**Last Update: January 22, 2003 -- THE HP REFERENCE**

What do you do with your old calculator if you want to buy a new one, such as the HP48GX? Some people sell the old one to help pay for the new one. Others keep it; as a spare, to use while they learn the new one, or out of nostalgia.

A few HPCC members do more than keep their old calculators -- they actually collect them. Even if you do not collect old HP calculators yourself, it is worth knowing about them -- if you come across one it might be worth buying; it is a shame to see it thrown away, and you could sell it to someone who does collect them -- though it will not get you 7 million pounds at Christie's!

If you do find an old HP calculator -- in your office, maybe, or in a second-hand shop, or offered to you by someone who no longer needs it, you will have three questions. "Is it worth getting it?" "How much is it worth?" "How can I sell it if I don't want it?" I shall give general answers here to all three questions, but Les says a series of short articles about old HP calculators and their history would be useful for future issues of DATAFILE, so I shall write some such articles as well as this one; if you have a favourite old HP calculator and can write a article about it for DATAFILE then please do so!

The first question was "Is it worth getting it?" and the answer is nearly always "Yes". If you see an old HP calculator in a second-hand shop, try to buy it -- for yourself or another HPCC member! Naturally, the price demanded will affect your decision, but at a low price, say about #10 for a working calculator, or #5 for a broken one, it is definitely worth getting. Even the HP28C and HP18C which were both soon followed by improved versions, are worth getting if cheap enough; I am looking for a spare HP28C just now! Old manuals and accessories are worth getting too, though some HP-41 modules such as the Games module or normal memory modules are not worth much.

"How much is it worth?" is the most difficult question. No-one publishes a catalogue of prices, as is done for stamp or coin collectors! Too much depends on the condition, the rarity, how much the vendor wants, and how badly a potential customer wants it. If you have no idea of the value, try talking to one of the people named in the next paragraph. Some models particularly worth looking out for are as follows. The HP-01 calculator/watch is especially interesting, whether in a steel case or in a gold-plated one, because of the model number, and because it is a watch as well as a calculator. All the desktop and handheld models with a built-in printer are worth looking for, in particular the HP-97S which is a rare version of the HP-97, and the HP-95 of which some prototypes were made, but which never went into mass production. If you find any of these, buy first and ask questions afterwards! Other models which seem to be hard to get include the HP-70 and the HP-22 (both are financial models), the HP-94 (any version) and the HP-10C.

Thirdly, "How can I sell it if I don't want it?" Colin Crowther who runs our library of books and old equipment can often help. So can I, because I write books about HP calculators, and therefore collect old ones. Colin and I also know other HPCC members who collect old HP calculators. You can also advertise in DATAFILE, or on The No Zone, or other electronic bulletin boards. So, check out that old HP calculator in your local junk shop and get in touch!

Just after I had written this article, I received a letter with the first issue of a small journal called "The International Calculator Collector". This is aimed at people who collect, or are interested in the history of, early handheld electronic calculators. By "early", they mean calculators with displays of types earlier than the current LCDs (liquid crystal displays). They invite subscriptions (US$12 for airmail delivery of the journal outside the USA, $8 in the USA). They also invite articles about calculators. The address is: International Association of Calculator Collectors, 1212 South Parton St., Santa Ana, CA 92707, USA. Maybe a collectors' catalogue will soon become available, contrary to the statement in my article!

Introduced on July 1 1972, this was the first handheld electronic calculator sold by HP, and the first ever to perform logarithmic and trigonometric functions with one keystroke. As opposed to later HP calculators, it has an x^y function, not y^x, and the trigonometric functions work in degrees only. The story goes that it was made after William Hewlett was shown a new scientific desktop calculator by his engineers, and asked for a version to fit in his shirt-pocket. At first, HP thought they would only make a few HP-35s for their own engineers, as no-one else would be interested. Then they decided to try selling it -- and sold hundreds of thousands. This means that the HP-35 is not particularly rare, but collectors will pay a good price for one because the HP-35 was the first HP handheld. We celebrated its twentieth anniversary last year, and articles about it were published in the proceedings of our 1992 conference and in DATAFILE V12N2. The latter article includes details of the three different types of HP-35 made.

Introduced on February 1 1973, this was the second handheld electronic calculator sold by HP, and the first business model. The HP-80 was introduced for financial users following the success of the HP-35 among scientific and technical users. It has the same physical design as the HP-35, and, like early HP-35s, early HP-80s do not have a model number on their label, only the words "HEWLETT-PACKARD". The HP-80 was the first HP calculator with business functions including the command row "n i PMT PV FV" below the display. It also had date arithmetic, bond and depreciation calculations, statistics functions including E+ and E-, and control of the display mode. It had a shift key to allow 11 extra functions to be placed on shifted keys. The ENTER key was labelled SAVE -- to avoid confusing business users! It had registers designed for use for specific business and statistics functions, but it also had STO and RCL buttons which allowed the user to store and recall numbers directly to and from a separate register. The x^y function of the HP-35 was replaced by y^x. Thus the HP-80 introduced many of the features which have since become standard on both scientific and business handheld calculators made by HP. The HP-80 is not particularly rare, but is of interest to collectors because it is HP's first business handheld.

Introduced on May 1 1973, this was the third handheld electronic calculator sold by HP, and the second scientific model. It built on the success of the HP-35, adding many new features. Some were taken from the HP-80, others were completely new, including the LAST X command, numbered data registers, store and recall register arithmetic, built-in conversion constants, and selection of trigonometric modes (DEG, RAD, GRAD). It is the first handheld known to have had a code name -- "Wizard". Early models had a built-in timer mode which could be accessed by bouncing the HP-45 on a springy mattress (!) or by pressing the RCL key and then pressing CHS, 7 and 8 all at once. This would start the timer, using the contents of register X as the initial time value. If your HP-45 has this mode, then while the timer mode is selected, you can press CHS to start or stop the timer, ENTER to quit the timer and clear the display, and to quit and leave the time in the display. Keys 1-9 store the current time in the numbered register if the timer is running, or recall the time from that register if the timer is stopped. CLX resets the timer to zero, EEX turns the display of hundredths of a second on or off, and E+ stops the timer. This timer was originally designed for use in the HP-45, but needed a crystal oscillator to provide sufficient accuracy. In the end, the crystal was left out of the HP-45, but early models still had the software built in, though it was not very accurate. The timer was later built into the HP-55. The HP-45 is a favourite among collectors; especially HP-45s with the timer. There was a large desktop version of the HP-45, with a built-in printer, called the HP-46.

Introduced on January 19 1974, this was the fourth handheld electronic calculator sold by HP, and the third scientific model. "Superstar" was its code name, and the HP-65 lived up to this name; it was the first programmable model, which led to its being dubbed "the world's first handheld computer". It had many new functions -- not just the functions needed for programming. It also had a built-in device for reading and writing magnetic cards; this allowed the user to save programs (but not data) on cards for future use, and to get programs on cards from HP, from other users, or from the HP User Library. HP sold sets of programs on particular topics, bundled together as "program pacs". The name "pac" lasted right through to the HP-41, for which a "program pac" usually meant a plug-in module. The arrival of the HP-65 prompted Richard Nelson to set up PPC, the first club for users of HP handhelds; indeed it is possible that this was the world's first club for users of computers, as opposed to the few clubs which already existed for people who were building their own micros at the time. Despite its many new features, the HP-65 was still very similar to the HP-35; obvious differences were the card reader, the slot to hold a card below the display (this allowed the user to write labels, on the card, for the row of keys directly below the card and the display -- the row of menu keys below the HP48 screen is a modern version of the same thing), and the second slider switch; to select program or run modes.

