Passing through Bletchley Park (near Milton Keynes) I dropped in to see The National Museum of Computing (TNMOC) in the UK. Bletchley Park is an intriguing place where WWII code breaking was done. Unfortunately I didn’t have time to stop, but I did pay a quick visit to see the UK’s version of a computer history museum.
It was housed in some small barracks in the back of Bletchley Park. You can tell that this place was a top secret military installation, because when I visited there were no signs showing its location—even some people who worked on the site there didn’t know where it was! Fortunately, I found it with about about 10 minutes left before it closed for the day: just enough time for a quick look at the centerpiece of TNMOC: Colossus.
(Note: that was back in the early years of the museum. Today, it is easier to find.)
Colossus was the first programmable electronic digital computer in the world. But very few people knew about it, because it was kept top secret until 1975.
Colossus was built during the Second World War for code breaking. It was top secret and after the war Colossus was destroyed, along with all of the plans. However, that didn’t stop the resourceful British: they rebuilt it from people’s memories and various scraps of information.
Colossus was decipher messages produced by the Lorenz machine, which was much more complicated and secure than the more common Enigma machine. It was used by the German Army High Command for longer and more secret messages. While the Enigma transmitted its encrypted messages using Morse code, the Lorenz used teleprinters.
Teleprinters have been around since the 19th century and by the 1920’s had standardised on an International Teleprinter Alphabet of a five-bit Baudot Code. From the received radio signals, the encrypted messages were punched onto paper tape. Each column on the tape had a small sprocket hole plus up to five larger holes, to represent the letter. The problem was speed. Normal paper tape readers used the sprockets to pull the tape through, but at high speed the tape would rip.
This part of the machine read the paper tapes quickly, by feeding them through using friction rather than the sprockets. There are two loops of paper tape: the cipher message tape, and a trial settings tape. The two tapes were compared by rolling them over each other and reading them with an optical sensor.
Because of the shape of its frame, the paper tape reader was nicknamed the “Bedstead”.
The Collosus was a programmable computer, with programs entered via switches. The paper tapes were only used for data, not for storing programs.
Colossus consisted of over 2500 values (vacuum tubes) and thyratrons (gas filled tubes). Over 500 thryatrons were used, in bistable pairs, to store data loaded from a second paper tape, so the next set of starting positions are ready for processing.
The computer has two banks of equipment.
A total of ten Colossus computers were built. This is a replica of the 9th one, which used to be in Block H where The National Museum of Computing now resides. The replica was built by a team led by Tony Sale, who was instrumental in saving Bletchley Park and establishing TNMOC.