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{{Short description|First pocket scientific calculator}}
{{About|the first HP pocket calculator|the calculator introduced in 2007|HP 35s|the pistol|Browning Hi-Power}}
{{Use dmy dates|date=April 2019|cs1-dates=y}}
{{Infobox Calculator
| name         = HP-35
| image        = HP-35 Red Dot.jpg
| caption      = HP-35 (1st version)<br />scientific electronic [[pocket calculator]]
| type         = [[Scientific calculator|Scientific]]
| entry        = [[RPN input mode|RPN]]
| introduced   = 1972
| discontinued = 1975
| processor    =
| display_type = Red [[Light-emitting diode|LED]] [[seven-segment display]]
| display_size = 15 digits (decimal point uses one digit), (±10<sup>±99</sup>)
| prog_lang    =
| memory_data  = Four-register operational stack with one memory register.
| memory_step  =
| memory_rom   = 768 bytes x 10 bits 
| supply       = Internal rechargeable three cell battery or 115/230&nbsp;V&nbsp;AC, 5&nbsp;W
| weight       = Calculator: {{convert|9|oz|g|abbr=on}},<br />recharger: {{convert|5|oz|g|abbr=on}}
| dimensions   = Length: {{convert|5.8|in|mm}},<br />width: {{convert|3.2|in|mm}},<br />height: {{convert|0.7|–|1.3|in|mm}}
}}
The '''HP-35''' was [[Hewlett-Packard]]'s first [[pocket calculator]] and the world's first ''scientific'' pocket calculator:<ref>{{cite web|url=http://www.ieee.org/about/news/2009/14april_1.html |archive-url=https://archive.today/20120905152953/http://www.ieee.org/about/news/2009/14april_1.html |url-status=dead |archive-date=5 September 2012 |title=HP-35 Scientific Calculator Awarded IEEE Milestone |publisher=Ieee.org |access-date=2012-05-17}}</ref> a calculator with [[trigonometric functions|trigonometric]] and [[exponential function]]s. It was introduced in 1972.

==History==
In about 1970 HP co-founder [[William Reddington Hewlett|Bill Hewlett]] challenged his [[France Rode|co-worker]]s to create a "shirt-pocket sized [[HP-9100]]". At the time, [[slide rule]]s 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 {{US$|395|1972|fmt=eq|round=-2}},{{Inflation/fn|US}} like HP's first scientific calculator, the desktop [[Hewlett-Packard 9100A|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. It was priced at {{USD|59.99}}.<ref>{{cite web|url=http://www8.hp.com/us/en/hp-news/press-release.html?id=170133 |title=Retro HP 35s Launched to Commemorate 35th Anniversary of First HP Handheld Calculator |publisher=Hp.com |access-date=2012-05-17}}</ref>

The HP-35 was named an [[List of IEEE milestones|IEEE Milestone]] in 2009.<ref>{{cite web |url=http://www.ieeeghn.org/wiki/index.php/Milestones:Development_of_the_HP-35,_the_First_Handheld_Scientific_Calculator,_1972 |title=Milestones:Development of the HP-35, the First Handheld Scientific Calculator, 1972 |work=IEEE Global History Network |publisher=IEEE |access-date=4 August 2011}}</ref>

==Description==
[[Image:HP 35 pocket calculator main board.jpg|thumb|right|HP-35 mainboard]]

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 [[Light-emitting diode|LED]] display was capable of displaying a ten-digit [[Significand|mantissa]] plus its sign and a decimal point and a two-digit [[Exponentiation|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 [[Nickel-cadmium battery|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 ([[1-bit architecture|1-bit]]) processor [[chipset]] dual sourced  under contract from [[Mostek]] and American MicroSystems Inc (pictured), processing [[Decimal floating point]] numbers with 10 digit mantissa and 2 digit exponent, stored in 14 nibble (56 bit) numbers as [[binary-coded decimal|BCD]] including two nibbles for the signs.

