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16/06/2022 Comments off

HP 41C front viewAround the time of the HP-67, an article in the Hewlett-Packard Journal, stated that electronic technology was no longer the only limitation of pocket calculator progress. The human interface was becoming an even greater barrier to adding more functionality. The HP-67 was an excellent example of the problem. It had three shift keys and most of its other keys had four functions. HP was running out of keyboard space for new functions, and many users found it difficult write and use numeric-only programs.

The HP-41C overcame these limitations by adding alphanumeric capabilities to both the display and keyboard. The keyboard had an « Alpha » key that toggled the keyboard between alpha and the normal calculator mode. (The alpha characters were printed on the slanted faces of the keys.) If the user needed a function not printed on a key, the name of the function could be typed in and executed. (About half of the HP-41Cs functions were preassigned to keys.)

Because typing out the name of a function could be cumbersome, the HP-41C added another toggle key called « user ». The user could assign any built-in function or user program to any key. Once the keyboard was placed in user mode, any assignments made by the user overrode the label on the key. (With the gold shift key, this allowed two user functions per key.) The user toggle state remained set even when the calculator was turned off allowing true keyboard customization.

To make it easy to remember keyboard assignments, HP provided keyboard overlays along with preprinted labels for all built-in functions and blank labels for user functions. In addition, whenever a key was held down, its function name was displayed. If it was the wrong key, the user could continue to hold it until the display showed « NULL » when meant the function was canceled.

Of course, HP didn’t just improve the human interface. The HP-41C had more memory (now non-volatile) than its predecessor, more functions, improved programming, and could be expanded with both RAM and ROM modules.

From The Introduction:

« The HP-41C represents a totally new concept in the design of Hewlett-Packard calculators. In fact, because of the advanced capabilities of the HP-41C, it can even be called a personal computing system. The HP-41C is the first Hewlett-Packard handheld calculator offering an exciting array of alphanumeric capabilities.With so many different kinds of calculator uses and applications in the world, we at Hewlett-Packard decided we could provide a significant contribution by designing and building you aquality calculator with expandable and flexible capability. The alphanumeric HP-41C is just the calculator. »

Alphanumeric Implementation

The HP-41C used a low power LCD display with 7 more segments than previous calculators. This allowed a full range of alphabetic characters. The characters were not as fully formed as a dot matrix display, but they had better contrast.

There was a special register on the HP-41C called the Alpha Register. It allowed up to 24 characters. This register was separate from the stack and the other storage registers. Alpha strings could be stored in normal registers or the stack but they were truncated to 6 characters when this was done. A function that shifted strings by 6 characters was included to make it easy to store long strings in multiple registers.


Cassette readerThe HP-41C came with 63 « registers » of memory. Memory could be dynamically reassigned between storage registers and program memory. (63 registers was equal to 200-400 fully merged lines of program memory.)

Four ports at the top end of the machine allowed the user to plug in 4 more RAM modules which would increase the maximum register space to 319 or allow 1000-2000 lines of program space. (Or any compromise between those.) Later, the entire register space could be provided via a single « Quad Memory » module and Extended Memory was added to allow even more space..

The Optional Card Reader/Writer (82104A)

Card reader, ROM module and faceplateBecause the HP-41C had continuous memory and because its application pacs came on ROM modules, a card reader was omitted. However, one could be plugged into the top end. (In less time then it takes to read this sentence.) The HP-41C was obviously not code compatible HP-67, however, a sophisticated translator was built into the card reader which translated programs to HP-41C code on the fly.

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