LCD Exploration Unwinds Back to CL84209

In my electronics teardown and salvaging adventures, I kept a few segmented LCD units with the intent to learn about them later. After I watched Joey Castillo’s Remoticon presentation on LCDs, I decided it was time to take some LCDs out of my pile and start learning how to use them.

In hindsight, I undertook this adventure in backwards order. I couldn’t make heads or tails of my first attempt, and the second didn’t fare much better though it gave me enough information to infer those two LCDs both came from an AT&T CL84209 wireless landline system. The third attempt was a car tape deck I managed to power up, but it was too advanced of a system and wouldn’t help me learn LCDs basics. The fourth item was an LCD salvaged from a food thermometer, but I had problems trying to attach wires to this display. I was finally successful on the fifth try, obtaining full control over a simple segmented LCD salvaged from an electric blanket controller.

After that success, I started unwinding my stack of failures. A bit of hackery and a lot of patience allowed me to map out the food thermometer LCD on my second try.

With these two examples of low-level LCD control, I returned to the tape deck and learned how to work at a higher level with its LCD controller. I even learned a new communication protocol (Sanyo’s CCB) and programmed an Arduino Nano to speak its language.

Obtaining a full understanding of how to interface with the faceplate was very satisfying. Two key factors made it possible: the LCD controller’s datasheet, and a Saleae logic analyzer.

Armed with this experience, I will now return to where I started: to the mystery LCD units salvaged from an AT&T CL84209 landline phone system. I found nothing to identify their LCD controller, so I would have to go without a datasheet. However, I do have one valuable datapoint: a risky experiment proved that while those two LCDs were different, they do communicate with the same protocol. This is enough of a starting point for me to dive in with my logic analyzer.

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