An 8-tube module from a 1954 IBM mainframe examined

IBM’s vacuum tube computers of the 1950s were built from pluggable modules, each holding eight tubes and the associated components. I recently came across one of these modules so I studied its circuitry. This particular module implements five contact debouncing circuits, used to clean up input from a key or relay. When you press a key, the metal contacts tend to bounce a bit before closing, so you end up with multiple open/closed signals, rather than a nice, clean signal. The signal needs to be “debounced” to remove the extra transitions before being processed by a computer.

This is so far before my time, it basically looks like 19th century machinery to me. The steps between this module and what we have today blow my mind.


  1. 2018-01-08 3:51 pm
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  7. 2018-01-09 3:08 pm