The early history of HP calculators

A calculator collector reflects on the early 1970s, when HP's programmable calculators revolutionized the everyday work of engineers, technicians, and financial professionals. Oh, and computer scientists, too.

At the current rate of rainfall, when will your local reservoir overflow its banks? If you shoot a rocket at an angle of 60 degrees into a headwind, how far will it fly with 40 pounds of propellant and a 5-pound payload? Assuming a 100-month loan for \$75,000 at 5.11 percent, what will the payoff balance be after four years? If a lab culture is doubling every 14 hours, how many viruses will there be in a week?

Those sorts of questions aren’t asked by mathematicians, who are the people who derive equations to solve problems in a general way. Rather, they are asked by working engineers, technicians, military ballistics officers, and financiers, all of whom need an actual number: Given this set of inputs, tell me the answer.

Before the modern era (say, the 1970s), these problems could be hard to solve, requiring a lot of pencils and paper, a book of tables, or a slide rule. Mathematicians never carried slide rules, but astronauts did, as their backup computers.

Everything the slide rule could do, a so-called slide-rule calculator could do better—and more accurately. Slide rules are really good at few things. Multiplication and division? Easy. Exponents, like 613? Easy. Doing trig, like sines, cosines, and tangents? Easy. Logarithms? Easy.

However, slide rules had limitations. They were good to about three digits of accuracy, no more, in the hands of a skilled operator. Three digits was fine for real-world engineering, but not enough for finance. With slide rules, you had to keep track of the decimal point yourself: The slide rule might tell you the answer is 641, but you had to know if that was 64.1 or 0.641 or 641.0. And if you were chaining calculations (needed in all but the simplest problems), accuracy dropped with each successive operation.

Hewlett-Packard unleashed a monster when it created the HP-9100A desktop calculator, released in 1968 at a price of about \$5,000. The HP-9100A did everything a slide rule could do, and more—such as trig, polar/rectangular conversions, and exponents and roots. However, it was big and it was expensive—about \$35,900 in 2017 dollars, or the price of a nice car! HP had a market for the HP-9100A, since it already sold test equipment into many labs. However, something better was needed, something affordable, something that could become a mass-market item. And that became the pocket slide-rule calculator revolution, starting off with the amazing HP-35.

The earliest days: Basic slide-rule calculators

If you look at the HP-35 today, it seems laughably simplistic. The calculator app in your smartphone is much more powerful. However, back in 1972, and at a price of only \$395 (\$2,350 in 2017 dollars), the HP-35 changed the world. Companies like General Electric ordered tens of thousands of units. It was crazy, especially for a device that had a few minor math bugs in its first shipping batch (HP gave everyone a free replacement).

The HP-35 was Hewlett-Packard’s first slide-rule pocket calculator. This one was a wedding present from my father-in-law. Note that exponents are uniquely calculated as xy with the HP-35. Yes, the nameplate fell off and is lost.

What did the HP-35 do? Well, basic slide-rule operations, plus addition and subtraction (something that slide rules didn’t do). The calculator performed exponents (uniquely in HP calculators, performed as xy instead of yx), roots, logarithms, roots, sine, cosine, tangent—plus the invest arcsine, arccosine, and arctangent. All this at an amazing eight digits of accuracy.

I am fortunate enough to have an HP-35. It was given to me as a wedding present by my father-in-law, who purchased it during his career as a naval and aviation architect. (And yes, that was a suitable wedding present—at least for techies!)

The market for basic slide-rule calculators was huge. A year later, HP offered an upgraded model, the HP-45, which approached the power of the HP-9100A desktop calculator. The HP-45 was introduced at the same price of \$395; the HP-35 dropped to \$295.

What was new? Stuff for more sophisticated computations, including those using polar coordinates, and trig in radians and gradians (1/400 of a circle), not only degrees (1/360 of a circle). The HP-45 could do factorials (n!) and simple unit conversations, such as liters to gallons, and decimal degrees to degrees/minutes/seconds. However, to me, the most impressive addition was basic statistics. Enter a succession of numbers, and the HP-45 would give you the average and the standard deviation.

The HP-45 was a more advanced version of the HP-35; the HP-55 was a programmable version that didn’t sell well. This HP-55 may be the last HP first-generation calculator manufactured. It was assembled from spare parts and gifted to me by HP’s Corvallis division after I keynoted a conference for its calculator design team.

This was the end of the slide rule.

Also in 1973, HP recognized the financial community with the HP-80, which focused on compound interest, also known as the Time Value of Money. Tell it four of these factors, and it computes the fifth: number of payments, interest rate, payment amount, present value, and future value. This technology allowed bankers and investors to put away their books of tables; no longer did they have to call the computation department to get values that didn’t fit their tables.

Alan’s HP calculator obsession

My collection of HP calculators began with the first model that I purchased, brand new, at my university book store. It was the HP-27, a second-generation model introduced in 1976 for \$200. The HP-27, still my favorite everyday calculator, did slide-rule math, statistics, and financial computations.

Two second-generation models. The HP-29C had “continuous memory,” meaning it didn’t lose its memory when you turned it off but would if you took out the battery without plugging it in first. Otherwise, it was similar to the HP-55.

Things got serious when I bought a used HP-65 card programmable calculator (more about this shortly) from an engineer who upgraded to the HP-67. That model introduced me to calculator programming, which augmented my studies as a mathematics and computer science student. Later, I bought an HP-45, and the collecting began in earnest, with a focus on the first-generation models (the focus of this article), and the second-generation ones, which were more powerful, smaller, and less expensive. I am fortunate to own a complete set of first-generation calculators: the HP-35, HP-45, HP-55, HP-65, HP-67, HP-70, HP-80, HP-91, HP-92, and HP-97. A big source of used calculators was the Lawrence Livermore Laboratories, which sold off a lot of equipment in the mid-1980s. Alan scores!

