I want to see how much I can understand of the scanner image sensor bar I pulled out of a Canon MX340 multi-function inkjet. A little research side quest helped me feel better prepared for the task of figuring out what happens over the twelve wires connecting the sensor bar to the printer main board.

Looking at the copper traces on the circuit board, I can see some of them are directly wired to the pins for LED illumination. Verifying continuity between those connector pins and LED pins identified the left-most five of twelve wires. Probing for continuity with chassis ground, I found one more ground wire. Measuring the remaining wires while the system is powered up, I found one pin that held steady at 3.3V. A good power source candidate for image sensor logic.

That left five unknowns whose voltages fluctuate. Too quickly for my volt meter to catch what's happening, hinting at data signals. My oscilloscope has four channels so I need to choose four to solder wires to and leave one for later.

This connector also has 1.0mm pitch and this time I managed to solder wires without creating bridges. Luck or improving skill I don't know yet, either way I expect my MX340 teardown project will give me many more practice opportunities.

I turned on the MX340 and saw this on my oscilloscope screen. The yellow wire is attached to a pretty steady wave at 2.375MHz, possibly a clock signal. Its adjacent red and blue wires seem to track pretty closely to each other, the green wire looks much the same but at roughly 1V higher.

Unfortunately, that's pretty much everything I got. I see this pattern as soon as the printer powers up, and it looks much the same during the homing sequence and afterwards. I waved my hand and shined a light at the sensor bar and saw no significant change in these signals. If I were looking at LVDS, according to Wikipedia I should see a 350mV change, but I don't.

That said, there is something that occasionally flashes past this screen, but I didn't know how to isolate it yet. Whatever it was, it occurred too infrequently to be image data. (Later I would determine it is a line sync signal on the red wire.)

Could the magical data wire be the fifth and final wire I had yet to examine? On paper it was a 20% chance I missed the most important wire, but Murphy's Law raises those odds. Besides, I'm so close and it'd be silly to give up now. I turned off the printer and oscilloscope and turned on the soldering iron. The green wire was moved one pin over, and I tried again.

Jackpot! I have a candidate image sensor data signal.

This teardown ran far longer than I originally thought it would. Click here for the starting point.