Introduced on August 1 1973, this was the fifth handheld electronic calculator sold by HP, and the second business model. To be specific, the first one -- the HP-80 -- was called a "financial" calculator whereas the HP-70 was called a "business" calculator. In practice this meant that the HP-70 had fewer functions than the HP-80, and was $120 cheaper than the HP-80 had been when introduced. The HP-70 does have some innovations -- for example there is a key for display setting control -- the display settings could be controlled on the HP-80 too, but there was no key specially for this purpose. The double-width key is now called ENTER, not SAVE as on the HP-80. There is a memory register for accumulating values -- this is the first (and only?) HP handheld with an M+ button. There is a second register, which contains the number 12 when the HP-70 is turned on -- for use in calculations for a fixed number of intervals -- usually 12 months in a year. These changes, and the manual which contains lots of example calculations, and even the codename "Scrooge" suggest that the HP-70 was introduced as a cheaper business model to fight competitors. It is not clear how successful this was -- the HP-70 is somewhat rare!

A lot of the information in these short articles about HP calculator history comes from an extensive database set up by Craig Finseth. Craig allowed us to include this database on the Goodies disk which we produced for the Anniversary Conference -- it is also available by anonymous ftp from various computer archive sites. The database contains an introduction, a list of HP's handheld electronic calculators sorted by the date they were introduced, an evolution chart, a summary of various features, a few other files, and finally a file for every HPnn calculator, where nn is any number from 01 to 99 for which at least one HP handheld or desktop model exists. (What about the HP100LX -- should that be added to the list since the HP95LX is already there?) I would strongly urge anyone who is remotely interested in the history of HP handhelds to get a copy of this database -- copies of the conference disk are still available. I am very grateful to Craig for all the work he put into collecting all these data, and to all the people who gave him information. My thanks to Craig as well for letting me use information from the database for this set of historical articles!

The first few HP handheld calculators (HP-35, HP-80, HP-45) were all known simply by a model number; they all had the same size and shape. But then HP introduced the smaller handhelds -- HP-21, HP-25 and so on. Soon after came the small desktop models with a built-in printer; the HP-91, the HP-- 97, and later the HP-92. (They are "small" compared to HP's older desktop calculators.) It became convenient to give each "family" of calculators its own name. The original family, or series, came to be known as the "Classic" series. The smaller models were called the series 20 or the "Woodstock" series. The small desktop printer models are the series 90 or "Topcat" series. The next series after that were the two handheld printer models, known as the "Sting" series, then came the series 30, or "Spice/Spike" series. As you cansee, some series are known by both a number and a name -- one more series like this is the series 10 or "Voyager" series; the HP-10C, HP-11C, HP-12C, HP-15C and HP-16C. Model numbers are not always a good guide to which series a calculator belongs to, though, so in these "history" articles series are usually referred to by their name, not by a number.

I have tried to keep these "history" articles short, but some will be a bit longer -- and this is one of them! It concerns those sometimes mysterious letters which we find after the model number of our HP handheld calculators and computers. The early HP handheld calculators were all known simply by a model number; HP-35, HP-80, HP-45, HP-65, HP-90, HP-25 and so on. At first, the model number was not even put on the calculator -- the early HP-35 and HP-80 calculators had only "Hewlett.Packard" on the label, later ones had "Hewlett.Packard 35" and so on. The first HP calculator with a letter as well as a number was the HP-25C -- this was a version of the HP-25 with "Continuous Memory", in other words the contents of memory were not lost when the calculator was turned off. The "C" obviously stood for "Continuous", in fact the HP-25C, its successor the HP-29C, and the HP-19C which was an HP-29C with a built-in printer, all had the words "Continuous Memory" written along the bottom of the keyboard.

The next model with a letter was the HP-97S -- this was a special version of the HP-97, with a serial interface added on -- you can guess what the S stood for! The first series of HP calculators which had a letter after the model number were the so-called "Spice/Spike" series, or series 30. The first four did not have continuous memory and all had the letter E after their model numbers, for example HP-31E or HP-38E. The next four did have continuous memory, and had a C after the model numbers, for example the HP-33C and the HP-38C. I assume that the E was there to distinguish the models without continuous memory from those with, but if anyone knows what the E stood for, please let me know! The next series of models were the HP-41C, the HP41CV, and then the HP-41CX. The "V" was a Roman five, to show that the HP-41CV had five times as many data registers as the HP-41C, and the "X" stood for "EXtended", because the HP-41CX was an HP-41CV with many extensions built in, especially Extended functions and Extended memory. This was the first time a model had two letters after the number. The HP-41CV was followed by the HP-11C and HP-12C, and soon after by the HP-15C, HP-16C and HP-10C.

The HP-10C was the first model which reused a model number -- there had already been an HP-10 -- a handheld with a printer built in -- a simpler version of the HP-19C. People therefore began to put the letter A after the names of models which had not had a letter, for example the HP-10 began to be called the HP-10A to distinguish it from the HP-10C (and later the HP-10B, the only case so far where the same number has been used for three different models.) Even the HP-35 came to be called the HP-35A. The next handheld model after the HP-10C was the HP-75C handheld computer, but this was soon followed by the HP-71B computer, and the HP-75D, a version of the HP-75C with a port for a barcode reader. I have read reports that the HP-75C had originally been planned to be called the HP-75D, presumably the D was to make it one better than a C. Then the original HP-75 became the HP-75C, but they already had HP-75D labels, and presumably used them for the later version. Once again, I would welcome details! The "B" of the HP-71B is to signify that it was a Basic language computer -- but then so were the 75C and 75D! The HP-94 handheld computer followed this pattern, with three models called the HP-94D, HP-94E and HP-94F, the D, E and F showing how much memory the three models had (64k, 128k and 256k). The desktop series 80 computers had just a model number -- HP-85, HP-81 and HP-83, but then an HP-85F was introduced; an HP-85 with extras, a bit like the HP-94F.

After the HP-94 series, HP went back to just a "C", with the HP-18C and the HP-28C, but these were replaced by the HP-19B and the HP-28S, where the "B" and "S" stood for Business and Scientific. After that, HP stuck to B and S for business models, such as the HP-10B, the HP-14B, the HP-20S, the HP42S and so on. (I have always thought that "Scientific" is a misnomer -- these are really "Technical" calculators, since they are used by mathematicians and engineers, not just scientists.) There seem to be two exceptions -- the HP-16C Computer Science calculator, where the C could stand for "Computer", and the HP21S Statistics / Maths calculator, where the S could stand for "Statistics". At this point, HP introduced a new twist -- the HP-19BII, the HP-17BII, and then the HP-32SII, all three were improved versions of earlier models without a II in the name. (In all three cases the version II could work with both RPN and algebraic notation.) The HP48SX went back to two letters after the name; they stand for Scientific EXpandable. The cut-down version of the HP48SX, without expansion ports, was therefore called the HP48S. Next in line came the HP95LX; the letters stand for Lotus EXpandable, since the 95LX has Lotus 1-2-3 built in. The later version, with 1 Megabyte of RAM memory, is called the HP95LX1MB -- the most complicated name so far. (Those readers who are keen photographers might ponder the fact that camera manufacturers seem to go for the same business of adding ever more letters to new model names -- though cameras are not the only other product identified by a number and a letter.) Finally, as I write this, HP have just introduced the HP100LX -- no change there -- and the HP48G and HP48GX -- these are new versions of the HP48S and HP48SX, redesigned to emphasise that they are Graphical calculators. Now, have you remembered all that? Then you'll be able to explain exactly what the name means if HP introduce an HP42XLII. Answers on a postcard, please!