Programmability and program persistence appeared in the HP-65, which also started my passion for programmable calculators. The HP-67 was a more powerful, less expensive version. Look at all those functions!

I also managed to score a very rare printing version of the HP-45, which was a huge desktop device called the HP-46. It came in its own suitcase carrying case, like a sewing machine or portable typewriter of that era. It’s huge, it’s noisy, and it works. Alas, there’s no room on my desk for the HP-46, so it lives in a closet.

My last first-gen calculator acquired was the commercially unsuccessful HP-70. It was a stripped-down financial calculator that was replaced by a second-generation model that did more for less.

The HP-80 was a powerful calculator for financial professionals, focused on the Time Value of Money. It’s the only model that had “SAVE” instead of “ENTER.” The HP-70 is a rare stripped-down finance model that didn’t sell well. It’s the only first-gen with a light-colored case, and it was the final piece of my first-gen collection.

I treasure all these and have worked hard to keep them running, which means constantly rebuilding their NiCad battery packs. (I also have all the second-generation models, but frankly, they’re not as interesting, since they lack any historical context.)

The next step: Programmability with persistent storage

When you’ve got powerful slide-rule and statistics functions in a pocket calculator like the HP-45, how do you make it better? Sophisticated computation requires chains of operations to solve complex equations. To make those computations faster and less susceptible to keystroke errors, these chains allow users to program the keystroke sequence and run them over and over with different input parameters.

That capability—a version of which existed in that old HP-9100A desktop calculator—appeared in 1974 with the new HP-65, which cost \$795 new. Not only was the HP-65 programmable (with the ability to record and perform up to 100 program “steps”), but it offered persistence storage for programs. Remember, this was the era when if you turned a piece of electronics off, it was off. It forgot everything.

With the HP-65, you could record your programs on small magnetic cards and store them for later. Then, when you needed to redo that calculation, you could reload the program from the mag card. The programs also let you do limited if-then branching—that is, if the result of this calculation is less than 0, jump to a labeled line to continue. In a word, wow.

The HP-65 ushered in a new era, with publications like HP Key Notes offering programming articles and sample code. HP and third parties offered libraries of cards for common problem domains, such as aviation, navigation, finance, medicine, and electrical engineering. Several of my programs were published by HP.

While there were some programmable models that didn’t offer persistence, like the not-great-selling HP-55 (a programmable version of the HP-45), the ability to store programs was revolutionary. (HP’s arch-competitor in this era, Texas Instruments, offered a similar capability in its SR-52 calculator.) In an era before VisiCalc (introduced in 1979), or Mathematica (1988), if you wanted to do sophisticated math and equation-solving, you did it on a card-programmable calculator.

The HP-65, while incredible, was limited, as it did not have much program space. It was succeeded in 1976 with the pinnacle of early calculator power: the HP-67, which had 224 program steps, better code editing and debugging, nested subroutines, better comparisons and branching, more memories, and indirect addressing (i.e., branch to the step number stored in a register). The HP-67 could also read/write data, as well as programs, from the mag cards. In addition, it could load and merge partial program data from cards, allowing for very sophisticated programs.

The HP-67 was paired with the simultaneously released HP-97, which did everything the HP-67 did but with a built-in printer. Program cards were fully interchangeable between the models. What’s more, they were much less expensive: \$450 for the HP-67 and \$750 for the HP-97—less than the HP-65 at launch.

The printing version of the HP-67; it was fully compatible with the HP-67’s mag cards. Look at all those buttons! HP made similar-looking slide-rule and financial printing calculators.

Around this time, HP brought out a few other models, including the HP-91, a simple printing slide-rule calculator, and the HP-92, a printing financial calculator. However, with the HP-67/97, there was a pinnacle of sorts in HP’s calculators, but only for a few years.

All of these were rendered totally obsolete in 1979 by a revolutionary new platform: the HP-41C. This model did everything the earlier calculators did and also included a large alphanumeric LCD screen, a battery life measured in weeks instead of hours, and tons of accessories—including a card reader that could import and run HP-67/97 programs. The HP-41C also introduced expansion ports, which would handle not only memory and preprogrammed program chips, but also accessories like printers, bar-code readers, and even a tape drive. The “C” in the HP-41C also indicated what HP called continuous memory; it didn’t lose its memory when you turned it off. That was huge!

Variations of the HP-41C platform were sold until 1990, and who knows how many millions of programs were run on these calculators. It’s still not uncommon to find an HP-41C (or its two identical-looking successors, the HP-41CV and HP-41CX, which added some more functions) on someone’s desk. The earlier ones? You’ll only find them in the hands of collectors like yours truly.

Just like the slide rule and the dodo

The era of these calculators is basically over. Yes, nearly every banker uses an HP-12C calculator every day. Yes, students use powerful graphing calculators from HP, Texas Instruments, or some other companies to do math. Those are specialized cases, however. For real computation, of the sort that drove engineers to slide rules and then to calculators, we have better tools.

Much of the math that people need to do can be performed using Microsoft Excel, Google Sheets, or the equivalent. For more powerful math, there’s Wolfram Research’s Mathematica. Thanks to cloud computing, you can run all of these in the cloud—that’s where Google Sheets is, of course, as well as Office 365 and Wolfram Cloud.

Not to mention smartphones. There are special-purpose mobile apps for computation—and you can even download and use apps that simulate the look and functionality of your favorite first-generation HP calculator. Right now, my phone has apps that simulate the HP-70 (a budget version of the HP-80), HP-45, and HP-41C. So no matter where I go, my calculator collection sort of goes with me.

That’s comforting, in case I need to suddenly calculate when, at the current rate of rainfall, the local reservoir will overflow its banks.