Excerpt from Dealers of Lightning:
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Starkweather and Rider worked together on coordinating the SLOT and character generator until early 1972, when they were stymied not by a technical obstacle but one entirely man-made. This was the relocation of more than twenty of PARC’s seventy scientists up the hill to a building newly rented from the Singer Company and known as Building 34 (because its address was 3406 Hillview). The Computer Science Lab, including Rider, got bundled off to the new quarters while everyone else, including Starkweather, temporarily stayed behind on Porter. The move separated the two by a kilometer of real estate—too far to string an overhead line and, with the four-lane Foothill Highway in the way, impossible to link via a ground cable.
“The administrators said, ‘Don’t worry. You’ll be back together in another year,’” Starkweather recalled. “I said, ‘Great, what are we supposed to do in the meantime?’”
But one Sunday afternoon shortly after the move Starkweather got a brainstorm while sitting at home. He immediately jumped in his car, drove to Porter Drive, and mounted a stairwell to the roof.
Just as he had thought, he could take line-of-sight aim from where he stood to the rooftop of Building 34. He might not be able to span the distance by cable or wire—but he could do it by laser beam.
The next day he ordered four telescopes from Edmund’s for about $300 apiece. He and Rider replaced the eyepieces of two with low-power lasers and the others with sensitive photodetectors. They bolted one laser scope and one detector on each roof, aiming each at its complement across the way, to create a visible light data link.
The circuit worked flawlessly in almost any weather, even fog, although minor adjustments were often necessary after a rainstorm, when the weight of accumulated water made the roofs sag slightly.
“When SLOT was running I’d send a pulse of light up the hill to signal the character generator to send a line of data down to the detector on my roof, which would send it down to this laser and then to the printer,”
Starkweather recalled. “After all, we were only encoding ones and zeros. It was like sending binary data on a long wire made out of light, instead of copper.”