The Colossus Rebuild Project by Tony Sale

Colossus Reborn

The Relay Rack

The four rows of relays in the centre of the picture hold the count left on the bi-quinary counter circuits when the stop hole on the Paper Tape goes through the reader. Six relays for each counter. One for each of the valves in the ring counter and one for the bi-stable. Thus a count of six is with the fourth valve conducting and the bi-stable set to 0, seven would be with the bi-stable set to 1.

The batch of relays at the bottom are used to send the correct pattern of 6 voltages needed by the Flexowriter for it to print one of the digits 0 - 9 or space or carriage return line feed.

The printing is sequenced by a uniselector on the back of the relay rack driven at 10 characters per second by a multi-vibrator.

The sequencing causes the current start positions for the K4 and K5 Thyratron rings to be printed first, followed by the four digits of the count.

This printing is done whilst the Paper Tape is going through its next revolution, an early use of double buffering.

In this first implementation of Colossus only one counter is connected up and its results typed out.

Eventually it will have five sets of holding relays like the original, which even had spare sets of holding relays in case printing was not completed before the next stop hole came round.

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.