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Hewlett Packard 9815A Programmable Calculator

Updated 1/18/2025


The HP 9815A is a fine example of mid-to-late 1970's programmable calculator technology. This particular machine was manufactured in early 1978, likely making it one of the later 9815A's produced, as the machine was introduced in 1976. The 9815A has a single-line Burroughs Panaplex II display panel that has a total of sixteen digit positions. Each display position consists of a standard seven segment digit, a decimal point (which is centered in the digit position), and a comma (which is used for grouping digits in threes for easier reading, located to the right of the digit position). The display is a bit unusual in that an entire digit position is used to display the decimal point.

Keyboard Detail

The 9815A also has a 16-character per line dot matrix thermal printer, which is capable of printing alphanumerics via program control or for indicating error conditions with human readable messages. The printer is somewhat noisy due to a solenoid-activated paper advance mechanism, and is also rather slow due to the fact that the thermal print-head has only one line of dots, so each line of output is printed a row at a time rather than a full character at a time. The thermal printer in the Tektronix 31 calculator, a similarly featured calculator to the 9815A, has an alphanumeric printer that forms all of the characters on a line simultaneously, making it considerably faster than the printer used in the 9815A. The 9815 also has a magnetic cartridge (DC-100) tape drive for program and data storage.

Interior View of HP 9815A with Keyboard Assembly Removed

The 9815A is the third generation of HP Reverse Polish Notation Desktop Programmable calculators, with the all-discrete component-based HP 9100A/9100B calculators from the late '60's making up the first generation; and the early IC-based 9810 and other machines in the 9800-series making up the second generation. A follow-on machine to the 9815A was introduced later, which expanded the amount of available program/data memory. This machine was designated as the 9815S.

The reverse side of the Keyboard module, where most of the logic of the 9815A is located

The 9815A has a comprehensive set of math functions, ranging from basic math, statistical, and scientific functions. Trigonometric functions (with arguments in degrees, radians, or grads) are included, with the unexpected omission of hyperbolic functions. The omission of hyperbolic trig functions was not lost on HP, though, as the company had a large library of programs which were distributed on tape cartridges which would allow the machine to perform just about any function. The 9815A/S also provides built-in base-10 and natural logarithms, square root, yx, polar/rectangular conversions, degrees/minutes/seconds conversions, 1/x, mean(average) and standard deviation, and integer extraction. The machine operates in any of three display modes in calculator mode, with fixed point decimal (with automatic shift to scientific display when the result of a calculation is beyond display in fixed decimal form), scientific (exponential) notation, and engineering notation (exponential with all exponents forced to be multiples of three).

A closeup of the Burroughs Panaplex Display on the 9815A in operation

The 9815A also benefits from a very rich set of programming functionality, with multi-level nest-able subroutines, nest-able "FOR/NEXT" looping, branching by address or label, and a comprehensive set of conditional test instructions. Program editing on the 9815A is simplified over earlier HP programmables, as the machine automatically keeps track of branch instructions and modifies addresses as necessary when program steps were inserted or removed. The memory in the 9815A is split between memory registers and program step storage, with the division between the two settable by an obscure key press sequence which is documented under the printer paper cover, along with a number of other non-intuitive key sequences. Base memory allows up to 472 program steps, with Option 002 expansion memory, that could be ordered from the factory, or field-installed. This expansion memory brought total memory storage to 2008 steps. Available program step memory is decreased by the number of memory registers defined, with the power-up default being 20 memory registers, which reduces the total number of program steps on an Option 002-equipped 9815A to 1928 steps. The 9815A combines multi-key program sequences into a single step in program memory, so programs on the 9815 consume less space than on earlier HP programmable calculators, which required a separate step in program memory for storing each key press.

Close-up View of Burroughs Panaplex II Display Module used in the 9815A

Unlike earlier HP calculators that used HP custom-designed integrated circuits to make up its calculating engine, the 9815 is built from mostly off-the-shelf technology. The 9815 is the first HP calculator to us an off-the-shelf microprocessor for its primary logic. The microprocessor is a Motorola 6800 8-bit microprocessor, with 14K bytes of ROM(Read-Only Memory) providing the firmware that directs the operation of the calculator. Gluing together all of the microprocessor logic is a number of standard TTL small and medium-scale integrated circuits. The 6800's working memory is via static RAM, which loses its content when power is lost. The magnetic cartridge tape unit, which uses DC-100 tape cartridges (a standardized magtape format, versus the proprietary designs used on earlier HP and other manufacturer's calculators), provides off-line storage of programs and data. A comprehensive set of tape manipulation functions make short work of saving/restoring programs and memory registers to/from tape. A special mode (selected by a slide switch on the keyboard panel) allows the machine to automatically load a program from tape on power-up and begin execution, allowing canned applications to be easily built.

Profile view of HP9815A

The 9815 is made up of three main circuit boards. The keyboard is constructed from a circuit board with snap-dome type switches that contact gold traces on the circuit board. Sandwiched on the back side of the keyboard circuit board is the main calculating logic, consisting of the Motorola 6800 microprocessor, static random access memory chips, seven 2K-byte Read-Only memory chips, as well as address decoding, bus buffers, clock generator, and display drive circuitry. The other board is mounted in the base of the calculator, and contains a rather complex power supply, circuitry for driving the magnetic cartridge tape drive, and circuitry to drive the thermal printer.

The 9815A is surprisingly fast for an early microprocessor-driven calculator, both for its math and program functions. It can cycle through a program consisting of all "NO OPERATION" instructions (the default content of memory when the machine is powered up) in less than 1/2 second (that's on a machine with the 1,928 step Option 002 installed). For math operations, trig functions take at most about 200 milliseconds. Other math functions come up with results virtually instantaneously. The printer is used to announce error conditions by printing out a text message indicating the error condition, for example, "ILLEGAL ARGUMENT", or "LOG OF # <=0", or "OVERFLOW" (with the display showing "9.999999999 99" on overflow conditions). Being as the printer is quite noisy, it is almost startling when an error occurs. Error conditions do not lock out the keyboard, but try to leave the machine in a state where calculations can be resumed (e.g., trying to calculate the Logarithm of a negative number generates the error condition, but returns the argument to the display). The printer can be selected so that it is "OFF" (error messages still print in "OFF" mode), "ALL", where the printer records all operations, and "NORM", where a press of the "PRINT" key (or its shifted function which prints the entire RPN stack) prints the current content of the display.

For much more detailed and comprehensive information on the HP9815A and other HP calculators, Dave Hicks' Museum of HP Calculators is a must-visit site.


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