The December 1992 meeting was our club's last chance to celebrate the 20th anniversary of the HP-35. I was sure HPCC members would like to have a close look at some HP-35s, and to push some keys on them, so I brought four to exhibit.

The HP-35 was the first handheld electronic calculator which could calculate transcendental functions automatically. Transcendental functions are those whose values cannot in general be expressed as solutions to an algebraic equation. (Isn't that why we use RPN instead? Sorry, silly joke.) If you want to know more about transcendental functions, look at the appendix to this article.

Several changes were made in the HP-35 design during the time it was made, before it was replaced by the HP-45. The HP-35s I brought showed these changes.

* The earliest 35s had a small hole to the right of the on/off switch. When the HP-35 was turned on, a red spot showed through this hole. This was soon removed -- you could see the HP-35 was on anyway, because the display lit up! Even if the batteries were low, you could see the switch was at the side marked ON.

* So long as the HP-35 was the only handheld calculator made by HP, there was no need to put the model number on it; the label below the keyboard just said "HEWLETT.PACKARD". The success of the HP-35 led HP to introduce the HP-80, the first business model, and a model number on the label became necessary. By the way, the HP-80 was a combined calculator and set of financial tables, in the same way as the HP-35 was really a combined calculator and set of log tables -- see the appendix for more about this. That is not at all surprising -- the first mechanical calculators, such as Babbage's machines, were designed specifically to print actuarial tables. Mind you, the HP-80 already had more features to exploit the availability of built-in functions, and even had sigma+ and sigma- keys.

* All HP-35s have the numeric digits, pi, and the operations + - * / and ENTER printed on the keys, but at first the other operations were printed above their keys, on the keyboard. Later models have the other functions on the keys as well. These later models also have the words OFF and ON moulded into the keyboard, not printed on it next to the on/off key. True HP-35 zealots will point out that these later models also had one silver-painted line below the display, not two.

* Early HP-35s had some bugs! (So, what's new? Or old?) I included an HP leaflet describing these bugs with my exhibits. One bug meant that typing:

gives the result 2. instead of 2.02 but this bug was soon removed. Another bug is that the sine of some small angles comes out completely wrong -- this was dealt with later than the other bug, but newer models have neither bug.

* At first the HP-35 was made only in the USA (at the Advanced Products Department in Cupertino -- the calculator division moved subsequently to Corvallis). Later on, the HP-35 was made in Singapore too, which is what HP do with all the handheld calculators they make.

* The electronics inside the HP-35 was updated as well during the lifetime of the product. I did not take any of these HP-35s apart to show this though!

All this should be very clear then -- a really early HP-35 have the red dot, will have the 2.02 bug, have key function captions above the keys, have a label which says "HEWLETT.PACKARD", and will have been made in the USA. This was not the case with the HP-35s on show, though! The one with the red dot did not have the bugs, and had a label saying "HEWLETT.PACKARD 35". The only one with the 2.02 bug was the latest model, with the new-style keyboard!

There are two explanations for this confusing state of affairs. First of all, some people sent their HP-35s to HP to be fixed. The ROM chips in these will have been updated, so the bugs are no longer there. At the same time, if the label was coming loose, it will have been replaced with a new one. (HP did the same to me once -- I sent in an HP-25 to be fixed -- it was returned with a note saying that they no longer had the required spare parts -- but the label had been overlaid with one which says HP-25C.) Secondly, the calculators were assembled from parts held in large bins -- it is quite possible that some older parts were used in the assembly of newer calculators.

Anyway, the four HP-35s exhibited had all these features between them -- a red dot and none, labels above keys and on them, bugs and no bugs, made in the USA and in Singapore. What's more, they all work -- 20 years later! That's HP quality for you!

I included some other items. Unlike modern HP calculators, the old models came in a plastic case, containing the calculator, a charger cable, the manual with a list of corrections, and even some "PROPERTY OF" labels so you could identify your calculator. You could buy spare battery packs and chargers. If you really wanted to secure your HP-35, you could buy a security cradle, which could be screwed down to a surface, or held in place with a security cable.

Well, that's all! I hope people had fun looking at them -- fortunately I got all four back at the end of the day -- not all of them were mine! (Many thanks to Dr Bob Speer of the Spectroscopy group in the Physics Department at Imperial College, where our club meets, who introduced me to the HP-35 more than 20 years ago, and who loaned me some of his 35 collection to show.)

Transcendental numbers are numbers which are the solutions of transcendental equations -- equations which transcend algebraic methods -- they cannot be solved purely by algebraic manipulation.

For example, SQRT(2) is not a transcendental number, because it is a solution to the algebraic expression:

SQRT(2) is an irrational number, because it cannot be expressed as the ratio of any two integers. The discovery that this is so caused ancient Greek mathematicians great grief, but that is another story.

On the other hand, pi is a transcendental number, because no expression can be written of the type:

to which x=pi is a solution, so long as a finite number n of terms is used. Pi can only be expressed exactly if n is allowed to be infinite.

Pi is the solution to the equation:

where C is the circumference of a circle and r is the radius, but getting pi exactly from this equation again requires an infinite number of steps. In this case, this is because measuring C exactly with a straight ruler requires that an infinite number of infinitesimally small pieces of the circumference be measured.

Why a straight ruler? A practical reason is that straight edges are comparatively easy to make -- and a straight edge can be converted into a ruler just by laying it next to another straight edge which has already been marked out, and copying the marks. The insistence on a straight ruler comes from the ancient Greek foundations of geometry, based on their philosophical notions, but in this case, the philosophy was based very sensibly on the practical limitations of their technology. Indeed the whole business of treating transcendental numbers as special in some way comes largely from philosophical notions.

In practice, calculating any irrational number, whether transcendental or not, is done by means of a series of repeated approximations. If you have only a simple four-function calculator then you can calculate SQRT(2) or pi by making a set of approximations. The earliest handheld electronic calculators could be used to calculate square roots and so on only this way. In fact, it was simpler to carry round a set of printed mathematical tables with your calculator, look up a square root, log, sine, or whatever, and type it into the calculator to use it. In effect the calculator had to be used with a book of tables. (These books were commonly called "log tables", though they usually contained tables of trigonometric functions and square roots as well.)

Then a few calculators were designed which could calculate percentages as well as carrying out the four basic operations + - * / . The step after came with calculators which automatically calculated square roots. This could be done because a short program was built into the calculator to make the required set of approximations. A program to do this can be very short, involving repeatedly halving the difference between the square of the current approximation and the number whose square root is to be found.

