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Hewlett Packard Model 9100B Electronic Calculator

Updated 2/22/2021

The HP 9100B was the second calculator made for sale by Hewlett Packard. HP's first electronic calculator, the revolutionary 9100A was introduced in March of 1968(March 11), with the 9100B introduced in fall of 1969.

The development of the HP 9100 calculators is an amazing story. Technology author Steve Leibson has done an incredible job of telling this story. For anyone interested in this fascinating read, check out The 9100 Project.

The Engineering Prototype of the Hewlett Packard HP 9100 Calculator
Note Lack of Magnetic Card Reader
Image Courtesy of the Smithsonian Museum of Natural History, Kenneth Behring Center
Used With Permission
Reference: HP9100 Prototype Desktop Electronic Calculator

The 9100 calculators were still being actively advertised as late as mid-1970, however, their marketability was rather shortened due to the arrival of reasonably-priced large-scale integrated circuitry, which could replace the complex and costly discrete transistor circuitry of the 9100A & 9100B. HP's follow-on 9800-series calculators utilized integrated circuit technology to increase capabilities and reduce cost.

The 9100A and 9100B were most amazing because they managed to combine speed, features, and reliability that, at the time, were simply unheard of in the electronic calculator industry. The machines did this with completely discrete transistor technology. There were no digital integrated circuits used in the calculator, though there are four linear integrated circuits are used in the magnetic card reader as amplifiers. HP used all DTL (Diode-Transistor Logic), magnetic core memory storage, magnetic wire-rope low-level microcode ROM, and innovative circuit board technology for the high-level microcode ROM. All of these technologies came together to form machines which were engineering triumphs, and still today garner great admiration from electronic engineers for the sheer elegance of their design.

Wang Laboratories, recognized leader in the high-end electronic calculator marketplace prior to HP's entry, had been very successfully (and profitably) marketing their 300-series calculators for almost three years before the introduction of the HP 9100A. Wang's calculators were also of discrete transistor design, and offered advanced math functions, but were somewhat slow, and had very limited programming capabilities. Dr. An Wang, the founder of Wang Laboratories was given a sneak peak of a pre-production HP 9100A at the New York IEEE Electro Show in March of 1968, courtesy of Bill Hewlett, one of HP's founders, and the driving force behind HP getting into the electronic calculator business. Dr. Wang, after seeing the incredible capabilities of this soon-to-be-announced calculator tour-de-force from HP, returned to his company in near panic, and rightly so, as his company derived the vast majority of its revenue from the sales of their LOCI-2 and 300-Series calculators. This peek at HP's new electronic calculator resulted in the frenzied development of Wang's next generation of calculators, the 700-series, to compete with HP's wunderkind.

The nameplate that panicked Dr. An Wang

The 9100 calculators use RPN (Reverse Polish Notation) method for entry of problems. The HP 9100 calculators were not the first calculator to utilize RPN entry. The Friden 130 was the first calculator to use stack-based logic for problem entry. However, Mathatronics, with their Mathatron calculator patented the stack principle for internal use in an Algebraic-Entry calculator. This patent was later acquired by Hewlett Packard when Mathatronics was liquidated.

The 9100's use a subtly different version of the RPN stacking notion than later HP calculators, with only a three-level instead of a four-level stack. The 9100's use a CRT display, similar to the CRT display on the ground-breaking Friden 130 calculator, though the HP calculators used an unusually compact electrostatic deflection cathode ray tube that was manufactured by HP exclusively for use in the calculators. HP's vast experience building oscilloscopes proved very helpful in the design of the CRT used in the calculator. The Friden 130 and 132 calculators used an "off the shelf" CRT tube made by Westinghouse. All three levels of the stack are displayed at once, with the "X" register (where numbers were entered) at the bottom of the display, the "Y" register in the middle, and the "Z" register at the top. The digits on the display are presented in seven-segment form. The digits are formed as vectors, by moving the CRT's electron beam to an endpoint of a lit segment, turning up the power to cause the phosphor to glow, moving the beam to the endpoint of the segment, then lowering the power to move to the next lit segment. This is all done at a high rate of speed (and the phosphor on the display tube has a significant amount of persistence) such that the display looks continuous to the human eye. This vector display is much different than the raster-scan displays we are used to nowadays on televisions and computer displays. The display presents ten significant digits, with two non-displayed guard digits for accuracy, and 2 digits for exponential notation. When an error condition exists, an annunciator to the left of the CRT glows, and can be cleared by pressing any key.

HP9100B CRT Display

The 9100B control panel is very well laid out, of extremely high quality, and provides easy access to all of the functions of the machine. Four paddle-switches above the keyboard provide settings for (left to right) Degree/Radian selection; Fixed or Floating (the Floating setting displays numbers in exponential notation) mode; Power On/Off; and Program/Run settings. The keyboard is of exceptional quality, with very nice full-travel keys, and key caps that have their legends molded into them so that the legends will not wear off with use.

The left-most group of keys handle the higher-level math functions, including absolute value; return the integer portion of a number; trigonometric functions with inverse and hyperbolic modifiers; polar to rectangular and rectangular to polar conversion; natural logarithm, base 10 logarithm; ex and add/subtract/recall functions for memory registers 'e' and 'f'.

