Faster toggling of the pin indicated a darker section of the image, slower corresponded to lighter. The scanner software simply looked at the rate at which new characters were becoming available on the serial port as the handshake pin was toggled at various frequencies by the output of the optical sensor. Instead, the output of the analog optical sensor was fed to a voltage-to-frequency converter, which was then hooked up to the handshake/clock-in pin on the serial port. It did precisely nothing with the serial data lines at all, these were left for the computer to command the printer. The hilarious part is how the scanner actually delivered data to the Macintosh computer it was hooked up to. In that carriage was an optical reflective sensor which was scanned across a page horizontally while it was fed through the printer. The actual scanning method was simple enough the device mounted a carriage to the printer head of the ImageWriter. Weird enough already, but this device hides some weird secrets in its design. It was known as the Thunderscan, and was a scanning head built for the Apple ImageWriter dot matrix printer. Back in these halcyon days, an obscure company called Thunderware built a device to convert the former into the latter. Scanners were rare, particularly for the home market, because home computers could barely handle basic graphics anyway. Back in the 1980s, printers were expensive things.
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