The HP-35 took the next step, providing keys to calculate not only square roots, but also transcendental functions -- trigonometric functions, and inverse trigonometric functions, exponentials and logarithms. Packing all this into a handheld calculator required far more programming! HP managed to fit their programs in a small, low-power, handheld unit by using an exceptionally clever and fast method of calculating these functions -- the Cordic technique. All HP calculators since the HP-35 have used this same technique to provide compact and fast (and therefore low-power) calculation. If you want to know more about the method, see the article from the HP journal which introduced the HP-35. [ Note from Craig: see the bibliography for another reference to an article on Cordic techniques. ]

It was this ability to calculate transcendental functions automatically and rapidly which made it a huge success, and led to HP's further advances in calculators. One thing to note is that the HP-35 was designed to work for the users who previously carried a table of functions with their four-function calculators. Instead of looking up a function and typing it on the calculator, the user could now type in a number and press a button to get the function value. The HP-35 was really a combined calculator and set of log tables -- it was only later models which began to go beyond this to a fuller exploitation of the things this made possible. But it was the HP-35 which began it all. Meditate transcendentally on that if you will!

After reading some of these articles about calculator history, you might decide to start your own collection. This article deals with two questions. 1. Why collect calculators? 2. How to start a collection?

Why, indeed, collect calculators? A few people who collect them run a museum, or a club library, or write books about them, and NEED such a collection. Some people collect them as an investment -- but collecting ANYTHING as an investment is dangerous -- once the investors outnumber the collectors, the market usually collapses, as has happened with stamps and antique paintings, for example. Most people collect things because they have inherited the collecting bug, or instinct. But then, why calculators? Well, to compare them with some other things people collect, they smell better than cheese labels, they take up less space than lawnmowers, and they are more exotic than stamps! Besides, if you are a member of HPCC then you already have an interest in HP calculators, and are well placed to collect them. In the long run, though, collecting calculators is likely to be just a hobby, and it might become an expensive one, so think carefully before you decide to collect them.

How do you start a collection? DATAFILE readers might well have a collection already -- many of us have older HP calculators which we have replaced with newer ones, but which we have never sold or given away. If you have a bunch of old calculators at home, this can form the basis of your collection, and it can determine the sort of collection you decide to set up. Deciding what sort of collection you want is important -- you should not begin with the aim of getting every single HP calculator ever made -- you would be unlikely to succeed, and the effort might bankrupt you! You could for example decide to specialise, collecting items related to one particular model, or one family of calculators. My suggestion is that a budding collector should begin with handheld calculators only, and try to get one calculator from each family.

This means you would ignore the computers, the printing calculators, and the watches. That cuts things down to a manageable level. Now, try to get just one model from the Classic family (HP-35, 45, 55, 65, 67, 70, 80). Similarly, try to get one model from the Woodstock family (HP-21, 22, 25, 25C, 27, 29C). The same goes for the Spice/Spike family (31E, 32E, 33E, 33C, 34C, 37E, 38E, 38C), the Woodstock family (HP-10C, 11C, 12C, 15C, 16C), the Champion family (HP-18B, 19B, 19BII, 28C, 28S) and the Pioneer family (HP10B, 14B, 17B, 17BII, 20S, 21S, 22S, 27S, 32S, 32SII, 42S). The three HP-41 models (HP-41C, 41CV, 41CX) also make up a family, as do the four HP-48 models (HP48S, 48SX, 48G, 48GX). You might like to add an HP-71B, since it is both a computer and a calculator, and you can get one for less than #20 from Tuscan Consultants at the moment.

That's a minimum of nine! If this is too many, you could choose to collect only older models, say ones with LED displays (the ones with shining red numbers). If even this seems a bit much, you might decide the whole thing is a bad idea. Don't give up at once though; you might be able to swap calculators you already have for others, or pick up some old ones from friends or local secondhand dealers. I'll make more suggestions on collecting in future articles.

Introduced on January 1 1975, this was the sixth handheld electronic calculator sold by HP, and the fourth scientific model. Its code name was "Merlin" -- something to do with wizardry, no doubt. It was introduced a few days less than a year after the HP-65, as an alternative, and cheaper, programmable calculator. It did not have the card reader of the HP-65, had fewer program steps, and fewer mathematical functions than the HP-65 -- no ABS function for example. Instead it had more data registers, a range of conversion functions -- to and from metric measure, temperature conversions, and so on -- and also statistics functions. It also had, for the first time in an HP handheld calculator, a real built-in stopwatch -- an "official" version of the stopwatch earlier found on some HP-45 models. The HP-55 is an interesting step in the development of the HP handheld range, especially the stopwatch, but it seems to have generated less excitement among the HP fans than the HP-65. Nevertheless, it is eagerly sought after by collectors.

The new HP48G and HP48GX models provide ample territory for people looking for features that are little documented or completely undocumented. One such feature has been mentioned on electronic bulletin boards and I described it during the 1993 HPCC one-day conference. Since the conference, Daniel Legendre has written to me about the same feature, which reminds me that it has not yet been seen in DATAFILE -- my thanks to Daniel for reminding me of it.

To see this trick, just type the command RULES on an HP48G or HP48GX and press ENTER. You will see a crossword-style layout of the names of the team members who worked on the HP48G/GX project! The first name across the top is DIANA -- Diana Byrne, who led the team, and who visited HPCC earlier this year. HPCC members might recognise some of the other names too -- BILL is Dr Bill Wickes, JIM is Jim Donnelly, DENNIS is Dennis York -- all have visited HPCC. As for the other names, and the mysterious ALCUIN, well that's for another short article!

Now for an article which I wrote tonight -- it's one I have been planning for a long time, and your expression of regret at not getting the ones I had not yet writen encouraged me to write it down!

If you collect HP calculators actively, not be the sheer force of the fact that you buy a new one every so often, then you should think about the sort of collection you plan to build up. I have already written in an earlier article that trying to get one of everything is likely to be futile. I suggested that one alternative is to specialise. How could you do this?

One way is to choose a particular model and try to collect different types. For example the HP-35 had three different keyboards, it was made in the USA and in Singapore, and early models had just the words Hewlett Packard on the front whereas later ones had the model number too. Furthermore, early ones had several bugs, later ones had some of these bugs removed, and still later ones had all the bugs removed. It could be very interesting to try getting several HP-35 calculators, showing all these varieties.

Another model that is interesting to specialise in is the HP-41. There are three basic versions -- the HP-41C, HP-41CV and HP-41CX. Each can be found in the original style, and also in the "halfnut" style -- with a rounded black mask around the display and with a much simpler internal layout. There are many different versions of the internal electronics and programs. There are models made in the USA and in Singapore. There is even the "blanknut" -- an HP-41 with no function names on the keyboard. Beyond all this, there is a range of accessories made by HP, such as plug-in modules, printers, card readers, most of which come in several versions themselves. Then there are the various manuals and solution books published by HP, and after that there is the enormous range of publications and accessories made by companies other than HP.

If you really want to study a model in detail you might find ways to get early prototypes, mock-ups, and production plans from HP. These are VERY difficult to get, but some items, such as detailed documentation, repair manuals, or diagnostic modules, were available from HP if you knew who to ask.

If you decide to specialise, do choose something which interests you, and something you can reasonably expect to collect. There is no point trying to collect many HP-35s if you live in a country where no-one ever used an HP-35, nor in trying to build up a comprehensive collection of HP-01 models if you are not willing to pay more than $5 each! Try to treat your collecting as fun!

Some collectors collect things out of interest -- others like to treat collecting as a challenge! (Most, of course, are somewhere in between.) Those who want to make it a challenge will want to get the rarest items -- the rarest stamp, the most perfect El Greco, the largest diamond .or maybe the rarest HP calculator. Of course if you want to get the rarest, then you first need to know what is rarest, and that is the subject of this article. Well, there is a second subject, but I'll come to that later.

