IBM mainframe tube module part II: Powering up and using a 1950s key debouncer
In the 1950s, before integrated circuits or even transistors, mainframe computers were built from thousands of power-hungry vacuum tubes filling massive cabinets. To simplify construction and maintenance of these computers, @IBM invented a pluggable module with eight tubes;1 failed modules could be quickly pulled out of the computer and replaced. I came across one of these tube modules and wondered if it would still work decades later. Could I power it up and demonstrate it in a circuit, or would the components have failed with time? ￼ The glowing orange filaments are visible in the tubes of this IBM tube module. This 8-tube module is a key debouncer from the IBM 705 business computer. Part I of my post discussed tube modules and described the IBM 705 that used this module. To recap, the IBM 705 was a large business computer introduced in 1954. It weighed 16 tons, used 70 kilowatts of power and cost $15 million (in 2018 dollars). A few dozen 705 systems were built, mostly used by large companies and the US government. For example, Texaco2 used the 705 for accounting applications such as payroll, marketing, and distribution. Even though the 705 was intended as a business computer, Texaco also used it for technical applications such as refinery simulation and pipe stress analysis. Below you can see the large CPU of an IBM 705 computer. Each of the four panels in the front held up to 80 tube modules. ￼ The CPU of an IBM 705. From IBM 705 Electronic Data Processing Machine brochure. The debouncer By tracing out the circuitry of the tube module and studying old IBM documents, I determined that the module consisted of five key debouncing circuits. When you press a key or button, the metal contacts inside the switch tend to bounce against each other a few times before closing, so you end up with multiple open/closed signals, rather than a nice, clean signal. To use a key signal in a computer, it needs to be “debounced”, with the multiple rapid transitions replaced by a single, clean transition. (Perhaps you have used a cheap keyboard that occasionally gives you double letters; this happens when the keyboard bounces more than the debounce circuit can handle.) In modern systems, debouncing is usually done in software, but back in the 1950s tubes were used for debouncing. ￼ This tube module from an IBM 705 mainframe computer, implemented five key debouncing circuits. The IBM 705 was controlled from a complex console with control keys and neon status lights (below). The console was used for manual control of the computer, monitoring status, detecting and correcting errors, and debugging. (While memory could be modified from the console keyboard, programs were normally read from punch cards.) Tube-based debouncing circuits were used on many of the console keys to ensure proper operation, so that’s probably the role of the module I examined.