
HP-35

The HP-35 was Hewlett-Packard's first pocket calculator and the world's first "scientific" pocket calculator: a calculator with trigonometric and exponential functions. It was introduced in 1972.
History.
In about 1970 HP co-founder Bill Hewlett challenged his co-workers to create a "shirt-pocket sized HP-9100". 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. Introduced at $395 (), like HP's first scientific calculator, the desktop 9100A, it used Reverse Polish Notation (RPN) rather than what came to be called "algebraic" entry. The "35" in the calculator's name came from the number of keys.
The original HP-35 was available from 1972 to 1975. 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 HP-35 was named an IEEE Milestone in 2009.
Description.
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 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 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. Light emitting diodes were relatively new at the time and were much dimmer than high efficiency diodes developed in subsequent decades.
The calculator used three "AA"-sized NiCd batteries assembled into a removable proprietary battery pack. 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. An external battery charger was available and the calculator could also run from the charger, with or without batteries installed.
Internally, the calculator was organized around a serial (one-bit) processor chipset made under contract by Mostek, processing 56-bit floating-point numbers, representing 14-digit BCD numbers.
The calculator had a four register stack (x, y, z and t), the 'enter' key pushed the displayed value (x) down the stack. Any binary operation popped the top two registers, and pushed the result. When the stack was popped the t register duplicated into the z register.
Descendants.
The HP-35 was the start of a family of related calculators with similar mechanical packaging:
Follow-on calculators used varying mechanical packaging but most were operationally similar. The HP-25 was a smaller, cheaper model of a programmable scientific calculator without magnetic card reader, with features much like the HP-65. The HP-41C was a major advance in programmability and capacity, and offered CMOS memory so that programs were not lost when the calculator was switched off. It was the first calculator to offer alphanumeric capabilities for both the display and the keyboard. Four external ports below the display area allowed memory expansion (RAM modules), loading of additional programs (ROM modules) and interfacing a wide variety of peripherals including HP-IL ("HP Interface Loop"), a scaled-down version of the HPIB/GPIB/IEEE-488 instrument bus. The later HP-28C and HP-28S added much more memory and a substantially different, more powerful programming metaphor.