The HP 48SX calculator input/output system

Hewlett-Packard Journal, June, 1991 by Steven L. Harper, Robert S. Worsley

WHEN HANDHELD CALCULATORS were first introduced in the early 1970s, they provided portable computation without much memory. The limited keyboard and one-line numeric display provided just about all the input/output needed to use this capability effectively. As the incorporation of advancing memory technology made possible the storage of larger amounts of data and even programs created by the user in these handy little machines, the keyboard and display became an intolerable bottleneck. The need to enter a thousand or so keystrokes manually to use a program that someone else has written is quite an effective barrier.

The first solution to this problem was to use small magnetic cards to store the data and programs. There were some difficulties with power consumption, physical size, mechanical wear and tear, and cleanliness, but this mode of input/output was the accepted standard for some time. Eventually, however, larger and larger memories began to outstrip the capacity of the cards. This, combined with a need to communicate with other types of devices that did not use magnetic cards, necessitated a new approach.

The HP-IL (Hewlett-Packard Interface Link) was an electronic interfacing system that was designed with the needs of calculators in mind. It allowed systems with several devices to be configured automatically and controlled by a calculator. These devices included printers, mass storage, adapters to other interface systems, and even instruments. In many ways, it was superior to existing electronic interfaces, but this was not sufficient to overcome the inertia of the massive installed base of these other devices. Prices of calculators continued to drop and patterns of use changed as personal computers became ubiquitous. For these and other largely nontechnical reasons, HP-IL was no longer the ideal input/output medium for the majority of HP calculator users.

One area of serious complaint with calculators and electronic interfaces had been the cost and inconvenience of the cables. In addition, some calculators were so thin that even the HP-IL connectors, designed for small size, were unacceptably large. The most pressing need for these machines was to provide some form of hard-copy output to a low-cost portable printer. Drawing upon technology similar to that used in infrared remote control of TVs and VCRs, HP introduced a printer and a line of calculators that met the basic need with an interface that gave customers what they wanted: no cables at all. The only disadvantage was that this infrared connection was output-only.

Input/Output Needs of the HP 48SX

Market research indicated that an overwhelming majority of high-end calculator users either owned or had access to a personal computer. Clearly, it would be an advantage to be able to perform the data-entry-intensive tasks with the large keyboard and display of the personal computer and the computation-oriented work with the simple and powerful applications resident in the calculator. In other cases, the portability of the calculator was essential for part of the job and the speed and capability of the personal computer and its associated peripheral devices satisfied the rest of the need. These considerations forced the primary objective for the input/output capability of the HP 48SX scientific expandable calculator: an easy-to-use connection to existing personal computers.

In addition, there were two secondary objectives. While most users would choose to satisfy their needs for hard copy with the printer connected to their personal computer, there would be some applications where portable printing was essential, and perhaps some users who needed simple hard copy without access to a personal computer system. Also, our experience with the HP 41 calculators had shown that users of this class of machines do a lot of sharing of programs and data. In consequence, it was felt necessary that the HP 48SX be compatible with the existing infrared printer and that there be a convenient way to do input/output directly between calculators. Fig. 1 illustrates the desired input/output connections.

The most common interface on personal computers is the RS-232 serial port. It would be much too costly and inconvenient to require the customer to buy a special card for the personal computer, such as an HP-IL or infrared interface, to connect to the calculator. Development of these cards would be costly, too, since there are several different types of personal computers. These considerations quickly led to the decision to design into the HP 48SX the capability to connect directly to the serial port already in nearly all personal computers. This required a custom connector and cable, since the traditional DB9 or DB25 connectors used with RS-232 were much too large.

The requirement for program and data sharing between calculators could also be satisfied with RS-232, but the need to have the cable and/or some special calculator-to-calculator adapter was a severe drawback in this situation. A one-way infrared interface was already needed to maintain compatibility with the infrared printer. Would some enhancement of this interface fill the need without requiring the user to carry around a cable? Some investigation showed that a simple infrared receiver circuit for very short distances could indeed be included with minimum impact on the design.