The second group of keys (from the left) control memory registers and stack manipulation functions. The content of the X or Y stack register can be stored to a given storage register, the X register can be recalled from any storage register, and the content of the Y stack register can be exchanged with a given storage register. Storage registers are numbered 0 through 9 (using the regular number keypad), and the special keys labeled 'a' through 'f'. The 'a' through 'f' registers can be recalled in the X register by simply pressing the key corresponding to the register...a step saver when writing programs. The 9100B provides additional memory versus the earlier 9100A, in the form of a second set of 16 registers (for a total of 32 memory registers) which are accessible by pressing the '-' key before the register selection.

The third group of keys provides access to square root, add, subtract, multiply, divide, the numeric keys(0-9), decimal point, sign change, and exponent entry keys, as well as a [Clear X] key.

The last group of keys (rightmost) are mostly for the programming functions, which include conditional branches; unconditional branch; a flag set; flag test conditional branch; program stop (for input); program pause (1/8th second pause with display enabled, useful for temporary display of results since the display is blanked during program execution); program end (for marking the end of a program); subroutine branch and return function (9100B only); a [CONTINUE] key for continuing execution of a program when 'stop' occurs; and a [STEP] key for stepping through programs one step at a time. Also included in this last group of keys is the [CLEAR ALL] key. At the far right of the control panel is a thumb-wheel switch which selects the decimal point position for the display. When the "Fixed/Floating" switch is in 'Fixed' mode, the calculator sets the decimal point at the position specified by the thumb-wheel switch. If a number is such that it can't be displayed with the decimal point at the position specified, then the display for that number switches to exponential notation. With the "Fixed/Floating" switch is in 'Floating' position, it forces all three levels of the stack to be displayed in exponential notation.

An HP 9100 Magnetic Card and its Storage Envelope

HP 9100 Mag-Card Storage Container

Above the "Program/Run" switch is the magnetic card reader/writer. This device allows programs and data stored in the 9100's magnetic core memory to be recorded to a magnetic card, just slightly smaller than the average credit card, as well as being able to read the data on the card back into the calculator. Two pushbuttons control the card unit, one for recording information, and another for reading the content of a card into the machine. The magnetic card can record both program steps and the content of memory registers.

HP9100B Control Panel

The insides of the 9100 are truly a testimony to brilliant engineering. Hewlett Packard had a reputation for building extremely high-quality instrumentation, designed and constructed to very high levels of detail and quality, with exceptional engineering. The HP 9100A/9100B calculators were no exception to this reputation. It is very evident that HP wanted these machines to be reliable and robust pieces of equipment. Mechanically, the machine is extremely well constructed, with thick cabinet castings and heavy gauge stamped aluminum parts. The machine is built with a clamshell design, with the upper half of the cabinet hinging upward from the front, with a latching mechanism to hold the upper half open, allowing operation of the calculator with the top up. The machine consists of a rather large main circuit board that takes up the majority of the bottom of the chassis. This board contains only diodes and resistors that make up the logic gates of the machine. This board also contains the edge-connector sockets for other circuit boards, making it also the mother board for the calculator.

Beneath the motherboard is another circuit board that is very specially constructed to form the high-level microcode read-only memory (ROM) that drives the calculator. This circuit board was an incredible development for the time, containing sixteen layers of traces arranged in a grid, with an inductive coupling between layers where traces coincided such that a zero or a one can be encoded simply by the way the circuit board is laid out. To encode all of the bits necessary for the calculators microcode, HP had to create a new state-of-the-art in circuit board design, pushing circuit board technology to a new level. The ROM board contains approximately 1000 bits per square inch, making it the highest density read-only storage element in existence at the time.

Hewlett Packard 9100B Core Memory Board (Core Array Beneath Daughter Board in Center of Board)
Image Courtesy of Mr. Douglas Jones, University of Iowa Department of Computer Science, Core Memory Web Page

The seven boards that plug into the motherboard make up the microcoded processing unit of the calculator, including timing, low and high-level microcode control, magnetic core memory subsystem, keyboard encoding, display formatting, and peripheral interface circuits. The CRT display and high-voltage drive circuits, as well as the main power supply are situated in the upper half of the cabinet, and are also modular in design for easy serviceability.

Inside the 'clamshell'

The 9100 machines use a microcoded architecture. Rather than having a hard-wired backplane that interconnected all of the various logic elements, the design is based on a logic engine with a very specialized instruction set. The instructions for this engine come from the microcode ROM. This microcode storage contains the instructions that essentially form the "operating system" for the calculator. This Read-Only Memory (ROM), combined with wire-rope ROM based sequencing circuitry, gives the 9100 its brains.

Closer view of diode-resistor gate array board

Side view of 9100B interior

Closer view of control board

The 9100A and 9100B calculators from HP were engineering triumphs for the time. The machines are very fast, with trigonometric functions completing in significantly less than 1/2 second, as contrasted to the Wang 360SE equipped with an optional trigonometric keyboard/display unit (Model 360KT), which can take up to ten seconds to perform the same operation. The 9100's were also built such that they, as in this example, can survive for over 50 years, and still work flawlessly.

If you want to learn more about the fascinating history of early HP calculators, including a great deal more detail about the 9100A and 9100B, not to mention a wealth of other information on Hewlett Packard calculators, you should take a visit to Dave Hicks' wonderful Museum of HP Calculators site. It is well worth the visit.

This exhibit is dedicated in perpetuity to the memory of Mr. Pete Lancashire, Tektronix employee from 11/1977-9/1988, from whom this calculator was purchased.

Text and images Copyright ©1997-2023, Rick Bensene.

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