The rarest HP calculators are one-off items, such as mock-ups of proposed new models, or test versions made to help at the design stage, or handsets which look like the planned calculator but have to be plugged into a box of electronics which contains the actual programming being tested. None of these are real calculators, though, so most collectors leave them alone. They do not really count! (Or should I say they really do not count? My thanks to Frank Wales for pointing this out to me!) Still, some people would give an arm and a leg for one -- for example HP thought of following the HP-01 LED watch with an LCD version to be called the HP-02. I don't know if any HP-02 mock-ups were ever made, but I know there are people who would try to get one the same way as the man in the parable who sold all his worldly possessions to get the pearl of great price.

The next rarest are working prototypes. HP sometimes design a product, make a preliminary batch, then decide not to put it into commercial production. In the case of calculators, the HP-95C is the only example I know where this happened, and, again, there are collectors who would (nearly) kill to get one. Still, if the product was never sold to the public then most collectors -- sensibly -- see no reason to try getting one for their collection.

Next come some very rare models which were sold to the public, but did not sell well. The HP-01 calculator watch is one of the rarest -- HP treated it as a luxury product and sold it through selected jewellers and watch sellers, not through calculator dealers. It was very expensive for a watch -- particularly the gold model -- you could get a Rolex for the same sort of price -- and it was introduced shortly before red LED (Light Emitting Diode) displays were replaced by the much more efficient, less battery-hungry, and permanently visible LCDs (Liquid Crystal Displays). So, it did not sell at all well. If you can get one, well done, but many calculator collectors do not bother with it, since even HP treated it as a watch, not a calculator.

The three different HP-94 handheld computers are very rare too. They were designed for companies to buy in bulk, to be programmed for special purposes such as stock-keeping. There was significant interest in this model, but each interested company initially bought only a few, to write and test their programs, before buying in bulk. By the time the companies had their programs ready and were ready to buy in bulk, HP had decided the HP-94 was not selling well enough, and withdrawn the product! Actually, the HP-94 was not made by HP -- it was made for HP by Fujitsu, and only had an HP label. So, it was not really an HP product, and is ignored by most collectors, except those lucky enough to have got one.

Another class of rare model is the "special". For example the HP-97S was an HP-97 slightly redesigned and with an add-on box which made it possible to connect it to electronic equipment to take readings or control that equipment. Many collectors would treat this as a "special" version of the HP-97, and would not bother to try to get one. A different example is provided by special versions of calculators which just have an extra logo on the case. A batch of HP14B and of HP32S calculators like this was made for the 50th anniversary of HP. Similarly, a special run of HP28S calculators was made at the introduction of the HP28S, with a logo commemorating the 100th anniversary of the American Mathematical Society -- these were presented to mathematicians at a conference organised on this centenary. (See the separate article in a recent DATAFILE by Bill Wickes about the introduction of this model.) There is nothing special about these calculators apart from the wording or pattern of the case, and some collectors want these for their collections, but most are not bothered.

With some models, HP make a few calculators which work as normal but are in a transparent plastic case, not the normal HP black (or dark green or dark blue) plastic case. These are designed to let people see the insides of the calculator -- maybe to let them see how well it is made, and how much gold plating goes into an HP calculator -- no doubt justifying its high price. It is nice to have one of these, but they really are a "special", not a different model, so most collectors do not feel a vital need to have one.

Well, that about covers the rarest HP calculators -- and at the same time gives you an excuse you can give in each case for not having one! That was the other subject of this article -- why you need not feel deprived if you can not get these rarities. In a future article I shall look at some other HP calculators which are hard to get, but definitely are worth trying to find.

Introduced on February 1 1975, just a month after the HP-55, this was the seventh handheld electronic calculator sold by HP, and the first to look different from the original HP-35 shape. The model number showed that it was to be the first of a new family -- the "series 20" or "Woodstock" series. Its code name was "Pumpkin" -- it was a small, non-programmable handheld calculator with lots of scientific functions. Its main attractions were its small size -- which made it easier to fit in a pocket than the "Classic" models, and easier to use with one hand -- and its low price, for those days, of $125.

In my article about calculator rarities I described those HP calculators which are so rare that collectors are entitled to ignore them! Beyond those, there are some other models which you should be able to find, but which can be hard to get. This is not to say that they are rare -- some are fairly common, but are mostly in the hands of people who refuse to sell them, or of people who do not know that anyone might want them. What counts is how easy or hard it is to get a particular model -- and that can be a question of personal experience -- so be warned that this article is based largely on my own experiences and those of a few friends. If I write that model x is very hard to get, and you happen to have access to 10 model x calculators, then please get one for me, and write to me -- I'll buy it off you and shall be happy to write another article saying that perhaps model x is not so rare after all.

In my experience so far, it is especially difficult to get hold of HP calculators with built-in printers, and it is moderately difficult to get most financial models. The ones with built-in printers are easy to explain -- they are always more expensive, so fewer are sold. What is more, those which are sold tend to stay in use for a very long time, since the investment in them was high. Eventually they break down and get thrown away. So, the desktop models HP-91, HP-92 and HP-97 are all difficult to find. The HP-97 is probably the easiest to find of the three -- it was a partner to the HP-67 and it would seem that a lot were sold -- HP found it difficult to cancel this model because customers kept wanting more. The handheld printing models, the HP-10 and HP-19C had an extra problem -- the printers tended to break down -- perhaps because they were just too small -- so fewer were sold, and more were thrown away.

I have difficulty getting financial models probably because I have access mostly to users of technical models -- HPCC members, university staff, maths teachers, and such people. Relatively few of my friends and acquaintances are financial wizards who would have old unwanted HP financial calculators! In those cases where I have managed to find financial models, there seems to be a rule which goes as follows. If there are two similar models, one at a higher price than the other, then it is the cheaper one that is rarer. Presumably financiers are willing to pay the extra to get the better model! Thus, the HP-70 was released a year after the HP-80, as a cheaper financial model -- the HP-80 is still fairly common, whereas the HP-70 is very hard to find. Similarly, the HP-27 was a powerful financial model, with technical functions too, and the cheaper HP-22 which had fewer functions is much rarer. Even quite recently, the HP14B sold much less well than the HP17B, its closest rival, and was discontinued. In this case, though, the even cheaper HP10B is still on sale.

One thing which puzzles me is the relative scarcity of series 30 models. I have found it easier to locate older models, of the Classic series or series 20, and newer models such as HP-41s or series 10 models. At the time when they were on sale, the series 30 models were criticised for being less classy and less robust than their predecessors. Maybe fewer were sold as a result, or maybe more broke and got thrown away. But they might still be in use, or in their owner's desk drawers -- anyway, most are still missing from my collection.

The handheld computers -- HP-75C, HP-75D, HP-71B are not that difficult to find, even though relatively few were sold. They were expensive and even now most owners prefer to sell an old one rather than throw it away. Besides, second-hand stocks of the HP-75D and the HP-71B are being sold by EduCALC and the HP-71B is being sold both second-hand and unused by Tuscan Consultants. The HP-75C is probably harder to find, and the HP-94D, 94E and 94F are extremely rare, as I mentioned in the previous article.

One surprise is that I have not been able to find an HP-10C yet. This was the cheapest model of the Voyager series (series 10) -- it was a student model designed to undercut even the HP-11C. I would have thought it should have sold well -- but I have only ever come across one -- and that was owned by a colleague who uses it all the time and refuses to part with it! I wonder -- perhaps most owners are equally attached to their HP10Cs.

