Input/Output

Aligning the Soroban permutation bar during the Computer History Museum PDP-1 restoration project
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I/O refers to any device that is used to get data into or out of the machine. ‘Peripheral,’ is another word frequently used—though I/O can include items (such as the front panel) that are mechanically part of the CPU or even ‘logical’ I/O devices that have no physical existence.

The PDP-1 being restored had six I/O devices, in various states of disrepair. The electro-mechanical devices required thorough cleaning and lubrication, followed by careful adjustments, often without service manuals. Sometimes, lots of experimentation was required to bring a peripheral back to peak performance.

A few of the I/O devices required new parts to replace worn or missing parts. Some parts, such as bearings, are still manufactured and thus readily available. Others, such as drive belts, had to be specially-ordered.

The most challenging device was the Soroban console typewriter. This was a typewriter modified to talk to a computer and was widely-known to be unreliable--even when new. While it is an intricate mechanical marvel, replacement sub-assemblies were no longer available, so cleaning, repairing, and adjusting the sub-assemblies was the only option. Luckily, a local typewriter repair shop was found that could fix the basic typewriter mechanism while the team fixed the parts that interfaced the typewriter to the computer.

Team members found that one of the most rewarding parts of the restoration was getting the various peripherals working, which enabled them to interact more easily with the system, the philosophy behind of the PDP-1’s design some 45 years ago.

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