and delta signals go to the J rack switch

panel where they can be selected.

The K rack big switch panel with a panel of cathode follower isolators above it. The vertical column of groups of five switches on the left is the Q bus. The same five bits are fed down vertically to all switches, bit 1 on the left. Putting a switch down tests if the vertical bit coming down is a 1, putting the switch up tests if it is a 0. With the switch central the vertical bits are ignored. The result feeds towards the right and to the switches to the counters. The five counters are connected to the five vertical lines of switches on the right hand side of the switch panel. Thus putting up all switches in a row tests for all Q bits being 0. If so the selected counter increments by 1.

The lower five rows of keys on the Q bus behave slightly differently to the top section.

These keys add (modulo 2) the results of a test of any keys in a row and pass the result to the counters.

The counter select keys on the right hand side are also divided into two sections and there are negate keys in rows

at the bottom of each section.

The final outputs to be counted go via cathode followers to the C rack on the left hand side of Colossus. This rack has

the five electronic counters on its rear side and the five counter control panels on the front. The electronic counters are

bi-quinary, a divide by two circuit followed by a ring of five circuit.