I have not yet mentioned the more recently discontinued models, except the HP14B. The HP17B, HP18C and HP19B -- predecessors of the HP17BII and HP19BII are fairly easy to find. The HP27S is more difficult, and the HP22S seems to be particularly rare -- I wish I had got one while they were on sale -- it is a very interesting attempt by HP to break into Casio's market of calculators with built-in formulae. The HP-16C -- a specialist computer science model -- seems to be fairly common, but is in great demand and fetches a very good price if you can get one. Many computer science people have bought a second one so as to have a spare -- I have a few spares myself! I wonder if the rapidly discontinued HP21S Statistician will become a desirable item in the same way. The HP-32S, predecessor of the HP32SII, came in two varieties -- the earlier models had a glass cover over the display whereas the later models have a bigger display without a second piece of glass above it. I am still looking for one of the latter -- they seem to be rare, but that might be because most are still in use.

Finally, there are the models which are still on sale -- none can be said to be hard to get, except that you naturally have to pay the full price for them, unless you are lucky enough to find someone selling a second-hand one. Anybody got an old HP-10B they would sell me?

Introduced on August 1 1975, six months after the HP-21, this was the eighth equal (with the HP-22) handheld electronic calculator sold by HP, and the first really powerful yet small technical model -- the same size as the HP-21 but with more functions, and programmable to boot. Its 49 step program memory and 8 data registers allowed a surprising amount of work to be done -- for example a 42-step integration program left 7 steps to define a quite complicated equation to be integrated numerically. The combination of small size and programmability made it an especially useful tool in cases where one-handed operation was needed. You could write a program to do a calculation, then hold the HP-25 in the palm of the hand, type in a number, and press the R/S key to run the program and get the answer, all with your thumb, leaving your other hand free. Only the Series 30 models will also let you work the same way -- just -- not even the later Voyager or current Pioneer models are small enough to allow this! It became very popular -- many people who could not afford an HP-65 or HP-67 used an HP-25 instead, and then moved to the HP-41 when that was introduced. Its codename, "Squash", might be an allusion to the amount of functionality squashed into such a small package; in any case it naturally follows the HP-21 name "Pumpkin".

Introduced on August 1 1975, six months after the HP-21, this was the eighth equal (with the HP-25) handheld electronic calculator sold by HP -- the business counterpart of the HP-21, and the same size. It was not programmable but provided a useful range of business functions, including statistics. Like other HP business models it had the n i PMT PV FV functions along the top row, but it also had an ACC command to compute accumulated interest and a BAL command to compute remaining balance. Its small size, 10 data registers and the fairly low price of $165 probably made it appeal to business users, but the HP27 introduced 5 months later at only $35 more, and with far more functions quickly reduced its sales, so it is rather difficult to find now. Its codename of "Turnip" carried on the tradition of calling these models after vegetables.

Introduced on March 1 1976, this was the tenth small electronic calculator sold by HP -- but the first one too big to be considered a "handheld". HP called it a "Scientific Portable Printing Calculator" to distinguish it from the smaller models and from their much bigger desktop models. It introduced HP's third family of calculators after the original Classic family and the small Woodstock models, the Topcat family. It was not programmable, had 16 storage registers, a standard set of scientific and RPN functions, statistical functions, and of course the printing functions -- print the X register, the stack, the statistics registers, and all registers. It had linear regression and H.MS arithmetic but no functions to convert between units. The printer was a thermal model -- using temperature sensitive paper, more expensive than ordinary paper but saving the trouble of fiddling about to change a ribbon. Since the HP-91 could not be called a "handheld calculator" the question arises whether it should be included in this series of articles about the history of handheld HP calculators -- see the article "HP Calculator History -- What Are We Talking About?". As befits a top cat, the HP-91 codename was "Felix".

In these short history articles we have come as far as the HP-91. This is too big to be described as a "handheld calculator", which leads to the question -- what exactly are we talking about in these articles? I began with the HP-35 -- the first handheld electronic calculator from HP; before that, HP made only electronic calculators nearly as big as a desktop computer. From the 35 to the HP-91 any HP calculator with a model number of two digits is a handheld calculator. The HP-91 was still a calculator, and much smaller than the big desktop models, but it can not really be called handheld. It is related to the handhelds, though, and worth studying with them, so instead of talking about "handheld" calculators, I shall use a different adjective: "small" calculators. A further ambiguity arises when we begin to ask if a particular model is a programmable calculator or a handheld computer -- we could call the HP-65 and HP-67 either, and we can certainly call the HP-41 both -- but that means we can include them in a history of small calculators. The HP-01 is both a wristwatch and a calculator, so it too can be included. The HP-75C causes more trouble -- it is a handheld computer, programmed in Basic, and the designers went to some trouble to distinguish it from a calculator -- for instance its display does not show the current angle mode nor the status of any flags. People who want to make a point of studying or collecting only calculators would not include it in their list, but those interested in the development of HP handhelds overall do include it. In any case, the HP-75C was closely related to HP handheld calculators, and led directly to the HP-71B which was a smaller handheld computer, but with a calculator mode. On the basis of taking an interest in any handheld related to HP calculators, the HP-75D, HP-71B, all three HP-94 models, and even the HP-95LX and HP-100LX should be included too. I shall even make a brief mention of the desktop series 80 models, which led to the HP-75C, and of the HP-81 and HP-46 -- desktop versions of the HP-80 and the HP-45. In fact, I plan to include in these articles any handheld or easily portable calculator or computer with a model number less than or equal to 100. The next size up, the HP-110 and HP Portable Plus, do not really fit. I am still not sure about the OmniBooks -- they are bigger but are in many ways enlarged versions of the HP95LX. What would happen if HP were to introduce an HP100LX successor called the HP105LX or the HP200WP? I suppose I would include them in this series if they proved to be of interest to enough HPCC members!

Introduced on May 1 1976, this was the eleventh small electronic calculator sold by HP and the fourth member of the Woodstock family. It was not programmable but provided a wide range of business, scientific and statistical functions. (In fact the nearest model to its concept since then has been the HP-27S!) One way to distinguish it from the HP-25 is to see that its two shift keys are gold and black, not gold and blue. To distinguish it from the HP-22, you can note that the standard business functions n i PMT PV FV are gold-shifted functions on the second row of keys. The statistics include computation of variance and normal distribution parameters in addition to the usual functions. It was only the second handheld, after the HP-45, to provide recall arithmetic. (See the separate article about this.) This was an ideal model for anyone who did not need programmability -- business users who sometimes needed technical functions and technical users who wanted to do business calculations, and the HP-27 apparently cut into sales of the HP-22. Its codename "Salad" seems to acknowledge that it was a mixture of various good things!

The HP-27 was only the second calculator to provide an unusual but useful feature called "recall arithmetic". Much more common on HP calculators is "storage arithmetic". Early calculators, especially those which did not have a stack like HP models, often provided an extra register, usually called M (for Memory), to let the user store information which would not be needed until later. Sometimes this register could be used to "accumulate" values, in other words add up a set of results -- this was done with a key marked M+ -- add to the Memory register. HP, in their usual thorough manner, expanded on this idea. First of all, they gave the user more than one M register -- usually 4 or more "storage registers". Secondly, they let the user not only add to these registers, but also subtract from them, and multiply and divide them. All these operations on storage registers are called "storage arithmetic" -- the number in register X is used to do arithmetic on a storage register. A simple example of the use of storage arithmetic might be the calculation of compound interest -- you store the original amount in a storage register, then use the stack to calculate the interest rate separately for each month in a year, and multiply the register by one plus that month's interest rate. I hasten to add that this idea is not restricted to old-style RPN calculators, exactly the same works in RPL, except that the number to use is stored in level 1 instead of stack register X, and works on a named variable instead of a numbered storage register. It even works on HP's algebraic calculators.

