The Colossus Rebuild Project by Tony Sale

Colossus Reborn

The J Rack

The signals from the R rack, both from the paper tape reader (Z) and the Thyratron rings (K) are routed to the logic circuits on the J rack. (possibly J for Jack field which is in the centre of the rack).

When, in 1996, I had managed to get the paper tape reader and the Thyratron rings working for tracks Z4 and Z5 and rings K4 and K5, I wired the outputs straight into the logic circuits in order to get signals to go into the counters.

We now know that the signals should be switched via the switch panel adjacent to the jack field panel on the front of the J rack.

The wiring to do this has now been done but connecting it up awaits my being allowed back into Bletchley Park.

Meanwhile my Virtual Colossus (now accessible from codesandciphers) shows how it did work and will work eventually on the Rebuild.

By selecting the delta outputs for Z and K, the double delta Tutte algorithm can be set up:

result = DeltaZ5 + DeltaK5 + DeltaZ4 + DeltaK4

Where the "+" is modulo 2 addition (XOR).

It is the result of this logic calculation for each character along the cipher text, which goes to the counter circuits for counting to get the score for that pass round the tape.

This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum, and Secretary of the Bletchley Park Heritage Society.