Reinventing The Harwell Dekatron

A huge number of modern replicas of retro computers pass our screens here at Hackaday, and among them are an astonishing variety of technologies. Those who weren’t lucky enough to be present in the days when the building blocks of computing were coming together may have missed out on understanding gate-level operation of a computer. Put your super-powerful and super-complex systems-on-chip aside sometime and dig into the details of their distant ancestors.

Most such machines follow a very conventional architecture, so it is something of a surprise to find a project recreating a modern version of something far more obscure. The Harwell Dekatron, also known as the WITCH, can be found at the National Museum Of Computing in Bletchley, UK, and [David Anders] is building a modern all-electronic replica of it.

The original machine is currently the world’s oldest working digital computer, a hybrid electromechanical computer built at the start of the 1950s  to perform calculations for British nuclear scientists. It was retired by the end of that decade and found its way — via a technical college, a museum, and a period of storage in a council archive — to Bletchley where it was restored to working order by 2012. Its special feature is the use of dekatron discharge tubes as memory, allowing an instant visual display of its working as it happens.

[David]’s replica uses modern logic chips to replicate the building blocks of the Harwell Dekatron, and his write-up is as fascinating for that as it is for his study of the real thing in the museum. We ran into [Dave] showing off this project at the Hackaday Dallas event last year and are excited to learn of the advancements since then from his Hackaday.io page. He’s put his research and designs on GitHub, and a series of YouTube videos, the introduction to which we’ve put below the break.

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World’s oldest functioning digital computer reminds us of a telephone exchange

This is the WHICH, the Wolverhampton Instrument for Teaching Computing from Harwell. It is the oldest functioning digital computer and thanks to a lengthy restoration process you can go and see it in person at The National Museum of Computing in Milton Keynes (Northwest of London in the UK).

The system was first put into operation in 1951. It’s function is both familiar and foreign. First off, it uses decimal rather than binary for its calculations. And instead of transistors it uses electromechanical switches like are found in older automatic telephone exchanges. This makes for very noisy and slow operation. User input is taken from strips of paper with holes punched in them. As data is accumulated it is shown in the registers using decatrons (which have since become popular in hobby projects). Luckily we can get a look at this in the BBC story about the WITCH.

According to the eLinux page on the device, it was disassembled and put into storage from 1997 until 2009. At that point it was loaned to the museum and has been undergoing cleaning, reassembly, and repair ever since.

[Thanks David]