Extensions are possible to this idea. One is to increase the number of operations which can be carried out on a storage register -- must we be limited to + - * and / ? If division is written STO/ nn to mean "divide the contents of register nn by the contents of X" then it should be possible to write STO^ nn to mean "set the contents of register nn to the power given in X". I made this suggestion in DATAFILE V3N3p22, the Summer 1984 issue (gosh, 10 years ago!), and I was delighted to find that HP did add this function when they brought out the HP-18C in June 1986. (It was kept in models derived from the HP-18C, the 17B, 17BII, 19B, 19BII and 27S, but was not put in other models.)

A second possible extension is to work in reverse, just as RCL nn is the opposite of STO nn -- it recalls the contents of register nn to X instead of storing X in nn -- so RCL+ nn could be used to recall the contents of register nn and add them to the contents of X, instead of adding X to the contents of register nn. This might seem less useful than storage arithmetic, but look again at the example of following a compound interest over a year. You could calculate the growth rate for each of 12 months, and store it in registers 01, 02 and so on up to 12. Then to see how your capital grows (or falls!) through the year, you would put the original sum in X, do RCL* 01 to see the growth in month 1, then RCL* 02, and so on up to RCL* 12. At each step, the compounded sum would be seen in the display, not hidden in some register as was the case when storage arithmetic was used. Recall arithmetic was introduced along with storage arithmetic on HP's third handheld calculator, the HP-45, but then someone lost enthusiasm for it -- it was next seen only on the HP-27, the eleventh model. The following model with recall arithmetic was the HP-15C, HP's 32nd model and the first programmable with RCL arithmetic -- which is very useful on programmable RPN calculators because it lets you do arithmetic on the stack without changing registers X, Y, Z, T or L. Since then only the HP-32S (HP's model 49), the HP-42S (model 51) and most recently the HP32SII (model 58) have recall arithmetic. Recall arithmetic is not vital on calculators which have the general "register exchange" command X<> nn, since for example RCL+ nn can be replaced with X<> nn STO+ nn X<> nn, but RCL+ nn is faster and more compact. It is an interesting feature, but clearly HP have not put much emphasis on it, probably because few users exploited it. Pity -- but at least we can say that two current models have recall arithmetic!

Following the HP-27, HP's 11th calculator, introduced on May 1 1976, HP announced three calculators all on the same day -- July 1 1976. These were the HP-25C, HP-67A, and HP-97A -- that's equal 12th position for 3 models, so they are really 12th to 14th. What's more, there was the HP-95C which would probably have been announced at the same time too, but was cancelled.

Three models introduced on the same day is not a record -- exactly a year later HP introduced another three models, exactly two years later they introduced two models, and then on the same day in 1979, three years later, they announced five models! What's more, another two models were announced on the same day in 1982. I wonder what it is about the first of July that made HP like it as a day for introducing new models. The Fourth of July might seem more appropriate for a company from the United States!

The HP-25C was introduced with the HP-67A and the HP-97A on the first of July 1976. It was the fifth member of the Woodstock family of small handheld calculators. Apart from one significant difference it was exactly the same as its predecessor, the HP-25A. Even its codename "Squish" and its $200 price were similar to the codename "Squash" and the $195 price of the HP25A. If the codename Squash suggested that a lot of features had been squashed into such a small package then the name Squish correctly implied that yet another feature had been squeezed in. This feature -- the significant difference -- was that the HP-25C was the first calculator with "Continuous Memory", as denoted by the letter C after its model number. This was very important -- for the first time users could turn their handheld calculator off without losing the contents of its memory. Naturally, as soon as you had this advantage you would want to write longer programs, since you would not have to retype them every time you turned on the calculator. The desire to store longer programs was dealt with in two ways -- the use of cards instead of continuous memory, as already provided by the HP-65A and extended by the HP-67A and HP-97A -- or the provision of more memory, as on the HP-25C successor, the HP-29C. The HP-25C also introduced the idea of an upgrade to a model instead of its replacement by a different model -- something we are familiar with nowadays, as in the case of the HP48 family.

Introduced on July 1 1976 with the codename "Hawkeye", together with the HP-25C and the HP-97, this was essentially an upgraded HP-65. The competition were catching up and HP had to overtake them (did the codename mean that HP were keeping a sharp lookout for the opposition?), so the HP-67 improved on the HP-65 with new functions, more memory, and the ability to store data as well as programs on magnetic cards. One interesting extra was a second set of ten registers, called the "secondary storage registers". The "primary" and "secondary" registers could be swapped so the secondary ones could be used to hide a set of values while some other calculation was carried out using a new primary set. These secondary registers were used by the statistics functions, avoiding the problem common on HP calculators that statistics operations destroy data in some of the numbered registers. Another interesting extra was "rapid reverse branching" -- in addition to going directly to program labels, the calculator allowed indirect GOTO commands, via an intermediate register "I", and if "I" contained a negative number then the program would jump backwards by that number of program steps. Since the negative number could be as large as you wished, it could be used to jump backwards past the first program step, and past the last program step, so this technique could even be used to jump forwards in a program without the need for a label. It also allowed execution of subroutines. The HP-67 had so many functions that the designers added a third shift key, in black marked h to the yellow and blue f and g shift keys already used by the HP-65. (The HP-65 had only a partial third shift key, [f-1], to give the inverse of some functions.) To allow the HP-67 to be fully compatible with its printing version, the HP-97, it had four special commands. The command [-x-] paused a program for 5 seconds and displayed the contents of x, whereas on the HP-97 this was [PRINTx] which printed the contents of register x. (Note that this developed into the VIEW function on the HP-41.) The command [STK] displayed the stack registers T, Z, Y and X sequentially whereas on the HP-97 this became PRINT:[STACK]. The command [REG] displayed the contents of each primary register (0 to 9) whereas PRINT:[REG] printed all the primary registers. (On the HP-41 PRSTK and PRREG were functions built into the printer, not the HP-41 itself.) [SPACE] did nothing on the HP-67 whereas on the HP-97 it became PRINT:[SPACE] which (obviously) printed a space (a blank line). (The HP-41 had a similar do-nothing function called ADV, which advanced the paper by a blank line if a printer was attached.) The HP-67 was one of the first calculators to allow the creation of "non-standard" instructions -- ones which were not mentioned in the manuals and could not be entered from the keyboard. One way they were entered was by very briefly interrupting the power supply -- in fact special "phase" boxes were made by keen users, these would introduce a switch between the battery and one of its contacts, and the box would be used to interrupt power for a selected (short) time to generate a particular instruction. This allowed specialists to study the way instructions were stored inside the HP-67. One useful tool was the saving of such non-standard HP-67 instructions on magnetic cards, which could then be used to add non-programmable steps to programs (the HP-67 had a program MERGE command) without the need to create the instructions again. These special cards, and tables of HP-67 instructions, were among the tools used to develop "Synthetic Programming" on the later HP-41 calculators. The HP-67 was the last calculator of the "Classic" family, and as the notes in brackets show, it was a precursor to the HP-41 family.

