There was no shortage of surprises as I'm learning from the control panel circuit board of a Canon Pixma MX340 multi-function inkjet. I followed a wire that I thought supplied power to multiple components, found it powered just a single LED, then realizing the LED on/off is actually controlled by a transistor further down the line. Weird! Fortunately the remaining pins on this connector (for ribbon cable to main board) were easier to understand, enough for me to generate a medium-confidence pinout chart.

Connector to main board pin 11 was soldered to a trace that went under the connector and out the top side. Tracing its path through a zero-ohm jumper resistor (JP117), a few check points (CP102, CP112), an unpopulated capacitor position (C110) and a 220 ohm resistor (R101) I arrived at the "Stop" button (SW101) at the control panel's far right edge. Canon engineers decided a user trying to stop something in a panic should have a direct line to the main board bypassing the NEC K13988 intermediary.

Connector to main board pin 3 was connected to a capacitor (C116) to ground typical of a decoupling capacitor, and a 470 ohm resistor (R104) typical for LED current limiting. Upon initial examination I thought it might lead to one of the LEDs, but I've accounted for all LEDs by now so I know it isn't that. I traced its route through CP114 and CP126 to arrive at K13988 pin 29.

Connector to main board pin 2 and pin 1 traveled side-by-side, but received slightly different treatments. Pin 2 signal had to travel through a 100 ohm resistor (R103) while pin 1 signal did not. Pin 2 also had a capacitor (C115) to ground while the pin 1 equivalent position (C114) is unpopulated. When they reached the K13988 chip, main board connector pin 1 signal went to K13988 pin 16, and main board pin 2 went to K13988 pin 18.

Pin numbering is right-to-left in the picture, as per arrow and number 1 printed adjacent to the right end of the connector. There is also a dot every 5 pins to aid in counting. After tracking down all of these traces, I repeated the exercise with pins connected to the K13988 chip.

Pin Number Connection
1 (arrow) K13988 pin 16
(Main board to K13988 asynchronous serial 250000 8E1)
2 K13988 pin 18
(K13988 to main board asynchronous serial 250000 8E1)
3 K13988 pin 29
(K13988 Chip Enable)
4 Ground
5 (dot) Alarm LED+ @ 3.3V DC
6 Power LED+ @ 3.3V DC
7 Power button
(Grounded when pressed)
8 Ground
9 3.3V DC power
(Always on, even in standby)
10 (dot) Ground
11 Stop button
(Grounded when pressed)
12 WiFi LED+ @ 5.5V DC
(Illumination controlled via transistor Q101.)

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