Industrial design of the HP 48SX calculator

Hewlett-Packard Journal, June, 1991 by Michael Derocher

The expressed needs and wants of high-end technical calculator users contributed to the design objectives for the HP 48SX scientific expandable calculator. Needs analysis and concept testing were conducted at the onset of the HP 48SX program, and stated requirements included:

* Handheld product shape (traditional vertical format)

* Large display 8-line desirable)

* Customizable and expandable through plug-in media

* Data I/O for mass storage, printing, programming, etc.

* High-quality tactile keyboard.

In addition to these stated requirements, the industrial design objectives for the HP 48SX included:

* Easily learned accessibility to functions and features through an organized keyboard and display interface

* Direct and easy access to expansion and customization media

* A visual and tactile experience consistent with the product's "next-generation" technological leadership.

Package Design

A softened, vertical-format package allows comfortable handheld use of the HP 48SX. Even the molded rubber feet are sculpted to match the case contour for minimal tactile intrusion. Overall size was determined by the handheld requirement for width, keyboard and display size for length, and component sizes for thickness. Components are located to allow a low leading edge for the keyboard, resulting in a three-degree wedge profile. The keys and keyshapes are leveraged from the previous series of low-cost, high-volume calculators. They are designed to provide comfortable yet positive tactile response, minimal entry errors, and high reliability. User access to batteries and plug-in media is provided through covers at the back of the calculator. Both the infrared and RS-232 I/O ports are at the back, directed away from the user. A molded arrow-shape indicator on the topcase, above the logo, communicates the position of the direction-sensitive infrared I/O feature, and the lens area of the cover over the infrared components is polished to communicate its light-transmissive function. Next to it is the RS-232 port, which accepts a small, custom 4-pin plug designed to match the visual theme and scale of the HP 48SX.

The goal of customization and expansion is accomplished through two plug-in slots for credit-card-size memory modules, available as RAM or ROM applications. The slots are located under a removable cover and infrared lens at the rear of the bottom case. Because of cost and size constraints, an eject mechanism was not feasible for user removal of the cards. Instead, user access is provided by offset-stacking the cards to expose custom, molded plastic grips, which are attached to the cards by the vendor (see Fig. 1 on page 25). These contoured, textured grips are molded with a hole in the center to provide visual access to a title graphic on the printed card surface while the card is inserted in the calculator. The constructed grip texture is consistent in scale and appearance with other grip textures used on the HP 48SX, and enables the user to remove the cards from their friction-fit edge connectors with a single finger or thumb.

A custom soft case was designed to be included with the HP 48SX. its objectives are to protect the product in normal transport, to provide storage for a quick-reterence guide and additional memory cards, and to reinforce the message of product superiority and quality. The final design is a fine-weave pouch. The case is lined and padded, and has a zipper closure. An internal pocket is provided for the quick reference guide and memory cards.

User interface Hardware Design

Although the HP 48SX approaches computer power and sophistication, it operates primarily as an application with a dedicated keyboard and display interface. The most significant industrial design challenge was to provide visually ordered access to the 21 00 functions using only forty-nine keys plus display menus.

The user interface surface of the HP 48SX is divided into two primary zones: the display and display bezel and the keyboard and keyboard overlay (see Fig. 1 on page 6). The windowless 8-line liquid crystal display is framed by a formed aluminum overlay, which "cascades" down to the keyboard in two steps. Located in the middle step is a row of six keys which operate in conjunction with menus presented on the bottom row of the display. The overlay color surrounding these keys is the same as the display bezel to reinforce the association of keys with display menus. The six menu keys are differentiated in size and color from all other keys to distinguish them as special. At the base of the second overlay cascade is the remainder of the keyboard, which provides quick, direct access to alphanumeric entry, math operations, cursor control, and function sets presented as display menus. The total of 49 keys was chosen based on the minimum number required to provide discrete alphanumeric entry. Keys are grouped and sized according to function and relative frequency of use to order the keyboard visually.

The majority of the function markings are printed on the aluminum overlay. Unlike preceding models, including the HP 41 C, there was no opportunity to place a second set of function markings on the keys. This is because of the leveraged keyboard design, which is limited to a single, integrally molded function marking. This presented a graphic challenge because the keys access up to four major functions each. As a result, the keyboard overlay is designed with up to two shifted functions above each key, positioned side by side and accessed by color-coded shift keys. The shift keys are also coded with arrows indicating left or right for the relative position of the shifted nomenclature. These symbols are used in the documentation in place of color to reduce printing costs. Twenty-six keys also have an alpha character at the lower right corner, accessed by an alpha shift key. Shifted functions that call up screen menus are distinguished by a black field behind the text and are grouped in two areas of the keyboard. The overlay has a total of six silk-screened colors and one tint. The keyboard is designed to accept snap-in custom overlays for user-programmed keys, custom application requirements, and so on.