This portable thermal printer is a prototype for the HP 82240A printer developed as an accessory for Hewlett-Packard handheld calculators. Connection to a calculator is through infrared radiation, not by any cord.
Hewlett-Packard announced that the HP-28C calculator would be supplied with a printer before actually developing the product. The donor, David Rabinowitz, was the project manager for the printer project. R&D specified that the signal between calculator and printer would be infrared. Rabinowitz decided that that it would travel only over about three feet (so as not to interfere with other electronic devices – this had been a problem with the infrared keyboard products like the IBM PC Jr.) Because the product had been announced, the printer was to be available within a year. The prototype dates from 1985 or 1986, and production began not long thereafter.
The object has a translucent plastic case. It has buttons for on/off, print contrast, and paper advance. A mark toward the bottom left of the case reads: hp HEWLETT (/) PACKARD.
The infrared filter, a black piece on the front edge of the printer, blocks out light of other frequencies. A small square black detector behind it detects the infrared radiation. The Intel-designed 8050 chip in the case has 256 bytes of RAM and 4096 bytes of ROM. Software in the ROM decodes the signals, times the circuits, and controls the heaters in the print head that heat the thermal paper behind it. These temperature changes caused chemical changes, darkening the paper to print letters, numbers or graphs. Depending on the paper used, the printing was either blue or black.
The infrared demodulator chip required an operating voltage of 5 volts. The four batteries in the printer produced 6.4 volts when new, decreasing with time to as low as about four volts. A CMOS chip in the printer circuit boosted the voltage to five volts, but printing could be slow when the batteries were low. Using an AC adapter plugged into the wall guaranteed a higher voltage so the motor ran faster and printing was more rapid.
References:
Yves Nievergelt, “The Chip with the College Education: the HP-28C,” The American Mathematical Monthly, vol. 94, # 9, November 1987, pp. 895-902.
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