{| class="wikitable plainrowheaders"
|+ HP-35 functions
|-
! scope="row" | Arithmetic
| +, −, ×, ÷
|-
! scope="row" | Trigonometry
| sin, arc sin; cos, arc cos; tan, arc tan (decimal degrees)
|-
! scope="row" | Logarithms
| {{math|log<sub>10</sub>&thinsp;''x''; log<sub>e</sub>&thinsp;''x'', ''e<sup>x</sup>''}}
|-
! scope="row" | Other
| {{math|1/''x'', {{sqrt|''x''}}, ''x<sup>y</sup>'',}} {{pi}}
|}

The calculator had a four-register [[Stack-based memory allocation|stack]] (x, y, z and t). The stack was represented in the operating manuals with the t register at the top, followed by the registers z, y, and x. The "enter" key pushed the displayed value (x) up the stack. Any binary operation popped the bottom two registers and pushed the result. When the stack was popped, the t register duplicated into the z register. The ‚roll down‘ operation treated the stack like a ring buffer by ‚rolling down‘ the register contents from t to z, z to y, y to x, and x to t.

==Descendants==
The HP-35 was the start of a family of related calculators with similar mechanical packaging:
* The [[HP-45]] added many more features, including the ability to control the output format (rather than the purely automatic format of the HP-35). It also contained an undocumented timer feature. The timer worked, but was not accurate enough to use as a [[stopwatch]] due to lack of a [[crystal oscillator]].
* The [[HP-65]] added programmability, with program storage on magnetic cards.
* The [[HP-55]], a less expensive follow-on to the HP-65, provided storage for smaller programs, but didn't provide any external storage. The timer that was already present on the HP-45 was now crystal-controlled to achieve the needed accuracy and explicitly documented.
* The [[HP-67]] expanded on the programmability of the HP-65, and added fully merged keycodes.
* The HP-80 and cheaper HP-70 provided financial, rather than scientific functions, such as future value and net present value.

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-41]]C 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 ([[Read-only memory|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.

== Calculator trivia ==
* The HP-35 was {{convert|5.8|in|mm}} long and {{convert|3.2|in|mm}} wide, said to have been designed to fit into one of William Hewlett's shirt pockets.
* Was the first scientific calculator to fly in space in 1973.<ref name="autogenerated1">{{cite web|url=http://h20331.www2.hp.com/Hpsub/cache/392617-0-0-225-121.html |title=Hewlett-Packard Calculator Firsts |publisher=Hewlett-Packard |archive-url=https://web.archive.org/web/20061105022711/http://h20331.www2.hp.com/Hpsub/cache/392617-0-0-225-121.html |archive-date=5 November 2006 |url-status=dead}}</ref>
* HP-35 calculators were carried on the [[Skylab 3]] and [[Skylab 4]] flights, between July 1973 and February 1974.<ref>{{Cite news|url=https://airandspace.si.edu/collection-objects/calculator-pocket-electronic-hp-35/nasm_A19850582000|title=Calculator, Pocket, Electronic, HP-35|date=2016-03-14|work=National Air and Space Museum|access-date=2022-02-26|language=en}}</ref>
* Is the first pocket calculator with a numeric range that covered 200 decades (more precise 199, ±10<sup>±99</sup>).<ref name="autogenerated1"/>
* The LED display power requirement was responsible for the HP-35's short battery life between charges&mdash;about three hours. To extend operating time and avoid wearing out the on/off slide switch, users would press the decimal point key to force the display to illuminate just a single LED junction.
* The HP-35 calculated arithmetic, logarithmic, and trigonometric functions but the complete implementation used only 767 carefully chosen instructions (7670 bits).
* One high-quality feature of the HP-35 was its use of double-injected keys. Rather than printing the function on the key surface where it could wear off over time and use as with cheaper calculators, the keys were constructed with two colors of plastic, providing durable key top labels for the labeled keys.
* Introduction of the HP-35 and similar scientific calculators by [[Texas Instruments]] soon thereafter signaled the demise of the slide rule among science and engineering students. Slide rule holsters rapidly gave way to "electronic slide rule" holsters, and colleges began to drop slide-rule classes from their curricula.
* 100,000 HP-35 calculators were sold in the first year, and over 300,000 by the time it was discontinued in 1975—3½ years after its introduction.<ref>{{cite web|url=http://www.hp.com/hpinfo/abouthp/histnfacts/museum/personalsystems/0023/ |title=HP Virtual Museum: Hewlett-Packard-35 handheld scientific calculator, 1972 |publisher=Hp.com |access-date=2012-05-17}}</ref>

