curiousmarc
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HP 9825 insane LED display chip
🕑 Added 2021-08-30 07:23:35 +0000 UTCAll the LED chips that make the beautiful 32 character display line in my HP 9825 are dead! They died from the 13V overvoltage. These chips, capable of displaying 4 characters, each a matrix of 5x7 dots, were specifically developed for the HP 9825. HP sold them later commercially, under the HDSP-2000 series.
They are amazing and probably insanely costly hybrids on ceramic, containing 28 bars of LEDs, stitch bonded to form 4 arrays, and 2 driver IC chips, under a cover of tinted glass.
The chip in the middle is a shift register, so you can load 28 bits for a row of all characters serially, then display that, then move to the next row.
Ken Shirriff reversed engineered the driver chip already and explains how it works on twitter:
https://twitter.com/kenshirriff/status/1431752029106409473
I'll be showing a short video of the bits dancing around the shift register as I test the chips in the next post.
Marc