Introduced on July 1 1976, together with the HP-25C and the HP-67, this was a printing version of the HP-67, in the same shape and size as the earlier HP-91. The code name "Kitty Hawk" combines "Kitty" to show this is a member of the "Topcat" family of calculators with "Hawk" to show the relationship with the HP-67 whose codename was "Hawkeye". Programs written for the HP-67 and the HP-97 are fully interchangeable -- a magnetic card written on one model can be read by the other. To make this possible, four printing commands on the HP-97 had non-printing equivalents on the HP-67, as mentioned in the history article describing the HP-67. A fifth command, PRINT:[PRGM] had no equivalent on the HP-67, but it was not programmable so this did not affect the compatibility of programs saved on magnetic cards. Like the HP-67, the HP-97 could be made to store "non-standard" instructions -- this led to some interesting displays and printouts, but the printer could be made to overheat and burn out its printhead if such instructions were applied carelessly. As on the HP-91, the printer could be set to one of three modes; MANual, TRACE and NORMal -- the HP-95C (see separate history article) and later the HP-41 printers had the same modes. The availability of a larger keyboard and more keys meant that the HP-97 had only one shift key, as opposed to three on the HP-67. (This use of just one yellow shift key was carried over to the HP-41; the ALPHA key could be treated as another shift key, but really it was a mode change key.) The large keys used for common functions, the large and bright LED display, and the ease of saving programs and data on magnetic cards, made this a popular calculator -- so much so that HP kept it in production for several years after the HP-41 had replaced the HP-67.

The HP-95C was developed at the same time as the HP-25C, the HP-67 and the HP-97, but was never sold to the public. This makes it extremely interesting to collectors and historians of HP calculators, so I include it in this set of HP history articles. I believe that fewer than 20 exist; most readers will never have the chance to see one, so I have put a photograph of it on the cover of this issue of DATAFILE. It was apparently meant to be a printing version of the HP-25C, much as the HP-97 was a printing version of the HP-67. Unlike the HP-97 it did not have exactly the same functions as its non-printing version -- some functions were unusual, and I shall mention them below. The HP-95C was probably not released because it would have competed with the HP-97 and would have required an extra production line, in addition to those set up for the HP-91 and HP-97. (This is not unusual; HP develop many new products which are then never sold -- it is a concern to some HP engineers that many of the most interesting and time-consuming projects they work on cannot be mentioned in their CV.) The units which were made were "pre-production" models, for testing at HP; one sign of this is that they do not have serial numbers stamped on them. Manuals and solution books for the HP-95C had already been printed before the decision not to manufacture it, so they exist in larger numbers than the calculators, but they too are rare -- I do not have either myself (and would be happy to hear from anyone who has any spares or could make photocopies for me!)

If you study the photograph you will see keys marked A B C D below the display. These were used to create labels, to execute and go to programs with these labels, and even to clear programs marked by these labels. No such keys were available on the HP-91 and HP-97, though the top row of HP-67 and HP-97 keys could be used to execute program sections beginning with labels A, B, C, D and E (and a, b, c, d, e). The idea of such keys below the display and used for a special purpose may have developed from the top row of keys used on HP's financial calculators. On the HP-95C, though, these keys were used differently -- a Label A, B, C or D started a new program whose steps were labelled A000, A001, A002 and so on, then B000, B001, and so on. (The same scheme is used on the HP-32S and HP32SII but I do not think it has been used on any other HP calculator.) Numbered labels could be used separately in each program -- label 1 in program A would be found by a GTO 1 step in program A, and label 1 in program B would be found by a GTO 1 in program B. This separated programs very neatly but meant that GTO (go to label) and GSB (go to a subroutine at a label) steps would not allow program A (for example) to use some program steps inside program B. To allow this, a separate JUMP instruction was provided on the HP-95C. (You might have noticed this in the photograph, below the GTO key next to the yellow and blue f and g keys in the bottom row.) This let you jump into a specified program to a chosen label in that program; for example the program step JPB1 meant "jump into the program B at label 1 in that program. I do not believe this has been used in any other HP calculator, and I even wonder whether some marketing people at HP considered it too confusing, which might have contributed to the decision not to sell the HP-95C.

One more feature of the HP-95C can be inferred from the "C" in its name -- unlike the HP-91, the HP-92 or the HP-97, it had continuous memory. This would have been a very useful feature, but the card reader on the HP-97 probably won out as a preferred alternative. A fascinating calculator, and in a way it is a pity it was never sold to customers.

A year after the introduction of three new models on July 1 1976, HP introduced three more models on July 1 1977. These models (equal 15th in the count of small calculator models made by HP) were the HP-10, HP-29C and HP-92 -- three very different calculators. The HP-29C, codenamed "Bonnie", was the last model in the Woodstock family or "Series 20". The codename bears no relation to the previous names of Series 20 models, which were all named after vegetables. In contrast, the HP-29C was followed by a printing version, the HP-19C, and these two were called Bonnie and Clyde. Or does someone know of a vegetable called Bonnie? The HP-29C was introduced as an improved version of the HP-25C, with more memory, more program steps (98) and a few extra functions. In fact it inherited many of the features of the HP-67, such as a second set of registers and even reverse branching when a negative number was put in the "indirect" register and used by an indirect GTO command. Though a considerable improvement on the HP-25 and HP-25C, it did not quite achieve the same popularity, perhaps because the HP-67 provided keen competition.

Of the three models introduced by HP on July 1 1977, the HP-29C was described in the previous issue of Datafile. I have just managed to get an HP-10, so I shall describe that this time -- to describe the HP-92 properly I still need to beg, borrow or buy one! (Does any reader have a spare?) The HP-10 was a complete departure from previous HP calculators -- neither a financial model nor a technical one, but instead an "adding machine". It was the smallest HP calculator so far to have a built-in printer -- easily held in one hand, it did not use RPN, it had only + - x / and % keys; not even a square root, though it had an automatic constant feature and a single memory register. No wonder the calculator's codename was KISS -- "Keep It Simple, Stupid". A fairly small label on the back describes all the HP-10 operations -- there was no need for a pocket guide, and the manual was not that big. Next to the ON/OFF switch it had a mode switch to select how results were to be displayed -- in the LED display, on the printer, or on both. Using only the printer or only the LED was a useful option as it saved power -- even so, the battery pack is larger than that used on earlier handheld models. The option to use both is called ALL, and is equivalent to the TRACE mode on other printing HP calculators. Oddly enough, the HP-10 looks very much like the original HP-35 -- white keys for digits, blue keys for simple maths functions, and black keys for the more advanced functions -- though on the HP-10 even x and -- are treated as advanced functions! There is also a yellow shift key, used for the automatic constant, for selecting display modes, and for a few printing operations, including printing a row of ###### symbols to separate one calculation from another. There are so few functions that the calculator has only 22 keys (zero occupies the double-width key) which means that the keys are larger than those on other HP handheld models -- making it easier to use this as a simple printing model. It seems clear that this was an attempt by HP to enter the small business printing calculator market, but judging by its rarity, not many can have been sold. Apart from HP's usual reluctance to advertise, further reasons for the low sales may be that the target market was not aware of HP calculators, and that the tiny printer was reputed to be unreliable. Despite its unusual format, I must say that I very much like the HP-10!

The Calculator Reference by Rick Furr (rfurr@vcalc.net)

Back to The HP Page

Back to The Calculator Reference