![]() ![]() The HP-35 was the start of a family of related calculators with similar mechanical packaging: When the stack was popped, the t register duplicated into the z register. Any binary operation popped the top two registers and pushed the result. The calculator had a four-register stack (x, y, z and t), the "enter" key pushed the displayed value (x) down the stack. ![]() Sin, arc sin cos, arc cos tan, arc tan (decimal degrees) Internally, the calculator was organized around a serial ( 1-bit) processor chipset made under contract by Mostek, processing 56-bit floating-point numbers, representing 14-digit BCD numbers. An external battery charger was available, and the calculator could also run from the charger, with or without batteries installed. Replacement battery packs are no longer available, leaving existing HP-35 calculators to rely on AC power, or their users to rebuild the battery packs themselves using available cells. The calculator used three "AA"-sized NiCd batteries assembled into a removable proprietary battery pack. Light-emitting diodes were relatively new at the time and were much dimmer than high-efficiency diodes developed in subsequent decades. The display used a unique form of multiplexing, illuminating a single LED segment at a time rather than a single LED digit, because HP research had shown that this method was perceived by the human eye as brighter for equivalent power. The fifteen-digit LED display was capable of displaying a ten-digit mantissa plus its sign and a decimal point and a two-digit exponent plus its sign. The calculator used a traditional floating decimal display for numbers that could be displayed in that format, but automatically switched to scientific notation for other numbers. The HP-35 was named an IEEE Milestone in 2009. In 2007 HP announced the release of the "retro"-look HP 35s to commemorate the 35th anniversary of the launch of the original HP-35. The original HP-35 was available from 1972 to 1975. ![]() The "35" in the calculator's name came from the number of keys. Introduced at US$395 (equivalent to $2,559 in 2021), like HP's first scientific calculator, the desktop 9100A, it used reverse Polish notation (RPN) rather than what came to be called "algebraic" entry. At the time, slide rules were the only practical portable devices for performing trigonometric and exponential functions, as existing pocket calculators could only perform addition, subtraction, multiplication, and division. In about 1970 HP co-founder Bill Hewlett challenged his co-workers to create a "shirt-pocket sized HP-9100". ![]()
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