The Colossus Rebuild Project by Tony Sale

Colossus Reborn

The R Rack

Here is the circuits of panels R1 to R5 on the top half of the R rack.

This is not very readable so here are the blown up parts.

This is the input circuits taking the 0-5v data channel outputs, amplifying them in V1 then level shifting in V21 and producing the balanced +- 80v levels from V22 and V23.

Here are the circuits first amplifying the sprocket pulses from the paper tape reader, then producing a narrow pulse from the trailing edge of the sprocket pulse. It is this narrow pulse which then samples the data signals to standardise them to this point in time relative to the sprocket hole pulses.

This is the sample and hold circuits and power drivers. The narrow sampling pulses derived from the sprocket pulses go into the suppressers of V3 and V6. V4 and V5 form a bistable pair. The right hand pair of 6V6 power tetrodes provide the resultant +-80v signals at low impedance to feed into the J rack.

But the code breakers want the delta of the data signals, the difference between the current and previous signal. This is achieved in this circuit by "remembering" the previous signal on the capacitors ringed on the image.
This gives the name to this rack, the "remembering" or R rack. The remembered voltages from the previous data signal are sampled on the suppressers of V6 and V7, held by the bistable pair V8 and V9 to go into the XOR circuit.

This is the heart of Colossus, the very elegant logic XOR circuit. This implements the modulo 2 addition with no carry (XOR) required by the code breakers to compare cipher texts with wheel patterns.

The grids of V10 and V11 get the remembered level from the previous data, their suppressers get the current levels. Because their anodes are joined together that joined point can only go positive if both valves are cut off. This can only occur if both grid and suppresser levels are different thus giving the XOR result.

1 + 1 = 0; 0 + 0 = 0; 1 + 0 = 1; 0 + 1 = 1;

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.