The HP9100A/B Scientific Calculator
The world's first programmable scientific desktop calculator, the HP 9100A could add, subtract, multiply, divide, take square roots with 10-digit accuracy, compute logarithms and compute the full range of trigonometric functions in all quadrants in either degrees or radians. In a unit the size of a typewriter, with built-in, read-only memory that stored calculating and display routines, and the ability to perform floating-point calculations, the HP 9100A could run programs recorded on wallet-size magnetic cards.
The Environmental Chamber Controller
In 1968, we purchased a HP9100 Calculator as soon as one became available. And then we performed a lobotomy on it. The case of the HP9100 was cast aluminum, which we very carefully sawed off.
The HP9100 was built prior to the use of integrated circuits, thus every transistor in the HP9100 was a discrete device and the "ROM" was composed of four stacked boards that completely consumed the base of the calculator. The logic of the ROM was created through the use of a resistor/diode matrix (not unlike the display/keyboard computer that was on board the Apollo spacecraft that was flying to the Moon at the same time, although that computer was completely encased in poured plexiglass for mechanical rigidity).
We opened the calculator up in order to use it for a purpose for which it was never designed: an environmental chamber controller. To do this, various signals had to be intercepted from the circuit boards and converted into then-new TTL logic levels. And an 8-bit, sharable bus structure was designed to allow a serially sharable talker/many listener protocol to transmit the calculator's programmed commands to up to eight environmental control chambers simultaneously and listen to their responses.
Our HP representatives at the time were Norm Matlock, Ralph Kotowski, Jim Kemp, and Bill Little. Norm was excited enough about the novel use of the HP9100 that he convinced a group of HP engineers from the Colorado Springs Division to fly down and see what had been done. Eventually, three groups of HP engineers visited the lab at separate times and asked if they could copy the design, although it was nothing that they wouldn't have devised on their own. With some modification, HP later released the design as an internal standard and called it HP-IB, which was later certified to become IEEE-488.
Insects are extremely elaborate behavorial machines, more elaborate than the most complex mechanical machine ever built by humans. Much of their behavior is driven by abiotic environmental stimuli (temperature, humidity, daylength, light levels, etc.). These chambers were designed to simulate those stimuli in an effort to tease apart the factors that promote the onset of such key behaviors such as ovipositional (egg-laying) rates.
In the graph above, the temperature was commanded to cycle sinusoidally precisely plus and minus 15 degrees Farenheit from the room's ambient 75 degrees while maintaining a relative humidity (atmospheric water saturation percentage) fixed at 85%. As you can see, control was nearly absolute. This level of precision control was accomplished by running small multi-stage refridgeration units (one per chamber) in opposition to air and water heaters. Although the technique was energy-expensive, the level of control necessary to the task demanded this design.
The sensors in each of the chambers were idiosyncratic, thus the HP9100 maintained a different calibration table for each temperature and humidity sensor in the chamber field. But it was the ease of generating elaborate polynomial equations using the HP9100's intrinsic scientific calculation capabilities for complex curve-fittings that made the HP9100 such an ideal controller.
Every calculator manufactured from HP following the 9100 incorporated a controller interface and became more and more elaborate in its capacities to control instruments. The later calculators incorporated a very fine BASIC language as their operating system. Nonetheless, for years following the HP9100, when HP advertised their new controller calculators, the example they always used in their advertisements was that of controlling environmental control chambers.