I have my Saleae Logic 8 logic analyzer set up to listen in on the communication between control panel and main board of a Canon Pixma MX340 multi-function inkjet. After picking a few scenarios to record, I decided to start by looking at its steady-state behavior. I expect this to provide a baseline I can compare against for examining state change behaviors. I think I can get my baseline behavior from the trace where I pressed four buttons, as there should be plenty of steady-state information between my button presses.

Here's a snapshot of steady-state behavior under the oscilloscope. Channel 1 (yellow, main board to control panel) is held high, transmitting no data. Channel 3 (cyan, control panel enable) is held high to keep the K13988 chip active. The only activity here is on channel 2 (magenta, control panel to main board) where a short burst of activity occurs every 9+ milliseconds.

Zooming in, it looks like a simple square wave.

When interpreted as 250000 baud 8E1 serial data, this pulse represents a single byte of data with value of 0x80. Saleae Logic software measured interval of these pulses at 9.2ms apart, and a different value is conveyed if a button is pressed.

Button Pressed Value reported every 9.2ms
(Hexadecimal)
(None) 0x80
OK 0xC9
Right/+ 0xCA
Left/- 0xCB
Back 0x93

Looking at these values, I noticed "OK", "Right/+" and "Left/-" generated consecutive values, but "Back" was very different. Looking at the button matrix I mapped out earlier, I see the three consecutive values were all associated with pin 1. This increases my confidence in my button matrix, and this reported value is probably the button's position in that matrix.

This button matrix state is sent every 9.2ms, even if there's additional communication between the main board and control panel. For example, during the ~70ms required to update information displayed on the LCD screen, button matrix state is still sent in between all the display data acknowledgements.

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