Preparing Cordless Handset LCD for Logic Analysis

I’ve decided to take another look at a pair of LCD units salvaged from an AT&T CL84209 landline phone system. I have a bare LCD from the base station, and an LCD still attached to the remnants of a cordless handset. I had wired those two LCDs in parallel, a crazy roll of the dice that paid off in knowledge that those two LCDs use the same communication protocol. Now I am armed with a logic analyzer (Saleae Logic 8) and intend to get more details.

The first step is to attach wires to the handset LCD so I could probe communication traffic, similar to how I attached the bare LCD earlier but with less risk and higher chance of doing something useful. I recycled the bundle of wires I had used to perform the same task on a Toyota tape deck faceplate, which had more wires that I folded out of the way for this project. Having done similar tasks a few times, I’m starting to think about buying PCBite probes that promise to let me skip this soldering task. But I’m not yet at the point where I expect to use them enough to justify the expense.

There are nine pins on this LCD, which given my lack of official datasheet I’ve arbitrarily numbered from left to right as shown in the picture above. Earlier I had probed these connection points with a voltmeter while the handset circuit board was running. All of the pins appear to be safely within Saleae logic analyzer 25V limit relative to ground, which I determined to be pin 4. That left 8 pins I don’t understand and conveniently, my Saleae has 8 inputs so I could connect them all and not have to make decisions on which pins to omit. Once connected, I could take my first look with the Saleae in analog mode.

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