Soemtron 220

Electronics

Discrete Diode-Transistor Logic

Architecture, Memory & Speed

15 displayed digits, 1 sign indicator and one decimal position digit = 16 digit word
Digits encoded as 4-bit BCD, digit-serial processing over 4-bit internal bus
Core memory, 6 registers x 16 digits x 4 bits = 384 cores
25 kHz master clock

Construction

The Soemtron 220 is constructed in a very solid fashion with a full die-cast chassis and case, giving the impression of a mchine designed for many years of continuous service.

Internal construction is quite modular with the power supply, logic cage, keyboard and display being separate and individually removable with plug connections between each.

The power supply is transformer based with series pass regulation of main logic levels. A thermistor mounted beside the core array regulates the core drive voltages.

The logic is contained on twelve very densely packed double sided phenolic boards. The component sides are printed with component identifications and while the component density is often very high the components are at least all mounted visibly and in contact with the board surface. This makes identification and circuit tracing much easier than with some of the more anarchic boards from other manufacturers.

The keyboard uses industrial microswitches but unlike Wang machines where these are directly actuated, the Soemtron uses spring loaded keystems that provide a longer and more satisfying keypress. The keystems also feature an item of low-tech, not seen in an essential role in any other calculator - a length of string, laced through all of the keystems to prevent them exceeding the limit of their upstroke and falling out.