== See also ==
* [[CORDIC]]
* [[France Rode]]
* [[Sinclair Scientific]]

== References ==
{{reflist}}

== Further reading ==
* {{cite book |title=Introduction to Computer Architecture |author-first=Thomas M. |author-last=Whitney |author-link=Thomas Whitney (computing) |editor-first=Harold Stuart |editor-last=Stone |editor-link=Harold Stuart Stone |date=1975 |edition=1 |chapter=Part I. Basic Computer Architecture. / Chapter 3. Introduction to Calculators: / 3-5. Example Systems / The Hewlett Packard HP-35 |series=Computer Sciences Series |publisher=[[Science Research Associates, Inc.]] (SRA) |publication-place= |id=ark:/13960/t8pc40t3q. Order-Code 13-4005 |lccn=75-14016 |isbn=0-574-18405-8 |pages=118–135 [123–135] |chapter-url=https://archive.org/details/introductiontoco00ston/page/123/mode/2up?view=theater |access-date=2023-09-29 |quote-page=124 |quote=[…] The HP-35 is a totally [[serial computer]]. The adder is a [[Binary-coded decimal|BCD]] serial type […] The serial structure means less integrated circuit area must be allocated to interconnection lines and gating functions and an interesting trade off occurs. A bit-serial, digit-serial architecture is inherently one fourth the speed of a bit-parallel digit-serial structure […] But the basic clock rate for a bit-serial structure can sometimes be increased since additional area can be allocated for larger integrated devices that are necessary for greater speed. In the HP-35, the execution time of the most complex functions is under one second, while the serial architecture permits an increased circuit complexity. […] Instructions in the HP-35 are transferred serially from the active read-only memory to the arithmetic and control circuits and to other ROMs if present. […]}}
* [http://codex99.com/design/the-hp35.html CODEX 99 'The HP-35 Consumer Electronics, an Origin Story']
* [http://www.hpmuseum.org/hp35.htm The Museum of HP Calculators' article on the HP-35]
* {{cite journal |url=http://www.hpl.hp.com/hpjournal/72jun/toc-06-72.htm |title=HP35 edition |journal=HP Journal |date=June 1972 |volume=23 |issue=10}}
* {{cite web |url=http://home.citycable.ch/pierrefleur/Jacques-Laporte/HP%2035%20Saga.htm |first=Jacques |last=Laporte |title=The slide rule killer: a milestone in computer history |access-date=2016-01-01  |archive-date=2022-02-11  |archive-url=https://web.archive.org/web/20220211065021/http://home.citycable.ch/pierrefleur/Jacques-Laporte/HP%2035%20Saga.htm |url-status=dead }} A thorough analysis of the HP-35 firmware including the CORDIC algorithms and the bugs in the early ROM.
* [http://www.hp9825.com/html/osborne_s_story.html Tom Osborne's Story in his own words - the invention of the HP 9100, HP 35 and HP 65]
* {{cite video |first=Steven |last=Leibson |title=Interview with Dave Cochran about the HP 35 Calculator |url=https://www.youtube.com/watch?v=0IlSjfgrHVM |archive-url=https://ghostarchive.org/varchive/youtube/20211212/0IlSjfgrHVM |archive-date=2021-12-12 |url-status=live |date=2007}}{{cbignore}}

==External links==
* [http://mycalcdb.free.fr/main.php?l=0&id=770 HP-35] pictures on [http://mycalcdb.free.fr MyCalcDB] (database about 1970s and 1980s pocket calculators)
* [http://hp35.wz.cz/HP-35RD.html A list of earliest HP-35 "Red Dot" calculators]
* [https://neil.fraser.name/software/hp-35/ HP-35 Calculator Simulator (JavaScript)]
* [https://github.com/AshleyF/HP35 HP-35 Calculator Emulator (JavaScript; original ROM or bug-fixed ROM)]

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[[Category:HP calculators|35]]
[[Category:Computer-related introductions in 1972]]
[[Category:Serial computers]]